OSU College of Engineering

ENG 181 - Introduction to Engineering I

 

Pre-lab Assignment for Camera Lab 4

 

 

What You Have To Do Before Lab 4

 

1.         Read the material for Camera Lab 4.  If you like, you can even visit a library or use the web to find out more about these things.

2.         Perform the pre-lab assignment described in the Camera Lab 4 material.  The pre-lab tasks are to be performed and turned in by your team.  It is a good idea to make plans now for when you will get together as a team, before coming to Lab 4, to complete this part of the assignment.  You will NOT have time to do it at the beginning of Lab 4.  In order for your team meeting to be effective you should try to read as much of this document as possible before the meeting.

3.         Turn in the team pre-lab assignment at the start of Camera Lab 4.

Note:  The pre-lab assignment is not a lab report.  It does not have to follow the lab report format and may be handwritten.

 

What You Have To Do After Lab 4

 

1.         Analyze the results of the measurements you performed in Lab 4.

2.         Prepare a team report on Lab 4.  You will be given a set of instructions that are specific to the Lab 4 report.  Follow these instructions when writing your report.

3.         Turn in the team report for Lab 4 at the beginning of Lab 5.


OSU College of Engineering

ENG 181 - Introduction to Engineering I

 

Camera Lab 4 - The Flash Circuit

 

               Table of Contents

 

Table of Contents............................................................................................................... 2

Camera Lab 4 - The Flash Circuit..................................................................................... 3

1     Introduction............................................................................................................... 3

1.1    Objectives of Lab 4............................................................................................... 3

2     Background............................................................................................................... 4

2.1    The Core and Auxiliary Functions of the Flash Circuit............................................. 4

2.2    The Circuit Board and Circuit Schematic................................................................ 7

2.3    A Few Basic Concepts for Electronic Circuit....................................................... 13

2.4    Operation of the Flash Circuit.............................................................................. 23

2.5    What's an Oscilloscope?...................................................................................... 25

3     Pre-lab 4 Assignments............................................................................................ 27

4     Summary of the Lab 4 Procedures........................................................................ 34

4.1    Introduction......................................................................................................... 34

4.2    Task 1  - Charging Transient Measurements with the Oscilloscope....................... 36

4.3    Task 2  - Discharge Transient Measurements with the Oscilloscope...................... 37

4.4    Task 3  - (Optional) Flash Trigger Measurements................................................. 39

5     Report Instructions for Camera Lab 4.................................................................. 43

5.1    Introduction......................................................................................................... 43

5.2    Experimental Methodology.................................................................................. 43

5.3    Results and Description....................................................................................... 43

5.4    Discussion........................................................................................................... 44

5.5    Summary and Conclusions................................................................................... 45

5.6    Figures and Tables.............................................................................................. 45

 


OSU College of Engineering

ENG 181 - Introduction to Engineering I


               Camera Lab 4 - The Flash Circuit

 

Helpful Studying Procedure

While reading the following material you are encouraged to have the flash circuit board out of the camera (with the battery removed and capacitor discharged.  As you are reading certain parts you may find it helpful to look at the circuit, see the items being discussed and take notes on your observations.  You may also find it helpful to have a highlighting pen handy when you read section 2.4.  You can use it to mark the components on the circuit schematic (Figure 1 of Figure 2) as you read about them.  Clues to the answers to some of the questions in the Pre-lab assignments are dispersed throughout the reading and you are more likely to remember those clues if you observe the circuit and schematic while reading and take notes on your observations.



1         Introduction

 

This handout contains:

·        A description of the core and auxiliary functions performed by the flash circuit in the Kodak Max Flash camera

·        A discussion of the circuit board and circuit schematic

·        A discussion of some basic circuits concepts including a description of the components of the flash circuit on which you will be performing measurements.

·        A discussion of how the flash circuit performs the core functions

·        The Pre-lab Assignment you are to perform before coming to Lab 4

·        An outline of the measurements you will perform in Lab 4

 

You should read this entire document before coming to Lab 4 as part of your preparation for lab.  A pre-lab assignment with work you are to turn in is included, starting on page 27.

 

1.1             Objectives of Lab 4

            The Kodak Max Flash camera contains a circuit for operating the flash.  We do not expect you to become an instant expert on the operation of this circuit.  We do expect you to:

·        Develop an understanding of the distinction between the core functions and auxiliary functions of the flash circuit and acquire a "big picture" view of what the circuit must do to perform the core functions.

·        Acquire experience with the relationship between the actual circuit board in the camera and the "schematic" representation of the circuit.

·        Acquire experience in using a voltmeter and an oscilloscope to measure voltages in the circuit.

·        Be able to calculate currents flowing in certain parts of the circuit using the measured voltages together with the values of parameters of some of the components.

·        Develop an understanding of how and why the speeds of various parts of the flash circuit are related to the speeds of other parts of the camera.

 

The components and operation of some parts of the circuit are more complex than others are.  Understanding the operation of the more complex sections requires more background than we will be able to give you in this quick touch on electronic circuits.  These more complex parts of the circuit will receive only a cursory treatment in this laboratory.

 



2         Background

2.1             The Core and Auxiliary Functions of the Flash Circuit

 

2.1.1       The Battery and the Flash Tube

The overall objective that the flash circuit must meet is to provide a short flash of light that is synchronized with the shutter of the camera.  This is basically a problem in synchronized energy conversion that engineers solved with the flash circuit.  Before we can look at the core functions of the camera circuit we must take a brief look at the battery at the input end of the system and the flash lamp at the output.

 

The energy for the flash of light comes from the AA battery in the camera.  The battery is basically an energy storage and conversion device.  It stores energy in chemical form and converts it to electrical form. 

 

In general, a battery makes its electrical energy available when the user "completes a circuit" between the battery's positive and negative terminals.  In this case to "complete the circuit" means to provide a path for electrical current to flow out of the positive terminal of the battery, through the device that the battery is powering, and back to the negative terminal of the battery.  The term "electrical current" refers to the motion of electrons through the wires and components of the circuit.  More will be said about current in the Basic Concepts section of this document.  Note that the current eventually comes back to where it started.  This is the origin of the term "circuit."  In general, current will not flow unless such a closed path exists. 

 

The AA battery is a 1.5 volt battery.  It has a voltage difference of 1.5 volts between its positive and negative terminals.  The battery supplies current to the circuit at an electric potential, or voltage, of 1.5 volts.  (For a discussion and definitions of voltage, electric potential and current see the Basic Concepts section.)  The current that returns to the battery comes back at an electric potential that is 1.5 volts lower than when it left the battery.  This difference is related to the energy delivered to the components that the current passed through in the circuit.

 

            The flash lamp in the camera is also an energy conversion device.  It converts electrical energy into optical energy.  The flash lamp is very different than a typical incandescent light bulb since flash photography requires a very fast and bright light pulse.

 

The flash lamp has the form of a sealed glass tube with an electrode at each end.  The flash tube requires a voltage difference of several hundred volts across the two electrodes for it to operate.  However, it is not sufficient to merely apply this voltage.  Unlike an incandescent light bulb, the flash tube does not have a wire inside it between the two electrodes.  Rather, the tube is filled with a gas.  Under normal conditions the gas does not provide a path for current flow from one electrode to the other.  That is, under normal conditions the gas is an "open circuit" and has a resistance to current flow that is so large we can consider it to be infinite for most practical purposes.  It is not our purpose to study the physics of what happens inside the flash tube during a flash.  It will suffice to say that under the right conditions the gas in the tube can be made to break down and form a glow discharge.  In the glow discharge some of the atoms have one electron removed.  The atoms that are short one electron have a positive charge and are known as ions.  The voltage applied between the electrodes of the lamp cause the negative electrons and positive ions to move (in opposite directions), and moving charge is current.  As a result the glow discharge has a relatively small resistance to current flow.  Light is emitted as the electrons relax back to their normal locations on the atoms.

 

The right condition to trigger the flash is a voltage pulse applied to a third electrode (a trigger electrode) placed very close to the side of the flash tube.  The metal mirror, or reflector, behind the flash tube has double duty.  In addition to reflecting the light that comes out of the back of the lamp toward the front of the camera, it serves as the trigger electrode.  The required trigger voltage pulse is more than a thousand volts and causes the gas in the flash tube to break down.  Once the flash tube is triggered the glow discharge in the tube provides a low resistance path for current to flow between the electrodes at the two ends of the tube. 

 

The trigger voltage is only needed to start the discharge.  Once the glow discharge is started the voltage applied across the tube will maintain the glow discharge.  The current flow will persist as long as the voltage across the two electrodes is above some minimum value (typically a few tens of volts).

 

2.1.2       The Core Functions

            It should now be apparent that there is a mismatch between the battery and the flash tube.  The battery provides a voltage of 1.5 volts, but the flash tube needs much larger voltages.  The flash circuit provides the interface between the battery and the flash tube.  It must take what the battery supplies and convert it into something that is useable by the flash tube.

 

            While it may be possible in principle to design a circuit that will directly match the battery to the flash tube as soon as the shutter is triggered, such a circuit would definitely be expensive compared to the low cost of the camera.  It would probably also be too large to fit inside the existing camera box.  A circuit that can accomplish the required goals can be made economical and small enough to fit in the camera by breaking the overall process into two parts.  These two parts are the core functions of the flash circuit.

 

The first core function starts when the user presses the charging button on the front of the camera.  As a result, chemical potential energy stored in the battery is delivered to the circuit as a relatively large current at only 1.5 volts.  The circuit steps the voltage up to a much larger value, but the process of increasing the voltage necessarily decreases the current.  The small current at larger voltage is delivered to a large capacitor in the circuit.  As a result of the small current the capacitor slowly "charges up" to close to 350 volts.  This takes several seconds, much longer than the time the shutter is opened, so it must be done before the picture is actually taken.  In Lab 4 you will measure how long this takes.  The capacitor is used to temporarily store the electrical energy delivered from the battery through the circuit, but now the energy is at a large enough voltage to be useful for the flash lamp.  The flash tube is connected "in parallel" (see section 2.3.6 for an explanation of the usage of parallel here) with the capacitor, so the voltage on the capacitor also appears across the flash tube.  However, since the flash tube normally has very high resistance, it does not flash yet.

 

The second core function starts when the shutter is opened in the process of taking a picture.  The circuit uses a small fraction of the stored electrical energy to generate a very quick flash trigger pulse to initiate the glow discharge in the flash tube.  Once the glow discharge is started the flash tube temporarily has a very low resistance, allowing the energy stored in the capacitor to quickly discharge through the flash tube, where much of it is converted from electrical energy into optical energy.  (The remainder is converted into waste heat.)  This happens very quickly, while the shutter is open.  In Lab 4 you will measure just how fast these processes are.

 

 

2.1.3       Auxiliary Functions

The circuit performs a few convenience functions in addition to the two core functions.  One is to illuminate a pilot light to let the user know when the circuit is charged and ready for flash photography. 

 

The two core functions and the pilot light were the only functions performed by the flash circuit in the first generation of the Max Flash camera.  The user of the first generation camera had to press and hold the charging button until the pilot light turned on.  A flash photograph could then be taken within a few minutes (before the pilot light went out).  In order to take another flash photograph the user had to press and hold the charging button until the pilot light came on again.

 

The current generation of the MAX Flash camera has a few additional features.  First, the user only has to momentarily push the charge button to start the charging cycle.  The circuit latches into an "on state" and keeps charging the capacitor until the flash capacitor is fully charged.  Since the charging circuit is latched on, additional modifications had to be added to automatically unlatch it after the capacitor is fully charged.  Otherwise the battery would be quickly drained.

 

Another convenience feature that was added causes the flash circuit to automatically recharge after a flash photograph has been taken.  In this way the user doesn't have to remember to press the charge switch between taking flash photographs.

 

2.2             The Circuit Board and Circuit Schematic

           

The flash circuit for the camera is made on a printed circuit board.  The body of the board is made of an electrically insulating material.  Much of the "wiring" in a printed circuit board takes the form of thin metal traces printed on the board, rather than actual wires.  That's why it's called a "printed" circuit board. 

 

Holes are drilled through the board in appropriate locations.  When the circuit is being manufactured components are connected to the metal traces on the board by inserting the wire leads of the components into the holes and joining them to the metal traces with solder.  In the original versions of the flash circuit board all of the components were attached to the board this way.  In some more recent versions of the flash circuit board some of the components are attached by a technique known as "surface mount."  Components that are attached by surface mount do not have wire leads designed to go through holes.  Rather, they have metal tabs that are directly soldered to the metal traces on the board, without holes.  Since the cameras are recycled, some of you will have boards with surface mount components and some of you will not.

 

The following discussion will specifically refer to two different generations of flash circuit boards.  Boards labeled "3B1573" do not have surface mount components.  (The code for these boards may have a suffix something like " L' " or " REV K ".  The suffix refers to revisions of the board within that generation.)  Boards in the other generation are labeled "5C3406" and have surface mount components.  Both of these generations of boards have the core and auxiliary functions described earlier, although there are differences in the two circuits besides the mounting of the components.

 

The 3B1573 boards have metal traces on both sides.  Connection of a trace on one side to a trace on the other side can be through a hole if a lead from a component is soldered to a trace on both sides.  The 5C3406 boards are one-sided, although they may have wires inserted through holes to the other side to provide crossover connections.

 

            The sides of the boards that have printed wiring probably have a greenish appearance, rather than looking like metallic tin or copper.  This is because the board has been coated with a semi-transparent insulating film.  The insulating film is not present at the points where components must be soldered or where other electrical connections must be made (e.g. to the shutter, which acts as a switch).  Since the insulating film is semitransparent you can see the metal traces through it.  You can follow the metal traces from one component to another to observe how the components are interconnected.

 

            The circuit board in the camera is a relatively simple one.  More complex circuit boards can have multiple levels of wiring on several insulating layers that have been sandwiched together.  For the one or two-sided boards in the camera you can directly observe all the interconnections.  For a more complex multi-layer circuit board you would not be able to see the middle layers.  You would only be able to see the top layer on one side and bottom layer on the other.

 

            A circuit schematic is a symbolic representation of an electronic circuit.  Each symbol on the schematic represents a component in the circuit.  The lines in the schematic represent the wires or metal traces on the circuit board used to connect the components together.

 

Figure 1: Schematic of the 3B1573 generation of flash circuit.

 

Figure 2: Schematic of the 5C3406 generation of flash circuit.

A partial schematic of the 3B1573 generation of flash circuit is shown in Figure 1 while a partial schematic of the 5C3406 generation of flash circuit is shown in Figure 2.  In both circuits, the more complex circuitry that we will not study in this lab is not explicitly shown.  It is represented by the box labeled "More complex circuitry for converting DC to AC."  Most of the differences between the two generations occur inside this box.  There are a few other differences that are important for this lab, and they will be discussed below. 

 

Before getting into the details of the circuit a few general features about some conventions used in these schematics should be understood.  Wires connecting the various components are represented by lines.  A connection between two devices is known as a "node."  Wire intersections that are marked with a dot represent a connection between the wires.  Wires that are connected together will have the same voltage.  They form a single node and will have the same "node voltage."  The term "node" does not mean the dot at the intersections; all of the wires that are connected make up the node and have the same voltage.  Four nodes at which you will be attaching probes for measuring voltages are labeled with  A ,  B ,  C , and  G .  Wires that cross without a dot are not connected together - one wire bridges over the other.  They are not the same node.  There is one such bridge in Figure 1 (from Node B to the box representing the more complex circuitry), but none in Figure 2.

 

Note:  There is an alternate convention for drawing wires in schematics.  In this second convention any wires that intersect as straight lines are connected (no dots).   To draw wires that cross without implying a connection one uses a small "loop," typically drawn as a half-circle, in one of the wires to show it "bridging" over the other.  This second convention is not used in these instructions.

 

Table 1: International System of Units

Quantity

Unit

Symbol

Dimensions

Length

Meter

m

 

Mass

Kilogram

kg

 

Time

Second

s

 

Temperature

Kelvin

K

 

Current

Ampere (Amp)*

A

 

Frequency

Hertz

Hz

1/s

Force

Newton

N

kg-m/s2

Pressure

Pascal

Pa

N/m2

Energy

Joule

J

N-m

Power

Watt

W

J/s

Electric charge

Coulomb

C

A-s

Potential

Volt

V

J/C

Conductance

Seimens

S

A/V

Resistance

Ohm

W

V/A

Capacitance

Farad

F

C/V

Magnetic flux

Weber

Wb

V-s

Magnetic induction

Tesla

T

Wb/m2

Inductance

Henry

H

Wb/A

* "Amp" is a commonly used abbreviation for "Ampere"

 

            The layout of the components in the schematic does not reflect the physical position of the components on the circuit board.  Rather, it shows the electrical connections between the components.  In the schematic the components are frequently grouped according to the function of various parts of the circuit.  On the circuit board the layout of the components might be chosen for a variety of different reasons.  Some of these might be ease of manufacture, minimization of distance between components for speed of operation, or fitting the circuit board inside its housing.  Can you think of others?

 

            The following discussion describes each of the components that appear in the schematic.  While reading this section you should find each of the components on both the circuit board and whichever schematic is appropriate for your circuit.

 

The components drawn as zigzag lines and labeled with 2K, 1M, 3.9M and 150K are resistors.  The electrical function of resistors is described in more detail on page 14.  The unit of resistance is the ohm.  The ohm and other electronic units are defined in terms of more fundamental SI units in Table 1.  The labels next to the resistor symbols in the schematic give the nominal values of the resistance.  For example, the resistor labeled 1M has a nominal value of 1 Megohm, (106 ohms = 106 W) and the resistor labeled 150K has a value of 150 kilohm (150 ´ 103 W).  Table 2 identifies M, k and other unit prefixes.  Upper case omega is an abbreviation for "ohm."  The software package used to draw the schematic in Figure 1 omitted the omega.  It is not uncommon to find the ohms symbol omitted in schematics, however its presence is implied.  The resistors on the 3B1573 generation circuit boards are roughly shaped like cylinders with wires protruding from the ends.  The wires are bent and inserted through holes in the circuit board.  The resistors are marked with color bands.  These bands are a code for indicating the value of the resistor.  The surface mount resistors on the 5C3406 generation circuit boards are in the form of small black rectangles soldered on the two ends.  They are marked with numbers that are a code for the value of the resistors.  You may need a magnifying glass to read the numbers, depending on how good your eyes are for close work.  An Internet site (http://www.btinternet.com/~dtemicrosystems/beginner.htm) has a nice tutorial on resistor codes.  It explains the coding system for both the color coded and numbered resistors.  Briefly, the colors of the first three bands each represent a digit.  (The fourth band is typically silver of gold and represents a tolerance.)  Table 3 shows the meanings of the different colors.  The third digit is the number of zeros to add behind the first two digits.  For example, a 3.9 MW resistor would have the number code "395" or the color code "orange-white-green."

 

Table 2: Unit Prefixes

Multiple

Prefix

Symbol

Multiple

Prefix

Symbol

1018

exa

E

10-1

deci

d

1015

peta

P

10-2

centi

c

1012

tera

T

10-3

milli

m

109

giga

G

10-6

micro

m (u)**

106

mega

M

10-9

nano

n

103

kilo

k

10-12

pico

p

102

hecto

h

10-15

femto

f

10

deka

da

10-18

atto

a

** The Greek letter mu is the formally correct symbol for micro.  In cases when m is not available in the typeset being used to prepare a schematic the letter "u" is substituted.

 

 

Table 3: Resistor color codes.

Color

Digit

Color

Digit

Black

0

Green

5

Brown

1

Blue

6

Red

2

Violet

7

Orange

3

Grey

8

Yellow

4

White

9

           

The components drawn as two parallel lines perpendicular to the wires and labeled 470p, 22n and 120u are capacitors.  (In some Max Flash Cameras there may be a 160 mF capacitor in place of the 120 mF capacitor.)  A discussion of the electrical properties of capacitors can be found on page 15.  The unit of capacitance is the farad and has the symbol "F".  For example, the large capacitor has a value of 120 mF = 120 microfarads = 120 ´ 10-6 F = 1.2 ´ 10-4 F.

 

The part labeled D1 is a rectifier diode.  It probably has a black cylindrical package with wires, but different circuit boards may have different model diodes.  A diode lets current flow easily in one direction, but tends to block its flow in the opposite direction.  One end of a diode is the anode and the other is the cathode. One end of the package is marked with a line drawn around the circumference of the cylinder.  The line identifies the cathode end.  For the diode symbol on the schematic, the bar perpendicular to the wire marks the cathode end.  The triangle forms an arrow that points in the direction of easy current flow when the diode is forward biased (turned on). 

 

The symbol drawn as a short horizontal line under a longer horizontal line and labeled with a + and 1.5 V represents the AA battery.  The plus sign indicates the polarity and the adjacent number indicates that the voltage of the battery is 1.5 V.

 

            The component labeled XFORM is a transformer.  The line drawn down the center of the transformer means it is an iron core transformer.  (It is not uncommon to see two parallel lines drawn to represent the iron core.  The software package used to draw this schematic uses a single line, however.)  Since a magnet would attract the iron in the transformer core you could use a magnet to help you identify which of the transformers is the iron core transformer.  This transformer consists of two coils of wire (windings) wrapped around the iron core.  One of the coils has many times the number of turns (wraps) that the other coil has.  The wavy lines on each side of the symbol each represent one winding of the transformer.  To a first order approximation the current and voltage difference between the two windings scale according to the ratio of the number of turns.

 

The component labeled FLASH_XFORM is another transformer.  This transformer has an air core, so its symbol does not have the line(s) through the center.  You may find flash transformers with different appearances on circuit boards from different cameras.  One winding of this transformer also has many times the number of turns of the other winding.

 

            The purpose of the components labeled "Charge Switch," "Flash Switch" and "Flashlamp" should be obvious from their labels.  The reflector behind the flash lamp actually serves as the "Trigger" for the flash.  A copper strap connects it to the rest of the circuit.  Actually finding the switches on the circuit board is one of the Pre-lab tasks you are to perform.

 

 

2.3             A Few Basic Concepts for Electronic Circuit

 

2.3.1       Objective

            The objective of this section is to provide you with an introduction to some aspects of electronic circuits so that you will be able to perform and understand measurements you will use to characterize the camera flash circuit.  Some terms are defined, the electrical characteristics of some components of the flash circuit are discussed and an overview of some aspects of circuits is presented.

 

2.3.2       Electric Current

You may have an intuitive grasp that when we speak of an electric current we are referring to the motion of electronic charges.  In this discussion we will confine ourselves to currents flowing through wires or electronic components.  Charge is measured in units of coulombs.  The current is the amount of charge that moves past a location on the wire per unit time (coulomb/second = ampere).  (Or the charge that moves into or out of a lead of a component per unit time.)

 

We can conceive of moving positive or negative charges.  Historically, the existence of electronic current was known before it was known what physical object was actually moving.  A convention was established in which positive current flowed from point of higher electric potential to points of lower electric potential.  However, they got the convention backward!  It was eventually discovered that the particle moving in metal wires is the electron and that it moves in a direction opposite to the established convention.  Therefore a positive current flowing in a wire actually corresponds to the motion of negatively charged electrons moving in the opposite direction.  By the way, an electron has a charge of ‑1.6 ´ 10‑19 coulomb.  A current of 1 Amp = 1 coulomb/sec therefore corresponds to a very large number (~ 6,250,000,000,000,000,000) electrons moving past a location on the wire each second.

 

2.3.3       Voltage

Voltage is related to the force that causes current to flow.  (Important: pay attention to the wording in the previous sentence, in particular to "related to."  While voltage is related to the force that causes charges to move, it is not the force itself.)  An analogy that you may find useful is water flowing in a garden hose.  (It's not a perfect analogy but ignore that for now.)  The current of water is analogous to the electric current in a wire and the pressure that causes the water to flow is analogous to the voltage.

 

            When someone quotes a number for pressure in a hose or air pressure in a tire, that pressure is always stated relative to some reference.  The pressure quoted is actually the pressure difference between the thing being measured and the reference.  A common reference is the atmospheric pressure of the surrounding air, in which case the pressure is known as gauge pressure (in units of psig - pounds per square inch gauge - for example).  Another reference that is sometimes used is vacuum, in which case the pressure is known as the absolute pressure (psia).

 

            In a similar fashion, when a number for a voltages is stated it is actually the difference in voltage (or difference in electric potential) between two different points.   Voltages are always stated relative to some reference.  A common reference that is often used is earth ground.  That is the reference used by electrical utilities for electric power generation and distribution to homes and industry.  The 120 V supplied to your residence by the electric company is measured relative to earth ground.  Some circuits are isolated from earth ground, and in those cases a reference point in that circuit may be picked.  This is often also called the "ground" for that circuit, although it may not actually be connected to earth ground in any way.

 

Another way in which voltage is used is to describe the difference in electric potential across an electronic component.  When we speak of the voltage across a component we are measuring the voltage on one lead (wire) of the component while using another lead of the same component as the reference point.  (In the water hose analogy we could talk about the pressure difference across a device, such as a filter, through which the water is flowing.)

 

2.3.4       Resistors

A resistor is an electronic device that obeys Ohm's law.  Ohm's law states that the voltage across a resistance is proportional to the current through it.  (Note the emphasis on the words "across" for voltage and "through" for current.)  Ohm's law can be stated mathematically as

 

                        (Ohm's law)                                                                       (Equation 1)

 

or in words

 

(voltage across resistor) = (current through resistor) ´ (value of resistance)

 

where V stands for the voltage across the resistor, R stands for the resistance of the resistor, and I stands for the current through the resistor.  The unit of resistance is the ohm (from Table 1, ohm = volt/ampere, which agrees with the equation).  The symbol for a resistor is shown in Figure 3.  Also shown is the polarity of the voltage drop across the resistor if the current is flowing in the direction indicated by the arrow.

Figure 3: Symbol of the resistor.

 

            EXAMPLE: A 10 kW resistor that has 10 volts across it will have a current of I = 10 V/10 kW = 0.1 mA flowing through it.

 

            EXAMPLE: If the same resistor had 0.5 mA flowing through it the voltage across it would be V = (0.5 mA)(10 kW) = 5 volts.

 

2.3.5       Capacitors

            A capacitor is an electronic device that consists of two metal plates separated from each other by a thin insulator.  Since an insulator separates the two plates a steady state DC current cannot flow through a capacitor.  On the other hand, if a positive current is applied to one plate of the capacitor a positive charge will build up on that plate.  Since like charges repel each other and opposite charges attract, this positive charge will force an equal amount of positive charge off of the other plate, inducing a net negative charge of equal magnitude on it.  There will be an electric field between the positive and negative charges, and a voltage difference between the two plates, and hence across the capacitor.  The voltage difference across the capacitor is proportional to the charge on the capacitor.  In equation form

 

                     or                                                                           (Equation 2)

 

where V is the voltage across the capacitor, Q is the charge on the capacitor, and C is the capacitance of the capacitor.  Restating this with words

 

            (voltage across capacitor) = (charge on capacitor) / (value of capacitor)

 

or

 

            (charge on capacitor) = (value of capacitor) ´ (voltage across capacitor).

 

Capacitance is measured in farads (farad = coulomb/volt).  The symbol for a capacitor is shown in Figure 4. 

 

EXAMPLE: In order to charge the 120 mF capacitor in the flash circuit up to 300 V we would have to add a charge of 0.036 coulombs [= (120 mF) ´ (300 V)] to the positive plate of the capacitor.

Figure 4: Symbol of the capacitor.

 

If a current is flowing into a capacitor the charge on the capacitor has to be changing.  Mathematically

 

.        (Equation 3)

 

 

 

 

In words:

 

(current entering capacitor)

= (time rate of change of charge on capacitor)

= (value of capacitance) ´ (time rate of change of voltage across capacitor)

 

 is the time rate of change of the charge.   is the time rate of change of voltage.

 

For example, if I force a constant current of 1 mA onto the 120 mF capacitor the voltage across the capacitor will change at a rate of

 

 

The current flowing into a capacitor is proportional to the time rate of change of the voltage across the capacitor.  If the voltage is not changing the time rate of change (derivative) with respect to time will be zero, and the current will therefore be zero.  Conversely, if a current is flowing into the capacitor the voltage across it must have a non-zero time derivative - the voltage must be changing with time.

 

A charged capacitor stores electrical potential energy via the electric field inside it.  The amount of electrical potential energy stored in a charged capacitor can be expressed as

 

.                                                                                 (Equation 4)

 

In words

 

(electric potential energy) = (1/2) ´ (charge on capacitor)2 / (value of capacitor)

=  (1/2) ´ (value of capacitor) ´ (voltage across capacitor)2

The stored electrical energy can be released by allowing the capacitor to discharge through a circuit attached across it.  The energy stored on a 120 mF capacitor charged to 300 V is (1/2) ´ (120 mF) ´ (300 V)2 = 5.4 Joules.  In Lab 2 you measured the speed of the flash.  You probably saw that it was on the order of a few milliseconds long.  If we could completely discharge that capacitor in 1 ms, the average power leaving the capacitor during that 1 ms would be 5.4 J/ 1 ms = 5,400 watts!  The reason I stated this is the average power is that the energy does not leave the capacitor at a uniform rate during the flash discharge.  The energy leaves the capacitor faster at the beginning of the discharge so the peak power is even larger!  The flash capacitor delivers power on the order of kilowatts to the flash lamp (But fortunately only for a very short time, otherwise components of the flash circuit would probably melt!).

 

 

2.3.6       Series and Parallel Combinations of Circuit Elements

Circuits are formed by combining electronic devices in such a way that current can flow from one terminal of an electrical energy source, through the devices, and back to the other terminal of the energy source.  In this paragraph the emphasis is on the "circular" nature of the current flow, in that the current must be able to return to the energy source.  This is the origin of the term "circuit" and the phrases "completing the circuit" or "closed circuit."  If a path does not exist for the current to return to the energy source there is an "open circuit" and current will not flow out of the energy source.

 

Figure 5: A simple series circuit example.

We will begin by looking at the example of three resistors connected in series with a five-volt supply, as shown in Figure 5.  The positive direction of current flow will be out of the positive terminal of the 5 V supply and then through the 10 KW, 2.2 KW and 4.7 KW resistors and finally back to the negative terminal (not labeled) of the 5 V supply.  Since all of the components are connected in series, the same current must flow through all of them.

 

IMPORTANT:  Although this example only has resistors in it the concept of series current flow applies to other components as well.  If two components are connected in series with each other then the current flowing through them must be equal.

 

Any point in a circuit can only have a single voltage at any instant of time.  That necessarily means that the voltage at any point in the circuit is independent of the path to that point from the reference point.  For the circuit in this example lets take the bottom of the circuit as the reference point (we define V = 0 there).  If a clockwise path from the bottom is followed the path goes through the 5 volt power supply.  That means the voltage on the node between the 5 volts supply and the 10K resistor must be 5 volts.  If we follow a counterclockwise path from the reference the path goes through all three resistors to the top of the 5 volt power supply.  Therefore the voltage across the three resistors must exactly add up to 5 volts.  Since the voltage across each resistor is equal to the current through the resistor multiplied by the value of the resistance, we can write

 

.

 

From this we can find .  From Figure 5 we also see that resistors that are connected purely in series add together.  The three resistors in series are equivalent to one resistor with a value of 16.9 KW.

 

To continue the example we can find the voltage at every node in the circuit.  You might want to mark the voltages on Figure 5 as we work through this example.  This circuit has four nodes, one between each pair of components.  We will use the node at the negative terminal of the voltage supply as our reference and say it has a potential of zero volts.  (Mark this on Figure 5.)  The other voltages will be referenced to it. 

 

The voltage across the 4.7 KW resistor is (0.296 mA)(4.7K) = 1.39 volts.  Since the node at the bottom of this resistor is our reference, the node between this resistor and the 2.2KW resistor will be at 1.39 volts (relative to our reference - mark it on Figure 5). 

 

The voltage across the 2.2 KW resistor is (0.296 mA)(2.2K) = 0.65 volts.  Since the node at the bottom of this resistor is at 1.39 volts, the node between this resistor and the 10 KW resistor will be at 1.39 + 0.65 = 2.04 volts (relative to our reference - mark it on Figure 5). 

 

Finally, the voltage across the 10 KW resistor is (0.296 mA)(10K) = 2.96 volts.  Since the node at the bottom of this resistor is at 2.04 volts, the node between this resistor and the positive terminal of the supply will be at 2.04 + 2.96 = 5 volts (relative to our reference), just as we expected. 

 

Before moving on to the next example it is worth making one more observation that we will use in Lab 4.  The circuit above is a form of a voltage divider.  In deriving the current we used

 

.

 

Then, in finding the voltage across each of the resistors we multiplied the current by the resistance, or in general

 

           

 

where VR is the voltage across resistor R and R has a value of either 10K, 2.2K or 4.7K.  This shows that the voltage across any of the resistors is a fraction of the voltage across all of them, and the fraction is given by the ratio of the resistor in question to the sum of the resistances.  This is known as voltage division, and a series connection of resistors can be called a voltage divider.

 

One very important feature of series connected components should be repeated for emphasis.  Components connected purely in series have the same current flowing through them.

 

Figure 6: A simple parallel circuit.

Our next example will be a parallel combination of circuit elements, as shown in Figure 6.  This circuit has only two nodes, one at the top of the figure and the other at the bottom.  As a result, both of the resistors and the capacitor each have identical voltage across them, 5 volts.  Using Ohm's law, the current through the 4.7 KW resistor is (5 V)/(4.7K) = 1.064 mA.  The current through the 10 KW resistor is (5 V)/(10K) = 0.500 mA.  Since this is a DC circuit, the current through the capacitor is zero.  However, since there is a voltage across the capacitor the charge on the capacitor is (5 V)(10 mF) = 5 ´ 10‑5 coulombs (Equation 2) and the energy stored in it is (0.5)(10 mF)(5 V)2 = 125 mJoule (Equation 4).  When the voltage supply in this circuit was first turned on there would have been a very large current flow to the capacitor for a very short time to charge it with 5 ´ 10‑5 coulombs and 5 volts.

 

The current flowing into the two resistors is flowing out of the positive terminal of the 5 volt supply.  The algebraic sum of all currents entering a node must be equal to the sum of all currents leaving a node. For this circuit the current flowing out of the 5 volt supply can be found by adding the current leaving the node at the top of the circuit.  It is 1.064 mA + 0.500 mA = 1.564 mA.  Note that if we use Ohm's law and divide the voltage by the total current we obtain a resistance of 3.20 KW.  This is the "parallel" equivalent resistance of the two resistors.  We did this using numbers, but a general formula could be derived following a similar procedure using variables.  When two or more resistors are combined purely in parallel they have an equivalent resistance given by

 

 

 

 

 

                                                               (Equation 5)

 

where one term is included for each resistor in the parallel combination.

 

Reiterating an important point for you to remember from this example, circuit elements that are connected in parallel have the same voltage across them.

 

Figure 7: A circuit with both parallel and series connections.

We will look at one more example, which has both parallel and series connections, before moving on to the specifics of the flash circuit.  The circuit is shown in Figure 7.

 

In this example the capacitor is starting out charged to a voltage of 5 volts.  While the switch is open there can be no current flow.  The voltage across each resistor is zero and the current through them is also zero.  As soon as the switch is closed the capacitor voltage will be applied across the resistors, and a current will flow.  That means that charge will flow out of the capacitor, and the voltage will therefore decrease, which will mean the current will decrease too.  Our objective will be to only find the current and voltage of each resistor and current leaving the capacitor immediately after the switch is closed (t = 0+).

 

First we note that there is a 4.7 KW resistor in parallel with a 10 KW resistor.  We can use Equation 5 to find the equivalent resistance of this parallel combination to be

 

.

 

We also see that this equivalent resistance is in series with the upper 4.7 KW resistor, so the total resistance connected across the capacitor is 4.7 KW  + 3.2 KW = 7.9 KW. 

 

As stated above, right after the switch closes a total of 5 volts will appear across this combination of resistors.  We can thus use Ohm's law to find that the initial total current flowing into the resistors will be (5 volts/7.9 KW) = 0.633 mA.  This current is flowing out of the capacitor and into the upper 4.7 KW resistor. 

 

The voltage drop across the upper 4.7 KW resistor is therefore (4.7 KW)(0.633 mA) = 2.975 V (from Ohm's law again).  That leaves 5 V - 2.975 V = 2.025 V across the parallel combination of the lower resistors. 

 

We will use Ohm's law two more times to find the current through the lower two resistors, which each have 2.025 V across them.  The lower 4.7 KW resistor will have a current of (2.025 V)/( 4.7 KW) = 0.431 mA flowing through it.  The 10 KW resistor will have a current of (2.025 V)/( 10 KW) = 0.202 mA.  Note that these two currents add up to 0.633 mA, as they should, since they must equal the current leaving the upper resistor.

 

As stated above, since there is current leaving the capacitor the total voltage will immediately begin to fall below 5 volts.  We can use the capacitor equations to calculate the initial rate of decrease at t = 0.  From Equation 3

 

.

 

The voltage will drop quickly! 

 

Something Extra

We know that the current leaving the capacitor must enter the stack of resistors (whose equivalent resistance we have already calculated).  We can write an equation describing this:

 

.

 

The left-hand side is the capacitor current and the right hand side is the current entering the resistors.  In this equation the voltage is a function of time.  This type of equation is known as a differential equation.  Differential equations occur in all sorts of engineering problems. The solution to this equation for the case that the capacitor initially has 5 volts on it and the switch closes at time t = 0 is

 

.

 

You will learn how to solve differential equations to come up with solutions like this in your math classes.  Don't worry about it now - this is "something extra."  The voltage as a function of time as described by this solution is plotted in Figure 8.

 

 

 

 

2.3.7       Other Circuit Components

Another circuit element that is important for operation of the flash circuit is the transformer.  We will briefly describe the operation of an ideal transformer.  A transformer basically consists of two coils wound so that the magnetic field generated by a current passing through one of the coils will be intercepted by the other coil.  A changing magnetic field, created by a changing current in one coil, will induce a changing current in the other coil.  This only works for a changing current (and voltage).  A transformer does not work for DC current.  The relative values of the currents (and of the voltages across each of the coils) will depend on the relative number of turns in the two coils.  One of the coils in the transformer is usually referred to as the primary, and the other is referred to as the secondary.  In an ideal transformer if the secondary has n times as many turns as the primary, the voltage across the secondary will be n times larger than across the primary and the current through the secondary will be 1/n times the current through the primary.  The actual operation of real transformers is a bit more complicated due to non-ideal properties, but this is enough to understand the basics of how the transformers are used in the flash circuit.

 

 

Figure 8: Voltage vs. time for the circuit of Figure 7.

 

A final circuit element that we need to discuss it the diode rectifier.  It is labeled D1 in Figure 1 and Figure 2.  As mentioned earlier, the diode allows easy current flow in the "forward direction", but almost no current flow in the "reverse direction."  The diode symbol is arrow-like, and the "arrow" points in the direction of easy current flow.  Large currents can flow with very small voltages in the forward direction.  Forward voltages will typically be between 0.5 and 1 volt for all forward currents that a diode is rated for.  In the reverse direction only very small currents can flow as long as the breakdown voltage of the diode is not exceeded.  In the flash circuit, the diode allows charging currents to flow between the capacitor and the iron core transformer, but blocks currents in the opposite direction that would discharge the capacitor through the iron core transformer.  This allows a large voltage to build up on the capacitor as the charging circuit delivers current to it.

 

2.4             Operation of the Flash Circuit

 

            Earlier in this document we described the core and auxiliary functions of the flash circuit.  We will now take a closer look at how the core functions are implemented.  You should refer to Figure 1 or Figure 2, the schematics of the flash circuit, while reading this section. 

 

2.4.1       The Charging Circuit

            The energy for the flash circuit is initially stored in a size AA battery.  The battery is an electrochemical cell with an electric potential of 1.5 V DC.  The battery stores potential energy chemically and converts it into electric energy when a closed circuit is attached to its terminals.  The battery also tends to deliver its energy relatively slowly (compared to the speed required for the flash lamp).

 

            The flash lamp requires several hundred volts DC across its terminals.  In addition, the energy must be delivered quickly (on the order of milliseconds) to be useful for flash photography.  The flash lamp converts electrical energy into optical energy.

 

The charging portion of the flash circuit therefore has to convert from the 1.5 V of the battery to the several hundred volts for the flash lamp.  It also has to store the energy in a way that it can be quickly released when needed for a flash photograph.

 

            Much of the charging circuit is enclosed in the box for which we have not shown details.  The components of the charging circuit that are shown in Figure 1 and Figure 2 are the 1.5 V battery, the charging switch, the resistor connected to the charging switch, the iron core transformer (XFORM), the diode D1 and the 120 mF capacitor.  The basic operational concept of the circuit is to convert the DC from the battery into AC. The components in the box and the iron core transformer perform the DC to AC conversion. They create a relatively large (relative to other currents in the circuit) AC current at a relatively small AC voltage (a little less than 1.5 V peak-to-peak).  The AC current is roughly in the form of a series of pulses.  This current is applied to the primary winding of the iron core transformer.  The secondary coil of the transformer has many times the turns of the primary winding, so a much smaller current at a much larger voltage appears across the secondary.  The current pulses from the secondary are applied through diode D1 to the capacitor.  Diode D1 rectifies the current.  That is, the diode allows the current  to flow in one direction, but blocks it in the other.  This unidirectional (but not constant) current through D1 causes charge to build up on the 120 mF capacitor.  Since the voltage across a capacitor is proportional to the charge on it the voltage across the 120 mF capacitor increases with time (see the capacitor discussion on page 15).  This process can continue until the voltage across the 120 mF capacitor approaches the voltage at the secondary of the transformer, or until something else (an auxiliary function in this generation of the Max Flash camera) turns the charging circuit off.  This "something else" is also included in the more complex circuitry inside the box of Figure 1 and Figure 2.

 

2.4.2       Flash Trigger Circuit

            The flash trigger circuit is made up of the 22 nF capacitor, a 1 or 2 MW resistor (depending on which generation of the circuit you have), the flash transformer, the flash switch, and the reflector of the flash lamp.  Note that the flash lamp is in parallel with the 120 mF capacitor.  In addition, the combination of the 22 nF capacitor in series with the 1 or 2 MW resistor is in parallel with the 120 mF capacitor. The series combination of the 22 nf capacitor and the resistor (1 or 2 MW) will have the have the same large voltage across them as the 120 mF capacitor.  When the 120 mF capacitor is charged the 22 nF capacitor will also slowly charge through the 1 or 2 MW resistor.

 

The shutter basically is one side of the trigger switch.  A metal clip that the shutter hits when it is fully open is the other side.  When the shutter is fully open it momentarily closes the flash switch.  This allows the charge on the 22 nF capacitor to be quickly discharged through the primary of the flash transformer.  You will measure how quickly in Lab 4.  The voltage generated across the primary is stepped up on the secondary, and this large voltage pulse is applied to the reflector around the flash lamp, which triggers a discharge in the flash tube, which emits light.  The 1 or 2 MW resistor prevents the 120 mF capacitor from being quickly discharged through the flash transformer.  That allows most of the stored energy to flow through the flash tube where it is converted to light (and waste heat).

 

2.4.3       Neon Pilot Light

The connection of the neon pilot light is different for the 3B1573 and 5C3406 generations of the circuit.  In both circuits the neon lamp is connected in series (at node  B  ) with a 3.9 MW resistor.  In Figure 1 the neon lamp is connected to node  G  and the resistor to node  A .  In Figure 2 the node connections are swapped.  In either case, the total voltage across the flash capacitor also appears across the series combination of the neon lamp and resistor.  Some fraction of the total voltage will be across the neon lamp while the remainder of will be across the resistor.  That fraction is different when the neon lamp is off than when it is on.

 

There is another difference between the 3B1573 and 5C3406 generations of the circuit.  The circuit in Figure 1 has a connection from node  B  to the more complex circuitry in the box.  This connection is used to sense when the flash capacitor is charged and then turn off the charging circuit.  In Figure 2 the connection for detecting when to turn off the charging circuit goes from node  A  to the more complex circuitry in the box.

 

2.5             What's an Oscilloscope?

 

An Oscilloscope is an instrument that is typically used to measure voltage as a function of time.  The oscilloscope you will be using is a virtual digital oscilloscope that has two "channels."  The actual hardware resides inside the computer plugged into the PCI bus.  The probes are connected to the inputs on the back of the PC.  Since there are two inputs you will be able to display the voltage at two different points in your circuit as a function of time.  You will use both channels in Task 1 of this lab.  You will only use one channel for Tasks 2 and 3.  An image of the main graphical user interface of the VirtualBench-Scope is shown in Figure 9.  You will note in this figure that there are buttons to toggle on/off states as well as knobs and selection windows to adjust other parameters.  We will discuss the details of some of these here and in the lab procedure.

 

 

Figure 9: The main window of the VirtualBench-Scope

 

Display Screen: The front panel of the oscilloscope is divided into sections by function.  The largest section is the display.  The results of the measurements will be plotted on a graph on the display.  The horizontal axis will be time, and the vertical axis will be voltage.  You should note that the Voltage per division for the vertical axis is shown in the upper left hand corner of the display.  Also, the time base for the horizontal axis is displayed at the bottom of the display grid.

 

Channels:  The buttons in this section toggle the displayed signal sources on or off.  You will only be using ‘Ch 0’ and ‘Ch 1’ for this experiment.

 

Timebase:  The Timebase knob lets you adjust the time base for the horizontal axis of the diplay.  The time base is the same for all signals plotted on the display.

 

Volts/div:  This knob adjusts the Voltage per division for the channel that is displayed in the small selection box directly above the knob.  Note that this means each source on the display can be plotted with a different voltage scale.  The scale for each channel plotted will be shown at the top of the display.

 

V. Position:  This knob adjusts the vertical position of the channel displayed in the small selection box directly above the Volts/div knob.  Each displayed channel can be moved up and down on the vertical axis independently.  The ‘zero’ position for each channel is indicated by a small ground symbol attached to the right side of the display grid.

 

Cursors:  The button in this section will activate the measurement cursors.  The cursors are labeled ‘C1’ and C2’.  They each give measurement from the source displayed in their respective selection boxes.  The time and voltage for each cursor will be shown at the bottom of the display panel.

 

Trigger: While the oscilloscope can be set to continuously display the voltages being measured, that will usually result in a jumble on the screen.  We may want to trigger the display at some point in time, so that the signal we want to observe gets displayed.  You will use the controls in this section to define when the oscilloscope will start to display the voltages (the t = 0 point of the measurements).  You will do this by setting a trigger level voltage, the position in the timeline, the trigger mode, the trigger source and the trigger slope.  The trigger level will tell the oscilloscope to define t = 0 at the point the voltage crosses some value you set for the trigger level.  The trigger mode can tell the oscilloscope to plot the voltage once and then stop.  The source tells the oscilloscope which channel to use for the trigger.  The trigger slope will tell the oscilloscope to trigger when the voltage crosses the trigger level with either a positive slope or a negative slope (and not to trigger if the voltage crosses the trigger level with the wrong slope.)  The trigger level and its position in the time line is set by a ‘+’ cursor which can be moved on the display panel.

 

Run:  This is an important button below the display panel.  This button should be in its off position while you are changing settings.  You will activate it when you are ready to collect data.  Once you have the data you want you should deactivate the capture immediately to avoid losing your current data.

 

 



3         Pre-lab 4 Assignments

 

            This section describes tasks that you should perform before coming to Lab 4.  You are required to turn in your pre-lab work at the beginning of Lab 4.  The Pre-lab Tasks should be done together with your teammates.  Include your Team number and at the names of all members of the team at the top of the first page.  The work turned in by your team should be handed in separate from any work you turn in for Lab 3.  (Two separate stacks).

 

            Guidance on performing these assignments can be found in various sections of the reading.  In most cases you will need information from different sections to perform each of the assignments, but to help you out the most important sections for each assignment are listed after the assignment number.

 

˙ Pre-lab Assignment 1: (See section 2.3)

 

Find the currents and voltages in the adjacent circuit by the following steps.

 

Refer to sections 2.3.4, 2.3.5, and 2.3.6 for help with this.

 

Purpose:  To get you familiar with using Ohm's law to analyze current flow in a simple circuit.

 

Team Assignment to be Turned In

a)     What is the current flowing into the capacitor in this DC circuit?

b)     The 3.9 MW and 10 MW resistors are in parallel.  Find their equivalent resistance?

c)      If we neglect the capacitor, the 1MW resistor is in series with the equivalent resistance calculated in part b).  What is the total resistance?

d)     Why is it OK to neglect the capacitor for part c)?  [Think about part a).]

e)     Use the resistance calculated in part c) to calculate the total current flowing from the voltage source into the 1 MW resistor.  (Use Ohm's law)

f)        What is the voltage across the 1 MW resistor?  (Use Ohm's law)

g)     The capacitor, 3.9 MW resistor and 10 MW resistor all have the same voltage across them.  Calculate this voltage.

h)      Calculate the currents flowing through the 3.9 MW and 10 MW resistors.  (Use Ohm's law twice)

i)        Add the currents through the capacitor, and the 3.9 MW and 10 MW resistors together.  Compare the result to the current through the 1 MW resistor.

j)        Calculate the charge on the capacitor.

 

˙ Pre-lab Assignment 2: (See section 2.2).

 

Read the part number on your circuit board.

 

Purpose:  Kodak keeps revising the circuit board.  With every student in each section of the class buying a camera there is a good chance that most varieties of circuit board still in use will show up in this class.  The instructors will use this information to check if any boards we have not seen before have started to show up.

 

Team Assignment to be Turned In

Your team should prepare a table with a row for each team member and two columns.  Each row should contain the name of a team member and the part numbers from that team member's circuit board. 

 

 

˙ Pre-lab Assignment 3: (See sections 2.2 and 4.3)

 

The schematics in Figure 1 and Figure 2 each have a charging switch and a flash trigger switch.  Identify the physical parts of the camera that make up both switches.  You will have to look at how the circuit board physically interacts with the rest of the camera to help locate the flash trigger switch.

 

Purpose:  To prepare you for lab so you can work efficiently.  You will need to operate these switches in lab 4 while the circuit is not in the camera case. 

 

Team Assignment to be Turned In

Answer the following questions.

 

a)      The charging switch is entirely contained on the circuit board.  How does the switch operate (mechanically)?

b)      The flash switch is not entirely on the circuit board.  Identify which parts are on the board and which are in the rest of the camera.

c)      Why is the flash switch implemented this way?

d)      You will need to close the flash switch with the circuit board outside of the camera.  How will you be able to do this?  (Read the lab procedures for help answering this one.)

 

 

˙ Pre-lab Assignment 4: (See section 2.2)

 

In lab you will be working as a team to perform measurements on the circuit board from one of your cameras.  When you meet as a team before lab you should decide whose circuit board you will use.  This pre-lab task should be done for that circuit board.  Pictures of a 3B1573 and a 5C3406 circuit board are included later in this section.  Although their are slight variations in the different revisions of circuit boards in these generations, the major test points we will be using are roughly at the same point for boards within a generation.

 

In Lab 4 you will be probing various nodes of the circuit with an oscilloscope and with a voltmeter.  The nodes you will need to find are the ones labeled in Figure 1 or Figure 2.  Remember that a node is not just a single point and you can connect the test probes to any convenient location on the node.  A node can include the metal trace on the circuit board, the wire lead of a component if the lead is soldered to the trace, or in some cases the metal clips on the circuit board if they are soldered to the metal trace for that node.  You should identify the locations of convenient points to clip probes before coming to lab. 

 

You will be using three varieties of probes.  All three types clip onto the circuit and, once clipped on, allow hands-free measurements.

 

One type of probe is a mini-probe.  It consists of a small hook with a spring-loaded retainer.  The hook can be conveniently looped around a device's wire lead.  This type of probe can reach into somewhat tight spots on the circuit board, but there are points on the circuit that are too tight even for these probes so you should look for the most accessible point for a particular node.

 

The second type of probe you will use is a small alligator clip.  While these alligator clips are small, they are significantly larger than the mini-probes.  They are best used for clipping onto more accessible points.  For example they can be clipped onto a contact at the edge of the board with a jaw on each side of the board, provided that either the same node is contacted on each side, or only one jaw is touching a conductor.  They also work well for clipping onto the metal parts that make contact to the ends of the battery, for example.

 

The third type of probe is really a combination of the other two.  The oscilloscope probes use coaxial cables.  The center conductor carries the signal you are measuring, and has a mini-probe.  The outer conductor is used as a reference, and has a small alligator clip.

 

Team Assignment to be Turned In

Label the picture from the same generation as the circuit board you will be testing with the points you plan to connect your probes.  You should draw arrows pointing to the contact points you plan to use.  Label the tail end of each arrow with the letter of the node and the type of clip you plan to connect there.  Also write specific comments by the tail of the arrow that will help you remember exactly where to clip the probe.  (For example: If the picture is not clear enough to tell which of two wires on a component you will connect the probe to, write a few words that unambiguously state which wire you will use.)

 

You also might want to use a fine tipped marker to label some of these nodes on the circuit board, if there is room.

 

Node  G 

q       The node that is common to the negative terminal of the battery, the positive terminal of the large electrolytic capacitor (120 mF in Figure 1), and one terminal of the flash lamp.  Many other components are also connected to this node.  All voltage measurements are really measurements of voltage differences between two points.  This node will be the reference point (hence "G" for "ground") for many of the measurements you will perform. 

q       You should find and label at least two excellent places to hook up an alligator clip to node  G .

q       You should also find and label at least one place to hook up a mini-probe to node  G .

 

Node  A 

q       The node that is common to the negative terminal of the large electrolytic capacitor, the anode of D1, and one terminal of the flash lamp.

q       You should be able to find and label an excellent place to hook up an alligator clip to node  A .

q       You should be able to find and label at least one convenient places to connect a mini-probe to  A .

 

Node  B 

q       The node that is common to the 3.9 MW resistor and the neon lamp.

q       Find a convenient place to connect a mini-probe.

 

Node  C 

q       The node between the flash trigger and the flash transformer.  

q       Find and label a convenient place to connect a mini-probe.

 

Battery terminals

q       Add arrows to the photograph pointing to the battery terminals and identify which is the positive terminal and which is the negative terminal.

 

Flash Trigger

q       You will need to connect an alligator clip to a point on the circuit that is one side of the flash trigger switch.

q       Add and label an arrow pointing to the location for this alligator clip.


Figure 10: Photograph of the front side of a generation 3B1573 circuit board - specifically, revision L'.

 

 

Figure 11: Photograph of the front side of a generation 5C3406 circuit board - specifically, revision G.

 

 

3B1573 Circuit Board

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Use this page to label nodes for Pre-lab Assignment 4

 

 

 

5C3406 Circuit Board

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Use this page to label nodes for Pre-lab Assignment 4


4         Summary of the Lab 4 Procedures

 

4.1             Introduction

In Lab 4 you will be performing measurements on various parts of the flash circuit while each of the core functions is being executed.  You will be using a digital voltmeter and the virtual oscilloscope to measure voltages.  The digital voltmeter will allow you to measure DC voltages or voltages that are slowly changing with time.  With the oscilloscope you will be able to display graphs of voltages that are changing rapidly as a function of time.

 

In many cases we would like to know currents flowing in the circuit, however, it is difficult to measure currents directly.  To measure current we would have to break the circuit and insert the measuring instrument in series with the component whose current we wish to measure.  That would require us to unsolder one end of the component from the circuit board and clip or solder the current meter between that end of the component and the point on the circuit board to which it was originally attached.  That is not practical in many cases, and the limited time for Lab 4 does not allow us to do that.  In addition, while the digital multimeter you will use to measure the voltages has a current measuring capability, the oscilloscope is not designed to measure currents.

 

To extract information about currents flowing in the circuit you will therefore need to use voltage measurements and the equations presented starting on page 14 of this document.

 

In addition to the digital voltmeter and oscilloscope you will be using a test board that will simplify some of your connections.  In this section of the lab instructions the letter codes in boxes refer to the nodes on the circuit.  The Roman numeral codes refer to connections on the test box.

 

Warning: To minimize the chance of accidentally contacting high voltage, only one team member at a time should be handling the circuit or connecting wires.  We don't want one person to trigger the charging circuit while another person is still connecting wires.  Follow instructions carefully.  We suggest that while one member of a team is making the connections the others carefully watch to make sure that the correct connections are made in a safe manner.

 

 

 

 

 

 

 

 

Warning: At various times during the performance of these measurements you will need to handle the circuit board either with a battery connected, or just after the battery has been disconnected.  Large voltages may be present on the board if the charging circuit is operating or has recently been operated.  Do not touch the metal parts of the circuit in those situations.  At some of these times you may need to manipulate the circuit board.  The 120 mF capacitor provides a convenient handle for doing so, but remember that it may be charged to over 300 volts!  When handling the board by the capacitor, be sure to touch only the insulated portions of the capacitor.  Keep your fingers away from the metal leads.

 

Warning: When one end of a test lead has been connected to the circuit, handle the other end only by its insulated portions.

 

Warning: When attaching test leads to the circuit, be sure that the clips do not contact more than one node at a time.  If the lead does contact more than one node it will short out part of the circuit.  This can lead to undesired and potentially dangerous results or permanent damage to the circuit board.

 

Warming up the test equipment

 

            Run the VirtualBench-Scope application.  There should be an icon on your computer’s desktop.  Also turn on the voltmeter and set the function selector to measure DC volts.  This option is marked as a V with a horizontal straight line and horizontal straight dashed line by it.

 

            An outline of the measurements you will perform follows.  Detailed step-by-step instructions will be displayed in HTML format on the computers at each lab station during the lab.


4.2             Task 1  - Charging Transient Measurements with the Oscilloscope

 

¨            You will start with the battery removed, the 120 mF capacitor discharged, and the circuit board removed from the camera.

¨            You will connect Channel 0 of the oscilloscope to measure the voltage at node  A  as a function of time.  You will connect Channel 1 of the oscilloscope to measure the voltage at node  B  as a function of time.

¨            You will connect a wire with alligator clip to serve as a replacement for the shutter switch.

¨            You will connect a test board across the 120 mF capacitor and connect a voltmeter to the test board.  The voltmeter will let you observe the voltage across the capacitor at all times.  (The oscilloscope will only display the voltage when it is triggered.)  The test board also has resistors controlled by a switch that will allow you to safely discharge the capacitor between measurements.

¨            You will load a settings file in VirtualBench for the time and voltage scales on the oscilloscope to allow you to measure and best display the voltages at node  A  and at node  B  while the flash circuit is being charged.  This will also set the oscilloscope up to trigger properly at the start of the charging cycle.

¨            You will connect a battery to the flash circuit and then start the charging circuit.  The voltages at node  A  and at node  B  will be displayed as a function of time on the oscilloscope screen.

¨            You will use cursors on the oscilloscope to extract times and voltages from the voltage vs. time graphs on the display and record them in the following table.

>          Record the capacitor voltages at the times shown in the following table.  Also record the maximum capacitor voltage and the capacitor voltage just before and after the jump in the Node  B  voltage.

>          Fill in the Node  B  voltages in the appropriate parts of the table as well.

>          Then use a similar procedure after you have selected the appropriate cursor source to measure the times of the jump in the Node  B  voltage.

 

 

Just before Node  B  voltage jump:

Just after Node  B  voltage jump:

˝VC˝ Max:

Time

(sec)

t =

t =

t =

Capacitor Voltage (V)

 

 

 

Node  B  Voltage (V)

 

 

 

¨           
Mark the axes of the grid above to correspond to the oscilloscope display.  Label the axes with the variable (and units) being plotted (e.g. voltage in volts and time in seconds).  Mark the locations of zero for both the time and voltage.  Record the volts/div and time/div and mark the gridlines on the axes accordingly.

¨            Make a sketch of the display.  Your sketch should be consistent with the data in your table.  Rather than sketching the display you could also use ‘Alt + Print Screen’ to copy the window to the clipboard.  This can then be pasted to Word for inclusion in you report.

 

4.3             Task 2  - Discharge Transient Measurements with the Oscilloscope

 

¨            For the remainder of the measurements we will only use Channel 0.  You will turn off Channel 1 on the oscilloscope.

¨            You will reset the time and voltage scales on the oscilloscope to allow you to measure and best display the voltage at node  A  while the flash is set off.  You will also load settings for the oscilloscope to trigger properly at the start of the flash discharge.

¨            You will restart the charge switch to make sure the capacitor has a full charge. 

¨            You will use the wire with alligator clip that you attached in Task 1 to take the place of the shutter to set off the flash. The voltages at node  A  will be displayed as a function of time on the oscilloscope screen.

¨            Use the measurement cursor to fill in the voltage column in the following table.  Also record the "smallest" capacitor voltage displayed at the end of the flash discharge.  Calculate the values for the other columns in the table.  Remember Time = 0 is at the start of the curve.

 

Time (ms)

Capacitor Voltage (volts)

Dt (ms)

DV (volts)

I (amps) - see below

 

0

 

 

 

 

 

200

 

 

200

 

200

 

 

400

 

200

 

 

600

 

200

 

 

800

 

200

 

 

1000

 

 

 

 

 

>          As described on page 15 the capacitor current is given by .

We can approximate the derivative in this equation to give  or in words (current) » (value of capacitor) ´ (change in voltage) / (time interval).  The approximation is reasonable provided the time interval Dt is small.

>          Repeat the calculation for each of the time intervals in the table.

>          Do you observe any trends?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

¨           
Mark the axes of the grid below to correspond to the oscilloscope display.  Label the axes with the variable (and units) being plotted (e.g. voltage in volts and time in seconds).  Mark the locations of zero for both the time and voltage.  Record the volts/div and time/div and mark the gridlines on the axes accordingly. Remember that the time scale for this trace is different than for the previous trace.

¨            Make a sketch of the new trace. Your sketch should be consistent with the data in your table.  Also, you should copy and paste the active window to a Word document for use in your report.  (Alt+Print Screen)

 

4.4             Task 3  - (Optional) Flash Trigger Measurements

 

¨            This task is optional.  If completed and included in the report additional points will be awarded

¨            You will disconnect the battery and use the test board to discharge the capacitor so that you can safely handle the circuit board while reconnecting the test wires for these measurements.

¨            You will disconnect the oscilloscope probes from the circuit board but leave the other wires connected.

¨            You will next measure the flash trigger signal, which is larger than 400 volts.  The oscilloscope has a maximum allowed input of 400 volts (with the 100:1 probes attached), so the signal must be reduced before being measured.  You will make additional connections as shown in the following table.

Type of test lead

Connection

Banana plug to small alligator clip

Node  G  on the circuit to socket  V  on the test board

Banana plug to mini-probe

Node  C  on the circuit to socket  III  on the test board

100:1 oscilloscope probe mini-probe

Test point  IV'  on the test board

100:1 oscilloscope probe small alligator clip

Test point  V'  on the test board

 

The purpose of these connections it to attenuate the flash trigger voltage by using a voltage divider (see discussion on page 18).  The attenuation is accomplished through a voltage divider comprised of a 100 kW resistor, an 820 kW resistor, and the 10 MW input resistance of the oscilloscope.  A schematic of the voltage divider is shown in  Figure 12.  The 100 kW resistor is in parallel with the 10 MW input resistance of the oscilloscope.  The 820 kW resistor is connected in series with the parallel combination.  The voltage to be attenuated is applied between nodes  III  and  V  (or  V' , which is the same node).  The attenuated voltage is obtained between nodes  IV'  and  V' .  The attenuation factor in this case is given by

.

 

Note:  We would really like the input resistance of the oscilloscope to be infinite, but since it isn't, and since its value is comparable to the other resistors in the divider we need to account for the current going from our circuit into the oscilloscope in calculating the attenuation factor.

¨            You will reset the time and voltage scales on the oscilloscope to allow you to measure and best display the voltage at node  C , after it is passed through the voltage divider on the test board, while the flash is set off.  You will also set the oscilloscope trigger to properly start the measurement at the start of the flash trigger.

 Figure 12: Schematic of the voltage divider for the flash trigger measurement.


¨            IMPORTANT:  For this measurement you will need to divide your measured voltages by approximately 0.0901 (or multiply by 11.1) to correct for the additional attenuation provided by the voltage divider.

¨            You will reconnect the battery to the circuit and recharge the capacitor.

¨            You will use the wire you attached in Task 1 to take the place of the shutter to set off the flash.  The attenuated flash trigger signal will be displayed on the oscilloscope screen.

¨            Mark the axes of the grid below to correspond to the oscilloscope display.  Label the axes with the variable (and units) being plotted (e.g. voltage in volts and time in seconds).  Mark the locations of zero for both the time and voltage.  Record the volts/div and time/div and mark the gridlines on the axes accordingly.  Remember that the true voltage for this graph is not the same as that shown directly on the oscilloscope screen.  You should re-scale the voltage axis by dividing by approximately 0.0901 (or multiplying by 11.1).  Also remember that the time scale for this trace is different than for the previous traces.

¨           
Make a sketch of the new trace.  Your sketch should be consistent with the data in your table below.  Again, you will want to copy the window to a Word document for use in your report.

 

¨            Use the voltage and time cursors to record the voltage and time of the first 3 peaks and first 3 valleys of the displayed transient.

Valleys

Peaks

Time

Voltage

Corrected Voltage*

Time

Voltage

Corrected Voltage*

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

*    For this measurement you will need to divide your measured voltages by approximately 0.0901 (or multiply by 11.1) to correct for the attenuation by the voltage divider.

 

¨            You will disconnect the battery and use the test board to discharge the capacitor so that you can safely handle the circuit board while reconnecting the test wires for these measurements.

¨            You will then clean-up and turn off the oscilloscope and voltmeter.


5         Report Instructions for Camera Lab 4

 

Your report should generally follow the lab report format handed out to you earlier in the quarter.  Specific instructions on what to include in each of the sections follows.  Write in sentences and paragraphs, not as a bulleted list.  True for all sections of the report.

 

Team Report: This is a team report.  Your entire team should submit one report.

 

Length:  The written part of the report, not including the cover page or the tables and figures (see below), should be about three to four pages long.

 

Cover Page: Include the part number for the circuit board you used in the title of your report.  Include the names of all members of the team and your team number.

 

5.1        Introduction

 

In addition to what is specified in the report format handout, include a sentence giving the part number for the circuit board you used and the value of the capacitance of the large capacitor.

 

5.2        Experimental Methodology

 

The only thing to put in this section is the following sentence: "This section is not required for Camera Lab 4."

 

5.3        Results and Description

5.3.1       Task 1: Charging Transient

·        Description of the charging graph and table from Task 1.  Be sure to refer to the appropriate Figure.

·        How is the neon lamp turning on related to the data in the figure and table?

·        Use Ohm's law to calculate the current flowing through the 3.9 MW resistor at the following three times: 1) just before the jump in resistor voltage, 2) just after the jump, 3) when the capacitor voltage was at its maximum value.

·        Channel 1 of the oscilloscope has an input resistance of about 10 MW.  It was connected in parallel with the 3.9 MW resistor and therefore has the same voltage across it.  Calculate the current flowing through the input of Channel 1 at the same three times (Ohm's law again).

·        Use the above two currents to estimate the current flowing through the neon lamp at the same three times.

·        Determine the voltage across the neon lamp at the same three times.

5.3.2       Task 2: Flash Discharge Transients

·        Description of the flash discharge graphs and tables from Task 2.  Be sure to refer to the appropriate Figure.

5.3.3       Task 3: (Optional) Flash Trigger Transient

·        Description of the flash trigger graphs and tables from Task 3.  Remember to correct the voltages for the attenuation caused by the voltage divider.  Be sure to refer to the appropriate Figure.

 

5.4        Discussion

 

Discuss at least the following.  You may add discussion of other interesting observations.

5.4.1       Task 1: Charging Transient

·        Any trends you observed.

·        What happens to the capacitor during charging that causes the voltage to change?

·        What voltage is required to turn on the neon lamp?

·        What else happens when the neon lamp turns on?  Why does the resistor voltage jump?

5.4.2       Task 2: Latent and Flash discharge Transients

·        Any trends you observe.

·        After the capacitor is charged and the charging circuit has turned off, but before the flash is triggered, the voltage on the capacitor will slowly decrease.  (Many minutes for it to fall so low that the neon lamp turns off.)  What differs in the path of the current for the flash discharge that causes the flash discharge to be so much faster?

·        Why doesn't the capacitor voltage go all the way to zero during the flash discharge?

5.4.3       Task 3: (Optional) Flash Trigger Transient

·        Make a table of estimates of the duration of charging, flash discharge and trigger transients.

·        Charging transient: The time it takes the voltage to go from zero to 90% of its peak value.

·        For the flash discharge transient calculate Vstart - (90%)(Vstart - Vend).  Use your graph to estimate the time at which this voltage is reached.

·        For the flash trigger transient use one-half the time between the first peak and valley.

·        Compare the time scales for these three events with each other.

·        Compare these time scales to the shutter and light flash speeds you measured in an earlier lab.

·        Why must the flash trigger be so fast?

 

5.5        Summary and Conclusions

 

Follow the guidelines in the report format handout.

 

5.6        Figures and Tables

 

Include your figures and tables in this section.  If you captured the VirtualBench window in lab then put the plots here otherwise hand sketches will suffice.  Start this section on a new page and attach it at the end of the report.  Give each figure a number and caption (e.g. Figure 1:  Plot from Task 1).  Also give each table a table number and caption.  If a table includes results of calculations, include a sample calculation in the caption.  It is OK to cut and paste the filled in tables and figures from your Camera Lab 4 instructions.  It is OK for the captions and sample calculations to be hand written.  However, you may want to familiarize yourself with the equation editor tool in Word.  It will be useful to you in your future course work.  Just like Tables and Figure, Equations get numbers too.

 

Example:

 

                                                (Equation 5)