EGH166 Hands-on Lab

## Lab 4: Analog Electronics

#### Introduction

##### Background

Analog electronics encompasses a vast range of electrical systems. Computer power supplies, antenna transmission, industrial power grids and telephone transmission are just a few examples of analog electrical systems. Engineers are forever seeking perfect efficiency; electrical engineers are no different. Since nearly everything is in some way powered by electricity achieving high efficiency with electrical devices will save resources and as a more immediate benefit, save money.

##### Purpose

The purpose of this lab is to familiarize you with Analog Electricity while building and analyzing the waveforms across various terminals of a DC power supply.

##### Basic Principles

In this lab write-up, we will cover some basic principles behind:

1)            Analog Electricity,

2)            Batteries,

3)            Analog DC Power Supplies,

4)            Voltage rectifiers,

5)            Smoothing Capacitors,

6)            Voltage Regulators,

7)            Output Capacitor, and

8)            Analog Circuits

##### Lab Experience

The lab experience will encompass:

1)            Building a DC Power Supply,

2)            Using an oscilloscope to obtain the waveforms, and

3)            Determining peak, RMS values, and frequency of signals.

#### Theory

##### Analog Electricity

Analog simply means continuous. Analog data changes continuously with time, the temperature of the room changes constantly even if it is to just a small degree. An analog clock is one with continuously sweeping hands, one that never stops whereas a digital clock changes in fixed, exact increments. Analog electronics deals with continually changing voltage levels and currents.

##### Batteries

Batteries are designed to maintain an imbalance of charge and so maintain a potential for electron flow. This is achieved by storing a large number of electrons on the negative terminal of a battery and eliminating a large number of electrons from the positive terminal. Since the electrons seek balance, the electrons try to flow from the negative to the positive terminal. A battery is dead when the charge has balanced out on each side of the battery.

Rechargeable batteries are widely used on many portable devices such as video cameras, laptop computers and cellular phones. Even though battery technology is impressive, it is having difficulty reaching the electric powered vehicle market. Gasoline still stores more useful energy per unit volume and per unit weight than rechargeable batteries. Storing enough energy to move a car at 60+ mph for a few hours requires a lot of battery weight.

An ideal battery would be able to deliver a fixed amount of current at a steady voltage level throughout its life. This unfortunately is not the case. As a battery is used, it loses voltage and drops in current capacity and so does not deliver consistent power. The quality of a battery is related to how closely it comes to this ideal behavior.

##### Analog DC Power Supplies

The DC power supply to be explored is a basic, linear power supply. A DC power supply found in a computer takes 120-volt AC power and converts it into +5, +12, -5, and -12 volts DC. The manner in which this is accomplished depends upon the style and cost of the power supply.  Most computers use a more complicated, more efficient type of power supply called a switching power supply. The end result of a switching power supply is the same (a constant, stable voltage), but the manner in which the voltage is arrived at is quite different.
To understand how an analog DC power supply works, several electrical components must first be understood. Here is a description of the most important ones:

q       Resistor: The simplest impedance device; it restricts the flow of current.

q       Capacitor: Capacitors can be thought of as a mini rechargeable battery. They store electrical charge that is released when there is a demand. Capacitors are also used to filter out voltage fluctuations or supply extra current when there is a large demand.

q       Diode: A diode allows flow of electricity in only one direction. This is the one-way street of electrical circuits. Positive current can flow only in the direction that the arrow points (from Anode to Cathode).

q       Voltage Regulator: A voltage regulator is a special integrated circuit designed to maintain a certain voltage level. Voltage regulators must be supplied with a voltage higher than the voltage they are designed to produce. For example a 5-volt regulator must be supplied with at least 8 volts to function properly.

##### Voltage Rectifiers

The first step of converting AC to DC is to reduce the voltage level to a range closer to the final DC value. A transformer is used to step down the voltage. Next, any negative voltage levels must be eliminated or inverted; this is achieved using diodes as voltage rectifiers.

##### Half Wave Rectifier:

A half wave rectifier is shown above. The load indicated is the rest of the DC power supply and the DC circuit that it is powering. The AC signal is 'chopped' using a diode. When the AC voltage is positive the diode allows current to flow to the load. When the voltage is negative no current can flow. The resulting signal is a half wave rectified signal (one half of the original AC signal is present).

Figure 1. Half wave circuits showing the output signal.

##### Full Wave Rectifier

Almost all power supplies use a full wave rectifier circuit so that none of the power available is wasted. This is achieved with the following circuit. For a positive semi cycle, the current path is through diodes 1 and 2 (3 and 4 are open), and for a negative semi cycle, the current flows through diodes 3 and 4 (1 and 2 are open).

Figure 2. Full wave rectifier including input/output signals.

##### Smoothing Capacitors

Once the voltage level has been converted to an entirely positive voltage level, the next step is to smooth out the voltage to an average level in order to obtain a smooth, consistent voltage level. A capacitor is used to maintain an average voltage level; it behaves similar to a shock absorber in a car, which attempts to maintain a consistent distance from the ground to the car. A car has one end of the shock absorber attached to the car frame and the other attached to the moving part (the wheel axle) and the distance between these two is maintained. A capacitor is connected between two wires that a consistent voltage needs to be maintained between. Think of the car frame as the electrical ground for our electrical circuit.

Capacitors come in different sizes and styles, just as shock absorbers are rated for different stiffness to accommodate vehicles of different weight and ride feel. The most common are ceramic and electrolytic capacitors. Ceramic capacitors are typically used for very low capacitance levels and they are a non-polar component (it does not matter how it is oriented). Electrolytic capacitors are used for higher capacitance levels; they are capable of storing much more charge. Electrolytic capacitors are polarized: of the two leads, one must always be at a lower voltage level than the other.

Figure 3. Symbols of:  a) ceramic capacitor, and b) electrolytic capacitor.

Figure 4. Full wave rectifier with a smoothing capacitor connected in parallel to the load.

##### Voltage Regulators

At this point the power supply could work for very simple DC voltage usage such as powering a flashlight or battery operated mechanical toys, however a voltage source like this is not adequately stable for use in digital electronic circuits. An integrated circuit should be added which will maintain a constant smooth voltage level. This device is called a voltage regulator. Voltage regulators can be purchased in a variety of sizes and ratings, the most common have output voltages of +5, +12, -5, and -12 volts. Adjustable voltage regulators are also commercially available

##### Output Capacitor

One final component typical for DC power supplies is an output capacitor. This final capacitor is present so that current is available for quick, high demands and so voltage spikes can be suppressed

##### Analog Circuits

An inductor is another basic element of linear or analog electronics that is used in many applications. It is simply a coil of wire that is used to store current just as a capacitor stores voltage. Inductors cause a great deal of trouble in digital circuits because they force current to travel in a direction which it may not be desired, but in analog circuits such as radio frequency decoding or television signal circuits, inductors serve a useful purpose.

The basic elements resistor, capacitor and inductor make up the vast majority of analog devices. Other components, which are important, are transistors and operational amplifiers. Transistors can be used in strictly analog circuits, as well as providing an interface between analog and digital circuits. Operational amplifiers (op-amps) are used to amplify very low power signals for example; the signal picked up by a cassette tape head must be greatly amplified before it drives a loud speaker. All electrical circuits consist of the simple components mentioned which when used in combination can perform complicated tasks.

#### LAB EXPERIENCE

Make sketches of equipment used in class; include them in your lab write-up.

##### WARNING

q       A CHEATER PLUG MUST BE ATTACHED TO THE THREE PRONG PLUG OF THE OSCILLOSCOPE BEFORE CONNECTING IT TO A 120V RECEPTACLE.

q       NEVER CONNECT THE OSCILLOSCOPE LEADS TO THE 120V RECEPTACLES. THIS MAY RESULT IN SERIOUS DAMAGE TO THE DIGITAL OSCILLOSCOPES!

##### Build a Power Supply

q       Design and build an analog 5V DC power supply using a transformer, a full wave rectifier, a 1000 uF electrolytic capacitor, and a 5V voltage regulator.

q       Draw a schematic of the DC power supply designed and describe each of the components used.

q       Use a voltmeter to obtain the turn ratio of the transformer. Determine the rms and the peak value of the signal on the low voltage side of the transformer.

##### Use the Oscilloscope

1.      With the smoothing capacitor disconnected:

q       Use an oscilloscope to obtain the waveforms throughout the circuit. That includes, across the terminals of each diode, the resistor, the output signal of the transformer, and the input/output of the voltage regulator. Record voltage magnitudes on each waveform.

q       Make sketches of these waveforms and label them. Describe why the waveforms appear as they do. Make sure to include measured voltage values.

q       Determine the peak value, rms value, and the frequency of the rectified signal (coming out of the full wave rectifier).

2.      With the smoothing capacitor connected:

q       Use a voltmeter to measure the input/output voltages on the voltage regulator.

q       Use an oscilloscope to obtain the voltage waveform across the capacitor. Determine the peak value, rms value, and the frequency of this signal. Determine also charging and discharging time of the capacitor.

#### LAB REPORT

##### Format

·   Lab reports must be done TEAM

·   Follow given lab report format.

·   Maximum 4-5 pages (including figures and tables)

##### General Guidelines

·   Cover page

·   Description of Experimental Apparatus

·   Introduction/Background

·   Show complete circuit labeled.

·   Include labeled waveforms, results, and provide explanations and information as required on the lab experience.

·   Analysis of results/Summary.