ENG H192: Hands on Labs

Analog Electricity

Batteries

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 are a common source of DC power—that is, they supply a constant current instead of a constantly changing (alternating) current like from a wall socket. Batteries are designed to maintain an imbalance of charge and so maintain a potential for electron flow. A chemical reaction takes place in batteries that creates extra electrons on the negative side. 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. In other words, the reaction is complete.

Rechargeable batteries are widely used on many portable devices such as video cameras, laptop computers and cellular phones. They are recharged by reversing the chemical process that created the potential int eh first place. 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

Power Conversion

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. To understand how an analog DC power supply works, several electrical components must first be understood.

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

q Capacitor: Capacitors can be thought of as a mini rechargeable battery in the sense that it stores charge. This stored charge can then be released when the capacitor is attached to a load, such a resistor. 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. 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.

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. Then the signal must be filtered to smooth it out. Right now it looks like a series of hills—the final result should be a straight line. A combination of a capacitor and a voltage regulator will achieve this result. The process is explained in the following pages.

…Power Conversion

Half Wave Rectifiers

A half wave rectifier is shown in Figure 1. 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 Rectifiers

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. (See Figure 2). 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.

…Power Conversion

Filtering

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. One end of the shock absorber is 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 may be connected between two points in a circuit between which a consistent voltage needs to be maintained. (See Figure 3). 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. Symbols for these can be seen in figure 4.

Figure 3. Full wave rectifier with a smoothing capacitor connected in parallel to the load.

Figure 4. Symbols of: ceramic capacitor (left) and electrolytic capacitor (right).

…Power Conversion

Analog Circuits

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

Other Types of DC Power Supplies

The DC power supply just explored is a basic, linear 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.

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.

Construction and Analysis of the Power Supply

Follow the list of steps to build and analyze your DC power supply. When you are finished you will have 5 sketches to include in your reports, along with a value for the transformer turn ratio.

Build the circuit shown in Figure 5. This circuit is a full wave rectifier with a resistor load.

Use a multimeter to obtain the turn ratio of the transformer used. Do this by measuring the output voltage from the transformer and assuming that the input is 120 volts. REMEMBER TO SET THE MULTIMETER TO VOLTS AC.

Use an oscilloscope to obtain the waveform at the output of the transformer. Sketch the waveform seen on the oscilloscope to include in your report. Be sure to include voltage and time measurements from the oscilloscope on your sketch. DO NOT USE THE OSCILLOSCOPE TO MEASURE THE 110V AC INPUT.

Use your oscilloscope to measure the waveform over the load resistor. Include a sketch of this waveform along with appropriate voltage measurements in your report.

Figure 5. The Full Wave Rectifier with Resistor Load

Change the load resistor to an electrolytic capacitor to create the circuit shown in Figure 6. Be sure to follow the capacitor safety warnings and diagram in Figure 10. Sketch the waveform of the signal across the capacitor and record appropriate voltage measurements.

Figure 6. The Rectifier with Capacitive Load

6. Add the load resistor back into the circuit as shown in Figure 7. Sketch the waveform of the signal across the resistor and record appropriate voltage measurements.

Figure 7. The Rectifier with both Capacitive and Resistive Loads.

Modify the circuit to include the voltage regulator as shown in Figure 8 to create the full DC power supply. A schematic of the voltage regulator is shown in Figure 9. Verify that the output of the voltage regulator is indeed a 5V DC signal using a multimeter. Sketch the waveform across the output resistor and record appropriate voltage measurements.

NOTE: THE OSCILLOSCOPE’S “AUTO SCALE” FUNCTION WILL PROBABLY NOT FIND THE SIGNAL. YOU WILL NEED TO TUNE THE OSCILLOSCOPE BY HAND TO SEE THE WAVEFORM.

Figure 8. The full DC power supply

Figure 9. 7805 Voltage Regulator.

Note that the input is the red wire, the common (or ground) is the black wire, and the 5 Volt output is the white wire.

Capacitor Safety

Be sure to orient the capacitor in the manner shown in Figure 10 below. The arrow points at the negative terminal, and there is an indentation at the positive terminal.

Failure to orient the capacitor correctly will cause the capacitor to explode, causing injuries!

Figure 10. Capacitor Orientation

ENG H192: Hands on Labs

Lab 3: Analog Electronics

Introduction

Background

Purpose

Lab Experience

Lab Report Guidelines