ENG H192 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.
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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.
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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. To understand how an
analog DC power supply works, several electrical components must first be
understood.

Resistor: The simplest impedance device; it restricts the flow of
current.

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.

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).

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.
Power
Conversion
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
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).
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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.
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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. 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 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.
Figure 3. Full wave rectifier with a smoothing capacitor connected in parallel to the load.
Figure 4. Symbols of: a) ceramic capacitor, and b) electrolytic capacitor.
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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 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.
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.
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LAB EXPERIENCE
Make sketches of equipment used in class; include them in your lab write-up.
Construction
and Analysis of
the Power
Supply
1.
Build the circuit shown in Figure 5. This circuit is a full wave rectifier
with a resistor load.
2.
Use a multimeter to obtain the turn ratio of the transformer used.
Determine the peak value of the low voltage signal. REMEMBER TO
SET THE MULTIMETER TO VOLTS AC.
3.
Use an oscilloscope to obtain the waveform at the output of the
transformer and over the load resistor. Include these graphs along with
appropriate voltage measurements in your report. DO NOT USE THE
OSCILLOSCOPE TO MEASURE THE 110V AC INPUT.
4.
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.
5.
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.
6.
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.
Note: You should have one sketch of the voltage waveform for each
component of the circuit. Label each sketch and describe why the sketch
appears as it does. Make sure to include voltage levels.
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Figure 5. The Full Wave Rectifier with Resistor Load
Figure 6. The Rectifier with Capacitive Load
Figure 7. The Rectifier with both Capacitive and Resistive Loads.
Figure 8. The full DC power supply
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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 6 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
LAB REPORT
Format

General
Guidelines

Lab report format (Individual or Team, Lab Report or Memo) will be
announced in lab.
DC Power Supply: Show complete circuit, waveforms, and provide
explanations.
 Show Explanation and Analysis of the power supply in the same order
given in the lab experience.
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