Laboratory #5: Unregulated and Regulated DC Power Supplies
Electrical and Computer Engineering
EE 204.3
University of Saskatchewan
Authors:
Denard Lynch
Date:
Oct 24, 2012
Nov 4, 2013, revisions – D. Lynch
Description:
This laboratory explores the construction and behaviour of unregulated and regulated DC
powers supplies. The student will construct a basic full-wave rectifier with a capacitive
filter, fed from a 120 VAC source through a step-down transformer. After verification
and performance testing, the student will use a linear regulator to construct and test a
regulated 12VDC/1A power supply.
Learning Objectives:
In this laboratory, the student will:
• Construct and test a full-wave rectifier DC Power Supply
• Calculate the expected ripple voltage given a filter capacitor
• Determine the required components and construct a 12VDC regulated power
supply
Safety Considerations:
In addition to general electrical safety considerations, the student should also be aware of
the following considerations specific to this laboratory exercise:
• Resistors carrying current will generate heat energy and can be overheated in AC
circuits. Power is based on RMS voltages and currents. In all other respects, the
same considerations as in DC circuits apply.
• You will be using AC transformers energized from a 120 VAC source.
Potentially high powers are available, and incorrectly connected components can
become very hot very quickly.
• You will also be using high-wattage load resistors to test your power supplies.
These are designed to dissipate significant power and can become very hot.
Observe all guards and monitor the devices for excess heat, disconnecting when
necessary to be allowed to cool.
• Polarized electrolytic capacitors are usually used for power supply filtering
applications. These components have a positive and negative terminal that must
be connected correctly. Reverse biasing this type of component, even briefly, will
damage the component. If left, it will quickly overheat, physically distort and
possible explode. Always ensure the positive terminal is connected to the
positive (higher) potential in a DC circuit (electrolytic capacitors are not
suitable for use in AC circuits).
Denard Lynch
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Laboratory #5: Unregulated and Regulated DC Power Supplies
Electrical and Computer Engineering
EE 204.3
University of Saskatchewan
Background and Preparation:
You will continue to use your Digilent Discover Module for testing purposes. You will
use some additional laboratory equipment: a 120VAC to 24VAC step-down transformer,
and a high power rheostat (a high power variable resistor) or fixed power resistors. To
connect this equipment, you will need to use leads with banana plugs, and alligator clips.
There is an assortment of leads with banana plugs on the walls of each lab, and there is a
supply of alligator clips in a drawer in 2C80. Your instructors can direct you to these
areas and demonstrate their use. Additional background information can be found in
Appendix A in this
Procedure:
The procedure will involve constructing several steps:
• Building a full-wave rectifier, capacitor filtered Power Supply circuit
• Verifying the ripple voltage and measuring the load regulation
• Building a 12DC Regulated Power Supply
• Verifying the load regulation compared to the unregulated supply
ModelingThis will involve using theory of design and operation of AC power supplies. The
required parameters are described in the detailed procedure below. Please read the entire
procedure over carefully before the lab and calculate circuit component values where
possible.
MeasurementsUse your solderless breadboard and set up the circuits, in turn, for each circuit.
You may be able to construct more than one circuit on your breadboard at the same
time. In some cases, it may be advantageous to maintain a circuit for later parts of
the laboratory. A good first step is to examine the circuit, make a list of the parts
you will need, obtain the necessary parts and construct the circuit. You may also
need to measure the actual value of your passive components (e.g. using the LRC
Test Unit or equivalent) versus their nominal value. For example:
Component
1/4W resistor
Nominal Value
Measured Value
1000Ω
986Ω
Capacitor 100WVDC
0.1µF
0.092µF
Etc.
You will verify parameters of active, solid-state devices “in-circuit” if necessary.
Start by assuming the values given in datasheets for the components. Your lab
instructor will indicate where to obtain the necessary parts if they are not already in
your parts kit, and how to measure their actual value. You can use your Analog
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Laboratory #5: Unregulated and Regulated DC Power Supplies
Electrical and Computer Engineering
EE 204.3
University of Saskatchewan
Discovery Module (ADM) as an oscilloscope to observe the output of your Power
Supply. Be careful not to exceed the input limitations of the device (±20V).
Terms:
Power Supply
Provides a specified voltage up to a specified current to
operate other electrical or electronic devices. Most
commonly used to convert AC power to a DC source.
Line Regulation
The ability of a power supply to maintain the specified
output voltage if the input voltage changes
Load Regulation
The ability of a power supply to maintain the specified
output voltage if the load current changes
Half-wave
Rectification of an AC signal that results in a polarized
output during only half of the full sine wave period
Full-wave
Rectification of an AC signal that results in a polarized
output during both halves of the full sine wave period
Diode Bridge
4 diodes arranged such that a full-wave output is obtained
from a 2-wire AC input
I. Unregulated Full-wave DC Supply
.
The objective in this part is to construct and verify the operation of a full-wave
rectifier power supply that can deliver approximately 1 A of current to a
resistive load at a voltage of at least 12 VDC. In addition, you will measure the
static load performance of this unregulated supply by varying the load and
measuring the output voltage.
Obtain parts for, and construct the circuit shown in Figure 1. The transformer
shown in the schematic can be found on the equipment shelves in 2C80 (be sure to
return it to the proper. Labeled location when you are done). The transformer is
housed in a metal case and has a 2-prong line cord already attached. It also has 2
fuses to protect the secondary output from over current. The fuses are in line with
the “outer” leads (pins 2 and 4 on the schematic) which appear on the 2 red banana
plugs on the case. The centre tap appears on the black banana plug. During this
laboratory, you should never draw enough current to blow the 3 A fuses, but you
should check the output voltage of the transformer with a DMM if you suspect it is
not working properly and replace the fuses if necessary.
AC1
.
C1
3
5
4
120:24
.
AC2
LOAD
TR1
1
2
.
.
Figure 1: Full-wave Unregulated DC Supply
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Laboratory #5: Unregulated and Regulated DC Power Supplies
Electrical and Computer Engineering
EE 204.3
University of Saskatchewan
This transformer is a 120VRMS to ~24VRMS transformer (note: the secondary voltage
is a few volts higher with no load connected) with a centre-tapped secondary. Use
this centre tap (black banana plug, pin 3 in Figure 1)) as shown to wire a full-wave
rectifier using 2 1N4007 power diodes. Use a short loop of bus wire to create loops
on your breadboard so you can connect alligator clips from the transformer to
energize your circuit. Be sure to check your diode connections carefully before
connecting the transformer secondary to your circuit! Use a very low current
load (i.e. high resistance) to ensure some current is drawn so the diodes “turn on”
so you can see the expected full-wave waveform. (Suggestion: 1kΩ , but measure
your output voltage and calculate the expected DC voltage and heat dissipation to
ensure you don’t overheat the resistor you use as a simulated load. Resistors
come in certain wattage rating: ¼W ½W etc. – obtain one with the required
dissipation rating.)
Verify the full-wave output visually using your ADM before adding the capacitor.
Use the 1000µF electrolytic capacitor from your parts kit, or from the Tech office.
Important reminder: these capacitors are polarized, so they must be connected
correctly (i.e. positive to positive) or they will overheat and cause damage to
themselves and possibly other circuit components and bystanders! After verifying
a full-wave DC output with a light load (1kΩ) and without the capacitor, unplug the
transformer and install the filter capacitor. If in doubt, ask your instructor to verify
the proper polarity before re-energizing your circuit.
Observe the output waveform with your Digilent oscilloscope and measure the
output voltage (average) and ripple voltage with the very slight load imposed by the
~1kΩ test load. Next, obtain a high power rheostat from the equipment shelves in
2C70 (or your instructor may have a selection available in the room for your
convenience). These are high-power adjustable resistors that can be used to draw
significant current from your power supply in order to test its performance under a
static (i.e. not changing) load. Once again, the connections to the rheostats are
through banana plugs on the device, Study the connections so you are using the
appropriate terminals, and connect to your breadboard where necessary using
alligator clips on the end of the banana-plug leads. The alligator clips are not
insulated, so be careful to place then so that they will not create an unintended short
circuit. Use a DC ammeter (or DMM) to monitor the current. Measure the average
output voltage and the voltage ripple at several load points, e.g.
1. IkΩ load (~12mA)
2. .25A
3. .5A
4. 1A
Be careful not to exceed the average current capability of your 1N007 diodes, or the
power dissipation capabilities of the rheostat (usually indicated by a label on the
device). You may want to create a graph of the output voltage versus load current.
One of the key parameters of power supplies is how “flat” this curve is. Of course,
you would not expect an extremely flat output with this simple supply. The figure
of merit often used is the voltage variation over the rated output current range, e.g.
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Laboratory #5: Unregulated and Regulated DC Power Supplies
Electrical and Computer Engineering
EE 204.3
University of Saskatchewan
“12VDC ± 120mV. 50mA – 1A”. This is a “static” specification, which means the
output current is held constant long enough for the output voltage to stabilize.
Things to Note in this part:
• The no-load (or very low-load) output voltage
• The forward voltage drop of the diodes
• The ripple voltage at different load currents
• The change in output voltage with changing output current
II. Linear, Regulated DC Supply
The object in this part of the lab is to use a commercial integrated circuit
voltage regulator to regulate the output of the supply built in Part I. You will
then test its static load performance the same way described in Part I.
Use the LM2940-12 (12V fixed) linear regulator IC from your parts kit. Add it to
your breadboard circuit as shown in Error! Reference source not found.. The
capacitors, C2 and C3 are there to ensure stable operation of the IC. The value for
C3 should be a minimum of 22µF, low ESR (equivalent series resistance) to ensure
stable operation (i.e. no oscillation). The large-valued C1 will help filter the input,
so a small size for C2 is all that is required to stabilize on the input side. R1 is used
to provide a voltage drop from the “raw” input so the regulator IC will not have to
dissipate all the excess power itself. It should be sized accordingly, but to start, the
small current drawn by a low load (i.e. the 1kΩ resistor again) should be less that
the dissipation capability of the regulator’s case. For instance, if the raw input
voltage is ~17VDC and the regulator output is 12VDC and the load is 12mA, the
power that must be dissipated in the regulator is:
P = (17V − 2VD − 12V ) (12mA ) ≤ 60mW
However, this should always be checked with the expected operating conditions to
ensure the device isn’t overheated and damaged.
3
C1
C2
4
1000
.1
VI
VO
GND
3
C3
5
120:26
22
.
AC2
.
.
LOAD
1
2
AC1
.
R1
TR1
1
2
.
IC1
LM2940-12
.
.
Figure 2: 12V Regulated Power Supply
Monitor the output of the supply with your ADM again, and verify that the
regulator is stable and that there are no oscillations in the output voltage.
Before you can test the static load performance by increasing the current, you will
need to consider the expected power dissipation. A check of the datasheet reveals
that the TO220 package can dissipate a maximum of 2W at room temperature
without a heat sink (a way to draw the excess heat from the device without the
device overheating). If we desire a maximum output current of 1A (the maximum
Denard Lynch
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Laboratory #5: Unregulated and Regulated DC Power Supplies
Electrical and Computer Engineering
EE 204.3
University of Saskatchewan
average for our 1N4007 diodes), we can allow a maximum of 2V across the device.
If we have a raw input of 17VDC, this means we need to reduce it to a maximum of
14V @ 1A load current. In this case R1 should be:
17V − 2VD − 14V
P=
≈ 1Ω
1A
On the other side, the LM2940 needs about 1V more input voltage than output
voltage to operate, so we can only have a maximum of ~2Ω or the circuit won’t
perform properly. Exact values may not be available, so some compromises will
have to be made, and it may not be possible to test the supply over the full design
operating range, or perhaps for only very short periods to avoid excess heating.
Also, the power capability of this dropping resistor must also be up to the task. In
the example given, a 1 or 2W device would be requred.
Once these decisions are made and the circuit modified accordingly, verify static
operation again with the 1kΩ load before using the rheostat to measure the
regulation over a broader range of load currents. Repeat the measurements you did
for the unregulated supply. Compare the V-I graphs and comment on how they
compare in “flatness” (i.e. load regulation).
This type of regulator is very simple to implement and is generally free from noise
and has a very stable output. However, the main disadvantage is that the same
current going through the load is coming from the higher voltage source supply,
thus wasting power. Estimate the efficiency of your regulated supply by observing
the input supply voltage compared to the desired output voltage and calculating the
output power compared to the input power. Note: when taking measurement with
higher output powers, try to take the measurement quickly and then reduce or
remove the load until you are ready to take the next measurement in order to
avoid overheating of the components.
Things to Note in this part:
• The no-load (or very low-load) output voltage
• The voltage drop across the regulator
• The ripple voltage before and after the regulator
• The change in output voltage with changing output current
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Laboratory #5: Unregulated and Regulated DC Power Supplies
Electrical and Computer Engineering
EE 204.3
University of Saskatchewan
ReportingUse your lab notebook (logbook) to document
• the key objectives of this laboratory,
• your theoretical calculations
• Parts List: your equipment and circuit components used
• any measured values of components
• your measurements verifying your theoretical expectations (you can paste in
screen shots from your ADM where appropriate),
• use Power Triangles and Phasor Diagrams to help illustrate results (e.g. the
relationship between voltages or currents or powers).
• your observations and comments about how closely your observations matched
your expectations,
• related comments on practical limitations for your observations and comments on
possible sources of error
Denard Lynch
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Laboratory #5: Unregulated and Regulated DC Power Supplies
Electrical and Computer Engineering
EE 204.3
University of Saskatchewan
APPENDIX A: Background Theory
You will once again have occasion to deal with RMS values, and convert between RMS
and peak values. Recall that the peak value of an RMS-valued sinusoid is 2VRMS . (Also
recall the average value of a 1/2 –wave rectified sinewave is A π , and for a full-wave:
2A )
π
Also, be cautious about connecting the ground points between AC and DC circuits.
The configuration and components used in this lab are those covered in class. Please
refer to class notes for any required background information.
The first part of the lab uses a full-wave rectifier with an output similar to that obtained
when you investigated diode behaviour in a previous lab. In this lab you will calculate
the “filtering” effect of a capacitor added in parallel with the output. Recall that the
relationship between the ripple voltage, capacitor value and frequency of the charging
“pulses” is given by:
CF =
I Out
,
fVRipple
where CF is the size of the filter capacitor in Farads, IOut is the average output current, f is
the frequency of the “re-charging” (e.g. for a full wave rectified 60 Hz source, the
capacitor is “re-charged” 120/s, so f = 120), and VRipple is the maximum desired (peak-topeak) variation in the output voltage. You will use this “raw” rectifier as an input to the
next phase, a regulated power supply.
A linear regulator will accept an input voltage and provide a regulated (i.e. will not vary
with load changes) output voltage. The LM2940-12 regulator you will use in this lab will
accept up to a 26 V DC input and provide a 12 VDC output. The drop-out voltage, the
minimum “Extra” voltage required of the input over the output is .8V maximum. The
rated output current is 1A. Remember, any power coming from the raw source that is not
delivered to the load must be dissipated in the regulator. In some cases the excess power
can be diverted from the regulator IC by using a dropping (or ballast)_ resistor in series
before the regulator.
One of the “figures of merit” you will test for your supply is its ability to respond to very
quick changes in load current. The LM2940 should only vary a slight amount and then
return to its rated output voltage. You can compare your test results with the
specifications given in the manufacturers datasheet.
References:
EE204 Course Notes
various datasheets (1N4007, LM2940CT-12, )
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