EGR 277

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Lecture 10 EGR 262 Fundamental Circuits
Lab
EGR 262 Fundamental Circuits Lab Presentation
for Lab 10 Getting Power off the Wall
Instructor Paul Gordy Office H-115 Phone
822-7175 Email PGordy_at_tcc.edu
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Lecture 10 EGR 262 Fundamental Circuits
Lab
Providing Power to Circuits For each experiment
so far in this course we have used DC power
supplies available in lab to provide 5V, 9V, and
other voltages. If we were designing a
microprocessor-based piece of equipment or
device, we might want to include our own DC power
supply so that we could simply plug the device
into an AC outlet. In this lab we will see how
to build a power supply that provides 5V DC from
a 120V AC source.
AC Voltages AC, or alternating current, voltages
are sinusoidal in nature. Lets begin with some
terminology used to describe such sinusoidal
waveforms.
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Lecture 10 EGR 262 Fundamental Circuits
Lab
Sinusoidal Waveforms In general, a sinusoidal
voltage waveform can be expressed as v(t)
Vpcos(wt) where Vp peak or maximum
voltage w radian frequency (in rad/s) T
period (in seconds) f frequency in Hertz (Hz)
Example An AC wall outlet has VRMS 120V and f
60 Hz. Express the voltage as a time function
and sketch the voltage waveform.
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Lecture 10 EGR 262 Fundamental Circuits
Lab

• Transformers Transformers are used to
• Change voltage values (our main interest for Lab
  10)
• Change current values
• Change resistance (or impedance) values
• Isolate circuits

f magnetic flux (in webers, Wb)
where a turns ratio
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Lecture 10 EGR 262 Fundamental Circuits
Lab
Examples of transformers (reference
www.allelectronics.com)
Primary 120V Secondary 28V, 1.5A
Primary 120V Secondary 40VCT, 0.25A
Utility pole transformer
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Lecture 10 EGR 262 Fundamental Circuits
Lab
Examples of transformers (reference
www.allelectronics.com)
PC Mount Transformer Primary 120V Secondary
16VCT, 0.8A or 8V, 1.6A
Toroidal Transformer Primary 120V Secondary
8.5V and 9.4V
Variac (Variable Transformer) Input 110V Output
0 to 130V
Up/Down Transformer (110V to 220V) or (220V to
110V)
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Lecture 10 EGR 262 Fundamental Circuits
Lab
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Lecture 10 EGR 262 Fundamental Circuits
Lab
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Lecture 10 EGR 262 Fundamental Circuits
Lab

• Rectifiers
• The transformer just illustrated is used to step
  down the voltage, but it is still an AC voltage.
  We need to convert it to a DC (direct current)
  voltage. A rectifier is used for this purpose.
  Rectification is defined as the process of
  converting an AC waveform into a DC waveform.
  There are three common types of rectifier
  circuits
• Half-wave rectifier (HWR)
• Full-wave bridge rectifier (FWR-bridge)
• Full-wave center-tap rectifier (FWR-CT)

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Lecture 10 EGR 262 Fundamental Circuits
Lab
Half-Wave Rectifier Discuss the operation of the
half-wave rectifier shown below.
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Lecture 10 EGR 262 Fundamental Circuits
Lab
Filter Capacitor A filter capacitor is often
added to the output of a half-wave rectifier in
order to smooth the output. Discuss the
operation of this circuit.
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Lecture 10 EGR 262 Fundamental Circuits
Lab
Full-wave Rectifier with Filter Capacitor Discuss
the operation of the circuit below (which will be
used in lab). Discuss ripple voltage and the
selection of capacitor values.
100 ?F _
About 20V with our transformers
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Lecture 10 EGR 262 Fundamental Circuits
Lab

• Regulators
• Note that even with the addition of a large
  filter capacitor, the DC voltage generated has
  some ripple (which is undesirable). Regulators
  can be used to eliminate most of the ripple
  voltage. Note the block diagram shown below.
• Two types of regulators will be used in this lab
• Zener diode regulator
• Integrated circuit (IC) regulator

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Lecture 10 EGR 262 Fundamental Circuits
Lab
Diodes - Review When LEDs were introduced in Lab
1, it was noted that a diode acts somewhat like
a voltage-controlled switch. When the diode is
forward-biased (VD gt 0) it acts like a short
circuit (or a closed switch). When the diode is
reverse-biased (VD lt 0) it acts like an open
circuit (or open switch). This is true to some
extent, but if the reverse-bias voltage becomes
too large, the diode can enter the breakdown
region where it again acts like a short circuit.
For many diodes (signal and rectifier diodes),
the breakdown voltage is to be avoided and is
often treated like a max negative voltage not be
be exceeded. A rectifier diode for example,
might become forward biased with a mere 0.7V, but
it might require 1000V to go into breakdown. It
can easily be used to rectifier 120V AC voltages
without ever entering the breakdown
region. Figures 5 and 6 below are from the Lab 1
section of the lab manual.
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Lecture 10 EGR 262 Fundamental Circuits
Lab
Zener Diodes A Zener diode is a special type of
diode that is intended to operate in the
breakdown region. The breakdown voltage is much
lower than with other diodes and Zener diodes are
designed to have particular breakdown voltages.
For example, you might buy a 5.1V Zener diode or
a 12V Zener diode, where the voltage listed is
the breakdown voltage. Also note that the Zener
diode has a special symbol.
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Lecture 10 EGR 262 Fundamental Circuits
Lab
Diode specifications The following tables are
from a Jameco Electronics catalog
(www.jameco.com). Note that Zener diodes are
purchased with a particular value of VZ in mind.
Rectifier diodes, on the other hand, list the
Peak Reverse Voltage (PRV) a max reverse
voltage to be avoided.
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Lecture 10 EGR 262 Fundamental Circuits
Lab
Zener Regulator A Zener diode can be used to
build a simple regulator circuit. It will
regulate the voltage to the value of VZ, the
Zener voltage. Discuss the operation of this
regulator circuit. Add on a load resistor and
calculate Vout for various values of RL using
voltage division. Graph the results and identify
the region where the Zener diode is regulating.
About 20V with our transformers
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Lecture 10 EGR 262 Fundamental Circuits
Lab
IC Regulators A wide variety of 3-terminal
regulators are commercially available. The 7805
is a common 5V regulator. The 7805 will accept
an input between 6V and 35V and deliver a
constant 5V at the output. 3-terminal regulators
are also available for 12V (7812) and other
voltages, including variable regulators (such as
the LM317).
About 20V with our transformers
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Lecture 10 EGR 262 Fundamental Circuits
Lab
7805 and LM317 Regulators
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Lecture 10 EGR 262 Fundamental Circuits
Lab
7.1. Pre-lab Tasks (1) Sketch of power supply
waveforms at node a, node b, node c (without the
capacitor) and node c (with the capacitor) in
Figure 6 (shown below). (2) Explain in your own
words how your power supply works. (3)
Schematic of the Zener diode regulator power
supply, including the AC input, transformer,
full-wave rectifier, filter capacitor,
Zener-diode regulator (with 1k? resistor between
filter capacitor and a 1N4733A (5.1V) Zener
diode) and a load resistance. The load
resistance should consist of a 100 ? resistor and
a 10 k? potentiometer in series (to make sure
that the load resistance never drops below 100
?).
About 20V with our transformers
100 ?F _
Figure 6 Full-wave rectifier
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Lecture 10 EGR 262 Fundamental Circuits
Lab
7.1. Pre-lab Tasks (4) Calculate the predicted
output voltage, Vout, of the Zener diode
regulator if the unregulated voltage is 20V (not
12V as shown in the lab guide) and Rload 100 ?
, 200 ? , 300 ? , 400 ? , 500 ? , 1k?, 2k?, ,
10k?. Show a sample calculation for Vout. Also
calculate the power dissipated by Rload for each
case and tabulate the results. What is the max
power dissipation seen in this table? Graph Vout
versus Rload. If the thumbwheel potentiometer
used in lab has a max power rating of ¾ W, will
the max rating be exceeded? (5) Schematic of
7805 regulator power supply, including the AC
input, transformer, full-wave rectifier, filter
capacitor, 7805, and a load resistance. The load
resistance should consist of a 100 ? resistor and
a 10 k? potentiometer in series. (6) Show the
predicted output voltage, Vout, of the 7805
regulator if the unregulated voltage is 20V (not
12V as shown in the lab guide) and Rload 100 ?
, 200 ? , 300 ? , 400 ? , 500 ? , 1k?, 2k?, ,
10k?. Explain how the predicted voltages were
determined. Also calculate the power dissipated
by Rload for each case. What is the max power
dissipation seen in this table? Graph Vout
versus Rload. If the thumbwheel potentiometer
used in lab has a max power rating of ¾ W, will
the max rating be exceeded?
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Lecture 10 EGR 262 Fundamental Circuits
Lab
7.2. In-lab Tasks (1) Build the power supply
circuit ?rst without the voltage regulator and
100 µF capacitor. Use the oscilloscope to view
the output of the transformer at node b. Use the
Measurement feature on the oscilloscope to find
the maximum voltage. Capture the image. (2) Use
the oscilloscope to view the output of the
full-wave rectifier at node c (with no
capacitor). Use the Measurement feature on the
oscilloscope to find the maximum voltage.
Capture the image. (3) Measure the value of the
capacitor (before adding it to your circuit) on
an impedance bridge and record the value. (4)
Now add the capacitor to your circuit and use
the oscilloscope to view the voltage waveform at
node c. Use the Measurement feature on the
oscilloscope to find the average voltage and the
ripple voltage. Capture the image. (5) Add the
Zener-diode regulator circuit and 10 k? load
resistor and measure the output voltage as a
function of load resistance for Rload 100 ? ,
200 ? , 300 ? , 400 ? , 500 ? , 1k?, 2k?, ,
10k?. (7) Remove the Zener-diode circuit and add
the 7805 voltage regulator circuit and 10 k? load
resistor and measure the output voltage as a
function of load resistance for Rload 100 ? ,
200 ? , 300 ? , 400 ? , 500 ? , 1k?, 2k?, ,
10k?.
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Lecture 10 EGR 262 Fundamental Circuits
Lab
7.3. Post-Lab Tasks (1) Discuss the waveform
captured showing the output of the transformer.
Was it as expected? (2) Discuss the waveform
captured showing the output of the full-wave
rectifier without the filter capacitor. Was it
as expected? (3) Discuss the waveform captured
showing the output of the full-wave rectifier
with the filter capacitor. Was it as
expected? (4) Create a table comparing Vout
(predicted) and Vout(measured) for each value of
Rload for the Zener diode regulator circuit.
Include error. Also graph Vout (predicted)
versus Rload and Vout(measured) versus Rload (on
the same graph and include a legend). Discuss
the results and explain any differences. (5)
Create a table comparing Vout (predicted) and
Vout(measured) for each value of Rload for the
7805 regulator circuit. Include error. Also
graph Vout (predicted) versus Rload and
Vout(measured) versus Rload (on the same graph
and include a legend). Discuss the results and
explain any differences. (6) Compare the two
power supplies built and tested in lab. Which is
better? (7) The power supply you built provides
5 volts for your MicroStamp11. However, the
op-amps used in Labs 6-7 require 9 volts. Explain
how you could modify your power supply to power
your op-amps.