Download 1. Pre-Lab Introduction

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Lab 2: Power Supplies
Laboratory Goals
This project will show some of the basic principles of power supplies using fullwave
rectifier, Zener diode, and fixed-voltage regulator circuits.
Reading
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Student Reference Manual for Electronic Instrumentation Laboratories by Stanley Wolf
and Richard Smith, Copyright 1990.
Oscilloscope User’s Guide (Copies of this reference book are available in the lab, or at
the website)
Tektronics 571 Curve Tracer Manual
BS170 Transistor Data Sheet
Read the pre-lab introduction below
Equipment needed
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Lab notebook, pencil
Oscilloscope (Agilent or Tektronics)
2 oscilloscope probes (already attached to the oscilloscope)
BNC/EZ Hook test leads
Tektronics 571 Curve Tracer
PB-503 Proto-Board
Workstation PC, with PSICE application
Parts needed
Bridge Rectifier (50 PIV, 1 A), Zener diode (10 V at 500 mW), 7805 regulator
Lab safety concerns
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Make sure before you apply an input signal to a circuit, all connections are
correct, and no shorted wires exist.
Do not short the function generator signal and ground connections together
Do not touch the circuit wiring while power is applied to it
Ensure you connect the correct terminal of the transistor to prevent blowing the
transistor
1. Pre-Lab Introduction
Most of the direct current (DC) power used in electronic devices is derived by converting
60 Hz, 115 V alternating current (AC) power to direct current power. This AC to DC
conversion usually involves a step-down transformer, rectifier, filter, and a regulator. The
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Lab 2: Power Supplies
step-down transformer is used to decrease the AC line voltage from 115 VRMS to an
RMS value near the DC voltage needed. The output of the step-down transformer is then
fed into a diode rectifier circuit that only outputs positive halves of the input sinusoid. A
filter is then used to smooth the rectifier output to achieve a nearly constant DC voltage
level. A regulator can be added after the filter to ensure a constant output voltage in spite
of changes in load current and input voltages.
Two different types of voltage regulators will be used in this project. The first involves a
Zener diode circuit and the second involves a voltage regulator circuit. A Zener diode can
be used as a voltage regulator when the diode is reverse biased and operated in the
breakdown region. To maintain voltage regulation, the Zener diode must be operated in
the breakdown region at a current greater than the "knee" current (IZK). For currents
greater than IZK, the Zener diode characteristic curve is nearly vertical and the voltage
across the diode changes very little. Of course there is a maximum current the diode can
tolerate, so good regulation is provided when the diode is reverse biased with currents
between IZK and IZMAX. Zener diodes are available with a wide variety of breakdown
voltages. Another type of voltage regulator is available with the 7800 series regulators.
This series of fixed-voltage regulators is
numbered 78xx, where xx corresponds to the value of the output voltage. Output voltages
from 5 to 24 volts are available. These regulators are easy to use and work very well.
2. Design
1. Find approximations for the DC voltage level and AC peak to peak ripple voltage for
the bridge rectifier and filter circuit of Figure 1-1.
2. For the Zener diode regulator circuit of Figure 1-2 assume that the Zener diode will
regulate at 10 V over a current range of 5 mA to 25 mA. Assuming that the current
flowing through R is always between 5 mA and 25 mA and the Zener diode is regulating
at 10 V, find the minimum values of R and RL required. You may assume the forward
di
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Lab 2: Power Supplies
3. Lab Procedure
1. Construct the bridge rectifier circuit of Figure 1-1 without the capacitor. Use the
Variac with the step-down transformer for the input voltage to the bridge rectifier. With
the transformer plugged into the Variac, adjust the Variac until the secondary voltage
from the transformer equals 12 VRMS. BE CAREFUL not to short the secondary
terminals! Observe the secondary waveform on the oscilloscope. Put the oscilloscope on
DC coupling and observe the load voltage waveform VL. Remember that both the input
source and the load cannot share a common ground terminal.
2. Remove power from the circuit. Insert the capacitor as shown in Figure 1-1 being sure
to observe the correct polarity. Energize the circuit. With the oscilloscope on DC
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Lab 2: Power Supplies
coupling
observe VL. Measure the DC voltage level using the digital voltmeter. With the
oscilloscope on AC coupling observe the ripple voltage VR. Compare these measured
values with the calculated values.
3. Observe the effect of loading on the circuit by changing the load resistor from 1 k
voltage with the oscilloscope set on AC coupling. Compare these values with the
previously recorded values.
4. Record the Zener diode characteristic curve from the digital curve tracer. Note the
value of the breakdown voltage in the breakdown region. Also note the value of the
"knee" current IZK.
5. After verifying your designed values for R and RL with the instructor, construct the
Zener diode regulator circuit of Figure 1-2. Measure the DC voltage level with the digital
voltmeter for the minimum value of RL along with several values above and below the
minimum value. Be careful not to overload the Zener diode. Comment on the circuit's
operation for these different load resistances.
6. Construct the 7805 regulator circuit of Figure 1-3 being careful to observe the correct
pin configuration of the regulator. Measure the load voltage for RL
oes the value of the load
resistor affect the output voltage?
7. Using RL
voltage (pin 3). Decrease the regulator input voltage by decreasing the setting of the
Variac. For each decrease in amplitude, record the regulator input and output voltages.
Continue decreasing the amplitude until the output of the regulator drops a measurable
amount below 5 V. What is the minimum input voltage needed for the 7805 regulator to
produce a 5 V output?
4. Analysis
1. Why can't the input source and load have a common ground in the bridge rectifier
circuit?
2. Can the Zener diode be used as a conventional diode? Explain your answer and verify
with a curve from the curve tracer.
3. Would the value of the output filter capacitor have to increase, decrease, or remain the
same to maintain the same ripple voltage if the bridge rectifier were changed to a halfwave rectifier? Explain your answer.
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Lab 2: Power Supplies
4. How would increasing the frequency of the input source affect the ripple voltage
assuming all components remained the same?