EXPERIMENT 10
Voltage Limitation and Regulation with Diodes
This experiment is a follow up to EXPERIMENT 9 where
we looked at a simple half-wave rectifier. A serious problem with
simple rectifiers is that the DC voltage they produce is dependent
on the load. A common way to make the rectifier less sensitive
to the load is to add some regulation. This we can do by utilizing
the avalanching effect that occurs if we reverse voltage the diode
too much. First, we will see what kind of voltage limitation can
be achieved with a forward biased diode. Such limitation of voltages
is usually applied to protect circuit components.
Part A PN Junction Voltage
Limitation
When you performed the triangular wave sweep of the
1N4148 diode in EXPERIMENT 9, you should have observed that the
voltage across the diode remained near 0.5 volts when it was on.
We can take advantage of this effect in the following circuit
which permits small voltages to pass without distortion, but clips
any voltage outside the range of about -0.5 to 0.5 volts.
Observe the diodes functioning as limiters (or
clippers)
Draw the circuit shown. Use VSIN for the source (VAMPL
= 10, FREQ = 1k, everything else = 0).
Perform a transient analysis in increments of 50us
up to 5ms.
Use PROBE to plot the input and output voltages.
The input is the voltage source while the output is taken across
the diode pair, as shown.
Print your PROBE plot.
Change the amplitude of the sinusoidal input, Vin, to 0.1 volt. Repeat the transient analysis.
Why do the two plots look the way they do? Why is
this circuit called a limiter? When and why might this circuit
be useful?
Assemble the diode limiter circuit on your proto-board
Use the function generator to supply a 1 kHz, 10V,
sinusoidal input.
Observe Vout and Vin with the oscilloscope. You will
need to use both channels and to make sure that you connect your
grounds to only one place. Be careful to adjust the function generator
output such that the signal you observe on the scope looks as
much like the PSpice output as possible. You might have to adjust
the amplitude of the signal and the voltage and time scales on
the scope.
Add six experimental points to your PSpice graph.
Repeat the measurements with a 1 kHz, 0.1V sinusoidal
input.
Comment on the differences in your results.
Part B Zener Diode Voltage
Regulator
In the circuit shown above, D1 is a Zener diode. Be sure you have its orientation correct.
Obtain Zener diode characteristic.
Draw the circuit shown.
Perform a DC sweep analysis from -10 to +20 volts
in increments of 0.1 volts. Be sure that you enter V1 for the
name of the device that is swept.
Plot the current through the Zener diode I(D1) vs
the voltage across the diode -V(D1:2) to obtain the I-V characteristic
of the Zener diode in the same manner as we addressed the standard
diode in the last experiment. Note that your plot should look
like a typical Zener diode characteristic. If, for some reason,
the current or the voltage look backwards, reverse their signs
until the plot looks correct. Print this plot.
Now plot Vout vs Vin for your DC sweep. Print this
plot.
Hardware Implementation
Assemble the circuit on your protoboard.
Use the DC source that can produce an output up to
25 volts.
Using the Digital Multimeter (DVM), observe the voltage
across the Zener diode at the following source voltages: -10,
-5, 0, 5, 10, 15, 20 volts. Put the seven experimental points
on your PSpice plot of Vout vs Vin.
Connect the DVM to measure the current through the
diode. Current measurements are made in series using the two DVM
inputs labeled with the letter "I." Adjust the DC source
voltage until the current reads 5mA. Measure the voltage across
the diode for this condition. Plot this point on your PSpice I-V
characteristic. Repeat this for a current of -5mA.
Sinusoidal Response
Repeat the PSpice simulation of the Zener circuit
with a sinusoidal source like the one you used in Part A (VSIN
with VAMPL=10 and FREQ = 1k). Do the same kind of transient analysis.
Print the plot of Vout vs Vin.
Replace the DC source on your protoboard with the
function generator set to produce the same conditions as in your
simulation. Observe Vout and Vin on the oscilloscope. Again, take
pains to make the scope image look as much like the PSpice simulation
as possible.
Discuss the similarities and differences between
the simulated and experimental results.
Discussion/Calculation/Simulation of Results
We have seen that the voltage across the Zener diode
will remain equal to the Zener voltage, as long as we provide
enough voltage and current from the source. However, the circuit
configuration we have studied does not include a load. Discuss
how this circuit will perform for a load that is much smaller
than 1k ohm (eg 50 ohms), equal to 1k ohm, and much larger than
1 k ohm (eg 1 megohm). That is, under what conditions will it
produce the desired regulated voltage? Support your discussion
with calculations, simulations, and/or experimental results. Hint:
If you choose to simulate the circuit, you might want to try adding
the load resistor to your PSpice simulation while you are in class.
Last Updated on 21 February 1998