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EXPERIMENT # 3 The Zener Diode Voltage Regulator
EXPERIMENT # 3
The Zener Diode Voltage Regulator

Zener Diodes

Rectifier or switching diodes characteristics were studied in lab # 1
and some circuits utilizing the rectifier diodes were built in lab # 2.
This lab deals with the study of the characteristics of Zener diodes
and their use in the circuits as a voltage clipper or voltage
regulator.

Zener diodes are similar to the Rectifier diodes as far as forward
bias characteristics are concerned. In the reverse bias the
breakdown voltage of Zener diodes is usually quite smaller than
the rectifier diodes and also they are usually meant to be operated
in their breakdown region so they are made in a special way that
they dont get burned even when a large reverse breakdown
current pass through them at the breakdown voltage.

Procedure

The experiment consists of two parts:
1. Measurement of Zener Diodes I-V characteristics
2. Application of Zener diode in the bridge rectifier as a voltage
regulator.

Measurement of Zener diodes I-V characteristics
Since the Zener diode is meant to be operated in the reverse bias region,
only reverse bias characteristics will be observed. Forward bias
characteristics are similar to the characteristics of the Rectifier diode
that were observed in Lab # 1.
1. Build circuit # 1 as shown below. Observe that the Zener diode is
placed in the circuit in reverse bias configuration but the voltage
shown across it is taken as positive. This is consistent with the
direction of the flow of current in the circuit. But always keep in
mind that the region of operation of diode is in the third quadrant
i.e. voltage and current across diode are both in the direction
opposite to the forward bias direction.
1 K
Ammeter
V
z
+
-
Circuit # 1
A
V
IN

2. Record 20 readings of diode voltage (V
Z,
not V
IN
) and corresponding
current. Take 7 readings until you reach the zener breakdown
voltage and 13 more readings after that point i.e. within the
breakdown region. Draw the I-V characteristics of zener in the
reverse bias region in your lab report and estimate the value of
zener knee voltage (breakdown voltage) from your graph.



Application of Zener diode in the bridge rectifier as a voltage
regulator
1. Consider Circuit # 2. This is the same bridge rectifier circuit that
you built in lab # 2. Now a zener diode is used at the output of this
circuit to regulate the output voltage.

V
out
+
-
110V/
60Hz
v
in
+
-
C = 100
microFarad
100
KOhm
RL
R1

2. Choose a value of R
1
using different values of resistors that sets
the zener voltage at the output with a load of 1K. Be sure the value
of R
1
you choose would be the maximum value for which there
would be the zener voltage at the output.
3. Measure the average DC current through zener. Calculate the
average power dissipated in the zener diode.
4. Measure the DC average and ripple components of voltage across
both the load and capacitor. Draw a rough labeled sketch of ripple
voltage riding on DC average. Calculate how much does the zener
reduce the ripple contents of capacitor.
5. Remove R
L
and measure the output DC average voltage.
6. Record DC average voltage at the output for load resistances = 100,
500, 900, 1.5K, 5K and 10K. Measure in each case
(i) Current across R
1
.
(ii) Current across zener
(iii) Current across load
Confirm KCL using these three currents. Also calculate the power
dissipated in the zener diode for each case.
7 Calculate the voltage regulation for each load resistance.