PURPOSE AND BACKGROUND
The purposes of this experiment are to demonstrate (1) the characteristics
of a zener diode and (2) its use as a simple voltage regulator. Unlike rectifier
diodes, zener diodes are normally reverse biased, so they maintain a constant
voltage across their terminals over a specified range of current. Like a
rectifier diode, a zener diode can be approximated by a constant dc voltage
source in series with a resistor. When used as a regulator, the zener diode
maintains a dc output voltage that is essentially constant even though the
load current may vary.
REQUIRED PARTS AND EQUIPMENT
Resistors:
 100 ohm, 1/4 W
 Two 220 C 1/2 W
1N753, 6.2V, 400mW zener diode
Signal generator
Two DMMs (preferred) or VOMs
Dual trace oscilloscope
Breadboarding socket
015 dc power supply
= = =
USEFUL FORMULAS
Maximum limiting series resistance
Output voltage
Zener diode power dissipation
Percent load regulation
Output ripple voltage
PROCEDURE
1. Wire the circuit shown in the schematic diagram of Figure 71A.
2. Increase the dc supply voltage in small steps while simultaneously measuring
the voltage across (Vz) and the current through (Iz) the zener diode. In
the vicinity of the zener’s knee voltage (approximately 6V), make these steps
approximately 0.05 V. Do not exceed a zener current or 40 mA. Record your
data, and plot your results for the corresponding zener current and voltage
values on the graph provided for this purpose. What do you notice about the
currentvoltage curve for the zener diode?
FIGURE 71 Schematic diagram of circuits.
Note that initially, the zener diode current is essentially zero for diode
voltages less than the knee voltage. You should find that as the voltage
drop approaches the diode’s knee volt age, the diode’s current increases
rapidly, while, at the same time, the voltage stays essentially constant.
Consequently, the zener diode maintains an essentially constant voltage drop
when it is sufficiently reverse biased.
The 1N753 diode is rated at 6.2 V with a tolerance of 10%. From your graph,
determine the voltage across the zener diode at a current of approximately
20 mA. Within 10%, your value should be 6.2 V.
3. Determine the internal resistance Rz of your 1N753 zener diode from your
data by taking the change in zener voltage, Vz, divided by the corresponding
change in current, delta Iz. Do this calculation only on the straightline
breakdown region of your diode curve that you plotted in Step 2. Record your
result for the internal zener resistance in Table 71B.
4. Now wire the circuit shown in the schematic diagram of Figure 71B.
5. Apply dc voltage (Vin) to the breadboard. Measure the source
current (Is), zener current (Iz), load current (IL), and fullload output
voltage VFL, recording your values. Using e zener voltage and the
internal zener resistance calculated in Steps 2 and 3, compare the measured
output voltage with the expected value (Equation 3).
6. Now disconnect the 220ohm load resistor. Measure the source current
(Is), zener current (Iz), and output voltage with no load VNL, recording
your values. Using the zener voltage and the internal zener resistance determined
in Steps 2 and 3, compare the measured noload output voltage with the expected
value.
7. For this circuit, determine the percent load regulation, and record your
result.
8. Now add a signal generator in series with the dc voltage source as shown
in Figure 7—1C. Adjust the output of the signal generator at 0.5 V peaktopeak
with a frequency of 1 kHz.
9. With your oscilloscope at point A, observe both the dc and the ac voltage
levels, using your oscilloscope set on dc coupling. You should see a 0.5
V peaktopeak sine wave sine on a 15V dc level above ground.
10. With your oscilloscope at point B, measure the dc output voltage of
the zener diode regulator, recording your value. At this point you should
see virtually no ripple voltage on the regulator’s output signal. How does
this voltage compare with that measured in Step 5?
11. Now set your oscilloscope to ac coupling and increase its sensitivity
to 5 mV/div. You should now observe a sine ripple signal, but now much smaller
than the 500mV input ripple voltage. Measure the output peaktopeak ripple
voltage and compare it with the expected value; record your results.
Notice that the zener diode regulator provides a relatively constant output
voltage as long as the input voltage is greater than the zener’s knee voltage.
If there is any voltage variation or ripple on the input voltage signal,
the output remains essentially constant.
WHAT YOU HAVE DONE
This experiment demonstrated the characteristics of a 6.2V zener diode.
The zener diode is normally reverse biased so that it maintains a constant
voltage between its anode and cathode terminals over a specified range of
current. This experiment demonstrated the concept of voltage regulation where
the output voltage remained essentially constant with changes in load current.
