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Zener diode
Aim :- To study the Volt- ampere (V-I) characteristics of a Zener diode, both in forward
bias and reverse bias. And also to find out the following physical quantities.
1. knee voltage or cut-in voltage or threshold voltage ( Vo ) in forward bias.
2. Dynamic or ac forward resistance (RZ).
3. The Zener break down voltage in reverse bias (Vz)
Apparatus :- Zener diode, battery (0-10 V), rheostat, plug key, dc voltmeter (0-10V),
milli-ammeter (0-100mA) and micro-ammeter (0-500µA).
Formula :- Dynamic or ac forward resistance RZ =
Where
∆V f
∆I f
∆Vf = Change in the forward voltage (V)
Ω
= AC
∆ If = change in the forward current (mA) = DF
Theory :- The Zener diode is an ordinary P-N junction diode ( generally Silicon is
preferable), except that it is properly doped to have a very sharp and almost vertical break
down. This is exclusively operated under reverse bias conditions and designed to operate
in break down region with out damage. As this is a heavily doped diode, the depletion
region is very narrow (150-200Ao). When the reverse bias across the diode is increased,
the electric field becomes intense ( in the order of 108 V/m) and all the covalent bonds
break at a time, (minority charge carriers come from valance band to conduction bond )
which is called Zener break down. Then a large number of
electron- hole pairs
production occur. This results a sharp increase in the reverse current.
Procedure :- In the forward bias the battery positive terminal is connected to p-region
and negative terminal is connected to N-region. The connections are made as shown in
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the figure-1. The voltmeter is connected across the Zener diode to measure the voltage
applied, the milli-ammeter is connected in series to the diode, to measure the current
flowing through the diode. The voltage applied is increased in steps of 0.1 V , starting
from zero, the corresponding current from the milli-ammeter is noted in the table-1.
Graph-1 :- A graph is drawn, in the 1st quadrant, by taking voltage on X-axis and
current on Y-axis. From the graph the threshold voltage, Vo is noted. The values ∆Vf and
∆If are noted as shown in the graph, to measure the dynamic resistance, Rz of the Zener
diode.
In the reverse bias the battery positive terminal is connected to N-region and negative
terminal is connected to P-region. The connections are made as shown in the figure-2.
The voltmeter is connected across the diode to measure the voltage applied, the microammeter is connected in series to the diode,,to measure the current flowing through the
diode. The voltage applied is increased in steps of 1 V, starting from zero, until the Zener
break down occurs, the corresponding current from the micro-ammeter is noted in the
table-2.
Graph-2 :- A graph is drawn, in the 3rd quadrant, by taking voltage on negative X-axis
and current on negative Y-axis. Here, in the 3rd quadrant, deferent scale may be taken for
convenience. From the graph the Zener break down voltage, Vz is noted.
Precautions :- 1. Check the polarities of Zener diode before making connections.
2. The connections should be tight.
3.The power supply should be ‘on’ only when the observations are taken.
Results :-
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Table – 2
Table –1
Forward bias
Reverse bias
S.No. Applied
Current
Voltage
Through
Vf (V)
diode
S.No. Applied
Current
Voltage
Through
Vr (V)
diode
Ir (µA)
I f (mA)
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
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