Reverse Characteristic of a Zener diode

Reset voltage to  zero.  Connect  zener diode as shown in fig. and reverse characteristics are demonstrated in fig.

Raise voltage slowly and observe the current meter. Set the value of zener current  of  1  mA,  2  mA,  5  mA,  10  mA,  20  mA,  30  mA  and  record  corresponding voltage drop across the diode and plot the points on graph.

Graph should show that very little current flows until the voltage has risen to a value  near  nominal  voltage  (5.1  V)  of  the  zener diode.   Though  a  very  slight increase in voltage causes a large current to flow. Once an appreciable current is flowing in diode, any alteration in that current would make little difference to the voltage. Terminals to which voltmeter is connected will look like a source of voltage having a low source  resistance.   This  voltage  may  be  referred  to as  the  'zener  voltage'  or  avalanche  voltage,  (this  being  a  reference  to  physical  process  inside  semiconductor).