Here is a handy zener diode
tester which tests zener diodes with breakdown voltages extending up to 120
volts. The main advantage of this circuit is that it works with a voltage as
low as 6V DC and consumes less than 8 mA current. The circuit can be fitted
in a 9V battery box. Two-third of the box may be used for four 1.5V batteries
and the remaining one-third is sufficient for accommodating this circuit. In
this circuit a commonly available transformer with 230V AC primary to 9-0-9V,
500mA secondary is used in reverse to achieve higher AC voltage across 230V
AC terminals. Transistor T1 (BC547) is configured as an oscillator and driver
to obtain required AC voltage across transformer’s 230V AC terminals. This AC
voltage is converted to DC by diode D1 and filter capacitor C2 and is used to
test the zener diodes. R3 is used as a seri- es current limiting resistor. After
assembling the circuit, check DC voltage across points A and B without connecting
any zener diode. Now switch on S1. The DC voltage across A-B should vary from
10V to 120V by adjusting potmeter VR1 (10k). If every thing is all right, the
circuit is ready for use. For testing a zener diode of unknown value, connect
it across points A and B with cathode towards A. Adjust potmeter VR1 so as to
obtain the maximum DC voltage across A and B. Note down this zener value corresponding
to DC voltage reading on the digital multimeter. When testing zener diode of
value less than 3.3V, the meter shows less voltage instead of the actual zener
value. However, correct reading is obtained for zener diodes of value above
5.8V with a tolerance of ± 10per cent. In case zener diode shorts, the
multimeter shows 0 volts
