3 zener diode tester using 555 timer IC

Sometimes we cant read the markings of the zener diode  because its either too small or its erased. When this happens,we got some problem in determining what is  the "breakdown voltage" or zener voltage of the diode. This simple circuit is designed to test zener diode to determine its voltage.
zener tester schematic
555 IC pins
Part List:
R1 - 27 kilo-ohms 1/4W
R2 - 1.2 kilo-ohms 1/4W
R3 - 1.2 kilo-ohms 1/4W
R4 - 120 kilo-ohms 1/4W
C1 - 100nF
C2 - 100nF
C3 - 470nF rated 100V or higher
D1 - 1N4007
L1 -30 to 40 turns of #22AWG magnetic wire in ferrite rod
Q1 - TIP31C, TIP41C or any NPN with min 5W power capacity
DC - 6V  to 9V battery
Voltmeter - as testing accessory

How the circuit works:
The 555 time IC deliver pulses at a frequency of around 490Hz at 95% duty cycle. When this voltage pulses (around 6V) is injected in the base of transistor, it will be converted to higher voltage (around 70V).
How to use:
Just connect the zener diode at the circuit's output as shown in the diagram above. Make sure you follow the zener diode polarity to avoid damage. Turn on the circuit and using your voltmeter, test what is the voltage across the diode. The measured voltage is the value of zener voltage.

Note: You can add 10k resistor in series to the output so you can protect you zener from damage when accidentally connected in reverse. This will help you also determine the polarity of zener in case the marking is erased.


  1. hi guys cool project,

    I would do this project also. Can someone name the coil in Henry unit.

    Thanks a lot and sorry for my bad English.

    Best regards

  2. Oh I forgot to say a big please.


  3. Hi very good circuit but I would like too build it too. But Idon´t now the value of the L1!!

    Please post the value of L1!!!

    Thank you !!