Photo-zener diode tester-with 6V zener diode

This is a circuit to test zener diodes. It hooks up to a 120V AC line and boosts the output voltage to over 300V allowing you to test zener diodes of any voltage. The circuit also has a push button switch to triple the current, if needed. The output is attached to 2 pairs of alligator clips. One pair is to attach to the diode. The second pair is to attach to a multimeter. The voltage across the diode is show on you multimeter.


It should be noted that this circuit could be dangerous. It is tied directly to an AC line, and has an output of 300V. The output has resistors on each line, which limits the current making it a bit safer, but will still give you quite a shock.

It should be switched off and light should be out while attaching and removing the zener diode



Circuit-schematic-zener diode tester

Photo-zener diode tester-internal circuit


to be added

How to use

Photo-zener diode tester-with 6V zener diode
Testing a zener diode
  • Turn off the power, make sure the light is out
  • Attach the multimeter, positive to positive (red), negative to negative (green)
  • Set multimeter to DC voltage, at the range you expect the diode to be
  • Attach the zener diode, positive to positive (red), negative to negative (green)
  • Turn on the tester
  • Read the output on the multimeter, and that's the zener diodes reverse voltage
    • If the voltage is less than 1V you probably have diode hooked up backwards, or its not a zener diode
    • If the voltage is not relatively constant, the zener diode is bad, or its not a zener diode
  • If you need to test it with more current, press the button, and the current will be tripled.
    • If you are testing high voltage zener diodes , you will probably need to use this
    • If you are testing lower voltage zener diodes, the voltage should only change a little.
Zener current verses zener voltage
  • Normal current mode: Iz = (320 - Vz) / (R4 + R5)
  • Triple current mode: Iz = (320 - Vz) / (R4)
Remove the diode
  • Turn off the power
  • Wait until the light turns off
  • Remove the diode
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