Here is a very simple circuit that can be used for testing Zener diodes. The IC1 NE555 is wired as an astable multivibrator and the output of the IC is stepped up to a high voltage AC using the transformer T1.The unloaded AC voltage will be around 120V. The AC voltage is rectified by the diode D1 and filtered by the capacitor C3.This voltage is applied across the Zener diode through the current limiting resistors R2 or R3 which can be selected by switch S2 for testing the diode at 1mA or 2mA respectively. The digital multimeter in DC voltage mode is connected across the Zener diode under test. The Zener diode is a fine one; the meter will show the correct Zener voltage specified by the manufacturer.

Circuit diagram with Parts list.



  • The circuit can be assembled on a Vero board.
  • Use a 9V PP3 battery for powering the circuit.
  • The capacitor C3 must be rated at least 150V.
  • Use a holder for mounting IC1.
  • T1 can be a 110V primary , 9V secondary step down transformer connected in reverse.


  1. Seetharaman

    Hi Rajesh Primary 110V AC secondary 0-9 volt @ 100mA. Hope with you can get the transformer.
    Hi Joomun you can use a 9-0-9 100mA transformer with 230 primary but ensure that you use end 9 and 9 volt not the centre tap ‘0’ volt. This will not give 100% performance like the one specified but will be sufficient.


    plese provide more information about transformer. thank u.


    Can T1 be replaced by 230V Primary and 9V secondary? Our country electricity supply is 230V 50 Hz. It is very difficult to find a 110V Primary transfo.

  4. please tell me the rating and the no. of turns of transformer used in zener diode tester.if not thn wat to tell about transformer while buying it??thnks a lot in advance.

    • I have added it in the Notes section of the page.
      T1 is 110V primary, 9V secondary step down transformer connected in reverse.