Using a single 555 Timer IC and a small transformer to generate a high voltage, this circuit will test Zenerdiodes of voltage ratings up to 50 volts DC (VDC). The 555 timer is used in the astable circuit mode (see our calculator...), the output at pin 3 drives a small audio transformer such as the LT700. This has a primary impedance of 1K and a secondary impedance of 8 Ohms. Used in reverse the unloaded AC voltage is around 120 volts AC. This is rectified by the 1N4004 (or minimum equivalent, higher is OK, such as 1N4007) diode and smoothed (filtered) by the 2.2uF capacitor which MUST be rated at 150 VDC. The Zener under test is measured with a multimeter set to DC volts as shown. The load current switch enables the Zener to be tested at 1 or 2 mA DC, the rectified DC load. However, a good Zener should maintain the reading on the volt meter.
