This zener diode tester can be used to check zener diodes of V to 18V. The tester can also identify the polarity of zener diodes. The tester described in this article is a simple two-transistor circuit operating from a 9V battery which tests zener diodes with breakdown voltages up to 52 volts. This circuit will test zener diodes up to 56v. See Talking Electronics website, left index, Transistor Circuits (circuits ) and go to Zener Diode (making) to.


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Output Current R4 and R5 are relatively large, and limit the output current to a small amount.


With a low voltage diode the max current is about 4mA. If you press the push-button the current is tripled.

Now, the current through T1-L1 and R6 begins to ramp up, zener diode tester magnetic energy in the core. When the voltage drop across R6 rises above 0.

This action reduces the voltage across T1-L1, which correspondingly drops the voltage across the feedback winding, T1-L4, further reducing base drive to Q1 and shutting Zener diode tester off rapidly through this regenerative action. At the same time, the secondary voltage reverses and D3 starts to conduct.

Zener Diode Tester

When the stored energy in the core is fully released through the secondary, the voltages on all windings collapse, turning on Q1 again via C1 the voltage at pin 11 goes from a negative voltage toward ground. Then the cycle repeats until C2 is charged to a voltage level where the DUT begins to conduct, at which point the oscillation stabilizes and continues to feed power into the DUT.

Steady-state oscillation waveforms are diagrammed in Figure 2. Timing diagram showing transformer voltages. Voltage levels referenced to ground are shown for zener diode tester generalized zener diode tester diode voltage, Vz, under test.

The voltages shown in parentheses are for a 12V zener diode tester diode as the DUT, and corresponding actual circuit waveforms are shown in Figure 3. Oscilloscope capture of actual circuit testing a 12V zener.


If no Zener diode tester is present when the circuit is operating, then the voltage zener diode tester C4 will continue to rise, as will the peak voltage at the collector of Q1. The voltage at pin 2 of T1 and the output voltage would keep rising with each cycle, as would the peak voltage half the output voltage plus 9V at the collector of Q1.

This is a feature of a flyback circuit configuration, which allows zener diodes to be tested well above the battery voltage of 9V. Figure 4 shows actual measurements of current and power for various DUTs on the tester as constructed. Measurement of these diodes with a direct current supply and the same currents produced identical results, so measurement accuracy is excellent.

Measured output power and current.

How to build a zener diode tester

The zener diode tester has virtual instrumentation that enables measurement of voltage, current, and power in every wire and component as zener diode tester function of time. Only components that strongly affect circuit behavior need to be modeled. This file is available at the article link.

A simulation screenshot showing output voltage waveforms from the output of the secondary winding at connection to D1 is shown in Figure 6. LTspice simulation — transformer output waveform.


Components in the LTspice library were used, which differed in some cases from the actual components on zener diode tester schematic.

Simulated circuit behavior was very close to real circuit results and was particularly useful to optimize component values. WhatsApp Zener diode tester This zener diode tester can be used to check zener diodes of 3. The breakdown voltage of the unknown zener diode is indicated on the precalibrated dial of potmeter VR1.

The tester can also identify the polarity of zener diodes. When linear potmeter VR1 is adjusted such that the voltage at its wiper arm red crocodile clip exceeds the breakdown voltage of the zener diode, the zener diode conducts and applies the bias voltage zener diode tester the base of transistor T2, which causes red LED1 to light up.

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 2mA DC.