# Why Zener diode doesn't fire at specified voltage?

I have the following schematic, which would be used as a over-voltage and reverse polarity protection:

simulate this circuit – Schematic created using CircuitLab

Both D1 and D2 are rated at 15V+-5%, which should theoretically produce maximum breakdown voltage of 15.75V. Lab power supply is used as a voltage source. If I'm using jumper wire instead of fuse F1, then PSU just goes into current limiting mode, but crowbar part works as expected, but not at the desired voltage of 15V. I've tried it several times, both with and without load R3 (I'm using a heater as a load). The results are the same:

1. Voltage at output drops to zero when input exceeds 16.9V. If PSU is set to 16.9V, same voltage is at output. When I add 0.1V more, crowbar fires and output drops to zero.
2. I've measured voltage drop across D1, it exceeds 15V. If input is 16.9, then D1 voltage is about 16.3V, way too high compared to spec.
3. I'm using second DMM to measure D1 voltage. When input is getting closer to 16.9V, sometimes when I touch D1 with probes, output voltage drops to zero, but crowbar part does't fire (PSU shows 16.9V at the output).

Questions are: how to explain what I see, and how to tune crowbar to fire at exactly 15V? I think I could use 13V Zener, but I don't like acting without clear understanding of what's going on.

• Have you looked at the SCR trigger voltage between the gate and the cathode needed to fire the SCR?
– G36
Oct 24 '19 at 15:02
• Also, consider of using the TL431 instead of a Zener diode. See the example electro-tech-online.com/articles/…
– G36
Oct 24 '19 at 15:10
• the 100 ohm resistor may be way too low. Also, the Zener does NOT fire; there is no sharp turnon in a Zener. Oct 24 '19 at 15:21
• @G36 SCR is VS-25TTS08S-M, datasheet says Vt is 1.25V, if I'm reading it correctly. Still have no idea how to explain what I see Oct 24 '19 at 15:56
• @analogsystemsrf how can I calculate proper value? Oct 24 '19 at 15:58