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