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I have a simple 12V circuit with PIR, 555, couple of transistors and an outside 220mA LED light in a circuit powered from a 12V lead acid battery battery with long leads so am expecting LC spikes that could fry the lot.

I'm thinking of using a TVS diode to clamp the spikes but not sure how the reverse standoff voltage Vrwm, works with the clamp voltage Vc. They seem to work against each other.

e.g. a P6KE15A has Vrwm 12.8V which seems ideal but Vc 21.2V which may not be good for the circuit.

P6KE Series

From my understanding Vrwm should be the normal working voltage of the circuit, i.e. around 12V. However, if the circuit cannot sustain a 21.2V spike, it seems the diode won't protect it. It would fry the 555 for a start.

I'm expecting spikes of 35-40V from the long wires from the 12V lead acid battery and cheap, large mechanical button.

I can see that breakdown occurs at 14.3V for this diode but with a 40V spike the circuit will see the max Vc of 21.2V.

Or have I misunderstood how TVS diodes work?

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  • \$\begingroup\$ How long of leads? Why not an electrolytic capacitor? \$\endgroup\$
    – Tim Williams
    Aug 13 at 11:46
  • \$\begingroup\$ leads will be about 1 metre long. Same length of leads fried a switched mode voltage regulator from a reputable company. So I looked into RC spikes, TVS and filter caps. Good point about capacitor. I could use 330uF 35V one. Can't seem to find a 40V one in case there is a 40V spike. Could a lower rated TVS bring the level of the spike down to a value an electrolytic capacitor could handle? e.g. TVS in parallel with a cap which would be nearest the main circuit? \$\endgroup\$
    – codebrane
    Aug 13 at 13:58
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    \$\begingroup\$ To choose a TVS and be successful, you have to understand the nature and energy of the surge. So, concentrate on what the surge energy might be and how it manifests. \$\endgroup\$
    – Andy aka
    Aug 13 at 14:29
  • \$\begingroup\$ I had previously bought a voltage regulator board for a different project with max Vin of about 40V and 5V Vout. I wired it to a 12V battery with 6 inch long wires, 1.5mm I think and it fried instantly. The topic of LC spikes was mentioned and in this sitiuation I'm expecting switch bounce and perhaps 40V inductive spike on 1.5mm wires about 1m long. I have a very cheap switch mode voltage regulator connected to the 12V battery on about 5m long 1.5mm wires and it's fine, powering a 10W LED. So not really sure whether the original regulator was just garbage though it was pricey. \$\endgroup\$
    – codebrane
    Aug 13 at 14:36
  • \$\begingroup\$ Am I not understanding how the capacitor would work? Would it not just pass the spike on to the main circuit as it's in parallel and would charge up to the spike voltage? Whereas the TVS would clamp well below the spike voltage. \$\endgroup\$
    – codebrane
    Aug 13 at 14:39

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