I've been trying to find a device that will ground out a magneto's coil (primary kill wire) in a normally closed state without using a mechanical relay because I want my circuit to be all solid state.

I began to search for normally closed solid state relays, and thought I found one that would work: Vishay LH1501AB This works fine for keeping a 120V light bulb on until I apply voltage to pins 1 and 2 (LED) of this SSR. Now I tried to do the same on an internal combustion engine, except I put the magneto's grounding wire to pin 6 and then pin 4 went to the block of the engine. No matter what, spinning over the flywheel by hand, the spark plug would still spark (I don't want it to until I apply voltage to the SSR's LED).

I believe this has something to do with the contacts of pin 4 and pin 6 aren't really closed, because there isn't a voltage applied to the SSR's FETs, whereas with my lightbulb experiment, the voltage was present.

I'd appreciate any help to get this working using a normally closed SSR and not a mechanical relay, or any other solutions you may have; I couldn't find a normally closed triac.

Overview pic of what I'm trying to do:

enter image description here

  • 2
    \$\begingroup\$ What is the voltage coming off of the magneto? It may be too high for the SSR. \$\endgroup\$ – Enemy Of the State Machine Nov 1 '14 at 5:31
  • \$\begingroup\$ The On-resistance of that SSR can be as high as 25 ohms. Do you know what the highest resistance is that will work to kill the spark? \$\endgroup\$ – brhans Nov 1 '14 at 12:23
  • \$\begingroup\$ ~170 volts on primary, so should be fine. \$\endgroup\$ – klcjr89 Nov 1 '14 at 15:20
  • 2
    \$\begingroup\$ Frankly, I would seriously reconsider using a plain old relay. You haven't mentioned why it's not acceptable. \$\endgroup\$ – Laszlo Valko Nov 3 '14 at 4:09
  • \$\begingroup\$ It's not acceptable because I want an all solid state circuit. \$\endgroup\$ – klcjr89 Nov 3 '14 at 4:11

Once the SSR has been in the magneto circuit does it still work with the light bulb? Or have you fried the SSR? (you could just measure on resistance at low voltage.)

As Enemy of SM hinted. I suspect the high voltage from the magneto has killed your SSR. There are likely to be kilo-volts on the line.

  • \$\begingroup\$ I tried moving the wire from pin 4 in my diagram picture to pin 5 and tried that, but that ruined the SSR. \$\endgroup\$ – klcjr89 Nov 1 '14 at 15:24

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