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I am trying to figure out in this circuit, which lets the Triac 170 to conduct normally, which prevents an engine from firing, how to use an NPN transistor to replace the seat switch 154, but I assume there is AC voltage on the 112a line, and I know that AC would not play nicely with the transistor.

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    \$\begingroup\$ As an aside you can get opto-coupled triacs, like this farnell.com/datasheets/301912.pdf \$\endgroup\$
    – sstobbe
    Commented Jun 23, 2017 at 0:40
  • \$\begingroup\$ An opto triac would replace 170? My experience was I could not get them to be normally conducting. \$\endgroup\$
    – klcjr89
    Commented Jun 23, 2017 at 2:13
  • \$\begingroup\$ Yes, that's correct, so you could clamp the 112a node with an opto-triac or replace the whole circuit. As I presume you now have a go/nogo digital signal. \$\endgroup\$
    – sstobbe
    Commented Jun 23, 2017 at 2:44
  • \$\begingroup\$ I am a bit confused on what would power the led side then since it's supposed to default to grounding the magneto out with the control circuitry being off. \$\endgroup\$
    – klcjr89
    Commented Jun 23, 2017 at 2:49
  • \$\begingroup\$ Would this replace the entire circuit? Supposedly it's normally closed: mouser.com/ds/2/315/semi_eng_pwr1b_aqz40-1075815.pdf \$\endgroup\$
    – klcjr89
    Commented Jun 23, 2017 at 2:56

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You could use the transistor to switch a small relay. Or use a MOSFET-output SSR. Both those solutions also avoid any issues with where the grounding is.

As this is apparently lifted from a patent for a safety device - if your application is similar be sure to undertake a proper engineering review of the safety aspects. Keep in mind that semiconductors most typically fail on (but can also fail off or the connections can go open).

Edit: I am suggesting you replace the switch with the SSR- 400V or 600V units can be found, such as the TLP797J. You have to confirm that the voltage rating is adequate. This particular one can switch 100mA.

In the case of the relay you can choose whether to use a normally open or normally closed contact of a sealed SPDT relay, and replace the entire circuit. I assume they didn't do that in the patent for some reason (perhaps just the wiring) and I would want to figure out exactly why.

Again, the safety aspects need to be examined carefully at a system level as well.

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  • \$\begingroup\$ Can you elaborate more? Would it replace the entire circuit, or where would it be placed exactly. (Not looking for relay based solutions, only solid state) \$\endgroup\$
    – klcjr89
    Commented Jun 23, 2017 at 2:14
  • \$\begingroup\$ Comment on your last edit: a 100mA rating won't be enough as I just measured the current. \$\endgroup\$
    – klcjr89
    Commented Jun 23, 2017 at 3:57
  • \$\begingroup\$ 100mA should be enough to shunt out the gate of the triac with 22K in series. It's not enough to replace the triac. \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Jun 23, 2017 at 3:58

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