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I made a rev limiter by using a 5V SPDT relay on motorcycle between the CDI and ignition coil. The relay works fine. However, the NC contact tip wears out very fast.

I added a 1N4007 at NC and COM to protect the contact pin, but the voltage still passing through from NC to COM although the COM already switch to NO pin. So this method is not working.

Am thinking to add an RC snubber to the relay, but wondering whether it will have the same problem as above.

Should I replace the mechanical relay with an SSR instead?

This is the relay that I'm using now:

enter image description here

Edit:

This is my current schematic.

enter image description here

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The CDI module puts out pulsed DC, in the few hundred volts range, but nowhere near 10A; if you can switch the INPUT power to that CDI module instead of the output, you'd be switching higher current (maybe several amps) but lower voltages. The relay is rated for that higher current at low voltages.

Much of the contact 'wear' is in the high voltage causing a brief, hot, arc.

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  • \$\begingroup\$ Hi @Whit3rd, thanks for the reply! Will it "kill" my CDI if switching the input power to CDI? Sorry, may I know the output voltage range of ECU to ignition coil? I tried google but cannot find the answer. \$\endgroup\$ – Flamethrower Jul 15 '20 at 10:44
  • \$\begingroup\$ @Flamethrower My old CD ignition unit (Heathkit, similar to Mark Ten) used about 400VDC, with a conventional sparkplug (about 0.035inch gap) and spark coil, circa 1980. Modern designs vary, but all are switched to stop the engine. \$\endgroup\$ – Whit3rd Jul 15 '20 at 21:48
  • \$\begingroup\$ Hi @Whit3rd! Thanks for sharing! I will give it a try later! Thank you! :) \$\endgroup\$ – Flamethrower Jul 16 '20 at 4:44
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Where you add the diode matters and it should normally be in parallel with the inductance that's producing the arc in the relay, not the NC-COM relay contact. The problem is since your circuit is designed to make a spark that can jump across an air gap so adding any snubber, either diode or RC, in the proper place to suppress that spark across your relay could also suppress same ignition spark. Even then, a 1N4007 is too slow. Silver cadmium oxide contacts help and are more resistant to erosion.

You might want to look at a thyristor which is unable to interrupt the spark while it is in progress (so none of the harshness invovled with interrupting something mid-arc), but will block any future sparks. This is because a thyristor actually latches on when triggered and can't actually interrupt current itself. However, if the current falls below a certain level (ideally zero) due to the external circuit while the trigger is removed then thyristor will turn off and block future current from flowing

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  • \$\begingroup\$ Hi @DKNguyen, Thanks for the reply! I have just added my current schematic to my post. I'm not familiar with Thyristor, but is it wire like this? link \$\endgroup\$ – Flamethrower Jul 15 '20 at 7:14
  • \$\begingroup\$ @Flamethrower That's one way. \$\endgroup\$ – DKNguyen Jul 16 '20 at 18:25
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Relay damage may be avoided by using it to switch a signal instead of power.

enter image description here

The NC contact of the Rev Limiter relay is to be used to disconnect the CDI thyristor gate, when the RPM limit is crossed.

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  • \$\begingroup\$ Hi @vu2nan, thanks for the schematic! Since the V and A is much lower than CDI output, do you think there's still a need to add RC snubber to the relay? \$\endgroup\$ – Flamethrower Jul 16 '20 at 2:30
  • \$\begingroup\$ Anytime, Flamethrower! No snubber would be required across the relay contact. \$\endgroup\$ – vu2nan Jul 16 '20 at 4:31
  • \$\begingroup\$ Okay! Thanks @vu2nan! I will update this thread again! :) \$\endgroup\$ – Flamethrower Jul 16 '20 at 4:48

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