Automotive switch ratings?

[glennpm]

I had a running and starting issue this week so I'm replacing my Petronix II with a Bob Drake rebuilt 1942 crab distributor on my 59AB. I'm planning to add a ceramic 1.5 ohm inline with the coil's primary resistance of 2.3 ohm. I have a 12v negative ground wiring system.


Running resistance = 2.3 + 1.5 = 3.8 ohm


Ohm's Law I = V/R


I = ~12v/3.8 = ~3.3 amps


For starting I want to add a push button momentary switch so the coil will see about 5.5 amps when the ceramic 1.5a resistor is bypassed.


My question is the rating marking on a switch I have with these rating values acceptable?

6a 125 VAC
3a 250 VAC
3/4a 125v

What is the 12 volt rating. I need 5.5 to 6 amps?

Please teach this ME with EE knowledge :-)

Glenn

[tubman]

Watts (VoltsXAmps) is a rating of total power, which would determine if the switch could handle the load.

So we have :

12X6=72 Watts required

6X125V=750 Watts
3X250V=750 Watts
3/4X125=93.75 Watts

All three should be more than adequate; it all depends on the safety margin you want. You may want to recalculate using typical battery voltage of 13.6 Volts.

[glennpm]

Thanks Tubman


I used a lower voltage due to the drop when I'm starting up with the foot mounted starter switch, no solenoid.


Even so 13.6v X 6a = 81.6 watts


a question of the ratings, when the switch marking shows 3/4a 125v, is that the DC rating?

[tubman]

I don't think current type matters, since it's not given in the rating. "Watts is watts", so it shouldn't be relevant. There may be some esoteric difference with the way the contacts handle the transfer of power, but that's above my paygrade. One more thing. These ratings are for continuous use. Since you will be using it momentarily, you are on solid ground no matter what you use.

[glennpm]

Thank you!

[Gene1949]

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get yourself a56 and later starter solenoid. It will have a tap to bypass the resistor. .