Dumb electrical question

[ebinary]

I have a high-power relay with a 36V activation coil. If I activate the coil with the battery itself, the coil will actually draw enough current to shut down a dewalt battery. However, if I add much resistance inline (to drop the current), the resistors get warm and the voltage is quickly dropped too far to activate the coil.

Is there a simple passive circuit that will only allow a maximum of, say, 0.5A to be available to the coil on the relay? I don't want to waste energy on the relay (plus, it will weld switches closed if it draws 33V x 60Amps).

 

[dirty_d]

you sure its 36V to activate the coil? i would think that they would use a really long thin wire for the coil so the resistance is enough to not need a resistor in series. do you have a datasheet for the relay?

 

[ebinary]

you sure its 36V to activate the coil? i would think that they would use a really long thin wire for the coil so the resistance is enough to not need a resistor in series. do you have a datasheet for the relay?

Yeah, you'd think. Its definately 36V (18V, for example, is what it is drawing with 25ohms in series, and that won't activate it). It will probably activate starting at about 20V to 21V.

Eric

 

[fechter]

I've seen a setup where the coil gets the full voltage to get it to pull in, then drops the current to save power. Something like your resistor with a large capacitor across it.

It almost sounds like the coil is just way oversized for the application (made for a golf car?).

 

[Link]

Yeah, that's a lot of current for just a coil. All of mine draw less than an amp.

If can get at the coil, you might want to try rewinding it with more turns.

 

[swbluto]

Voltage reduction sounds like it might help you but that would also reduce the current(to something below which it might need). But, if this "coil" relay is based on electromagnetism(which it is, I'm guessing), then the magnetic field is determined by the current and if that's the current it needs to operate, it looks like you're looking at getting a lower-rated-current coil like one that link has.

You could also use something to store energy for those current demands, like a capacitor. An inductor might also help, but it depends on how you implement it and inductors are usually more problematic/expensive than capacitors.

 

[ebinary]

I've seen a setup where the coil gets the full voltage to get it to pull in, then drops the current to save power. Something like your resistor with a large capacitor across it.

It almost sounds like the coil is just way oversized for the application (made for a golf car?).

It is probably a golf-cart relay - its from an old kango electric go-kart kit, and I think it can switch 200A.

I ordered a 40A 12-480VDC solid state relay to replace it.