DC relay on AC?

DC is more problematic to switch than AC and a switch or relay designed for AC must be derated (often massively) to use with DC.

But what about the reverse?

I have an automotive type relay rated 40A at 14V.

That should handle 10A (actually less) at 120VAC okay, yes?

Consequence of either failure to open or failure to close is near zero (maybe a little wasted electricity). But I don't want it burning up either. I would think something with a 40A DC rating is, if anything, overkill. But the voltage is higher. However, I cannot imagine that the contacts are left close enough together that the 120V (170Vpp) is an issue as they'd be more likely to draw out and sustain an arc when used on the DC it was designed for. So I think I am good.

Reply to
Chet Kincaid
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** As far as the contacts are concerned, you are probably correct.

But relays have another job which is to safely isolate the coil connections from the switched circuit.

There is no guarantee that a relay specified for use in an automotive, 12V DC environment is suitably insulated for connection to the AC supply - where spike voltages in the thousands occur.

.... Phil

Reply to
Phil Allison

People who do not know that should not be able to get a screwdriver license or a permit for pliers. Scissors I am not so sure. If they have them they can grab their Darwin award but it is pretty hard to hurt anyone else. So I guess let them have them.

Reply to
jurb6006

Do you mean the specs that are printed on parts - like relays - should be respected?

What an odd idea!

John ;-#)#

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Reply to
John Robertson

I am old and have funny notions like that.

But I do have to admit to putting foil on the fuses in my amp so it could drive 2.3 ohm loads. One eight ohm and one four ohm per channel in parallel. Actually, never blew the outputs, it didn't even get that hot. Sansui 771.

Reply to
jurb6006

Phil Allison wrote in

An excellent point however in this instance the controlling circuit was to be another 120V circuit with the only reason for 12V being involved at all was the availability of the relay. I would have just used some old 12V charger from the junk box. So if hypothetically some big spike had come down the switched line, jumped over to the coil circuit, and blown through the P/S it would just have ended up on another 120V circuit which is just as prepared or not prepared for that as any other. Likewise spikes coming the other way.

However, it's a moot issue anyway as I realized I need NC contacts while the 12V relay is SPST NO. Most likely I will dig up a DPDT or 4PDT and parallel the poles for added ampacity. My "wtf was I thinking" is all about forgetting I need NC not about why I thought the 12V relay might be suitable.

Reply to
Chet Kincaid

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