This question has come up for me before (not here) and I've never gotten a satisfactory answer. If you've noticed my other post here, I'm trying to build something and part of the fun is seeing how much I can do on a budget.

I need to control a few 120vac components with a 24vdc switch. I found a 24vdc 4PST OMRON (four NO contacts rated 20amps, 120vac) mechanical relay for seven bucks @ surplus. This should do me fine. I'll just order it along with some other parts today from surpluscenter.com.

What I'm wondering is this. I have a junk drawer full of assorted relays, many of them are solid state 12vdc automotive types with contact ratings on the load side of 30plus amps.

Wouldn't such an amperage rating cover any range of AC/DC .001v to 250v without a sweat as long as it's kept at say ten amps or less?

And would 24vdc actually harm a 12vdc solid state relay on the control side? I'm guessing that automotive stuff is probably very conservatively rated. I know I can just go ahead and experiment, but I'm hoping I might get a little real insight here. Thanks so much.

 

For "lower" voltages, the rating of electromechanical relays is determined by how much current the contacts can carry. For "higher" voltages, the rating is determined by how much current they can interrupt or close on. At higher voltages, the main problem is arcing across the contacts. The relay must be able to break this arc on opening, and the arcing itself is a major cause of wear.

The terms "lower" and "higher" above are relative terms. The voltage at which arcing becomes a significant factor tends to be rather poorly defined. Relay manufacturers target their products at particular markets, so they do their product testing at specific voltages. Generally, at 12 volts arcing is not usually a major problem, while at 120 volts it is. However, if you have an inductive load then even at 12 volts you can get significant arcing.

DC circuits tend to arc more than AC circuits, as AC has natural zero voltage crossings twice per cycle which help to shut off the arc.

For solid state relays (SSR), the transistor (or SCR, etc.) circuits in the relays are designed to work up to specific voltages. If you exceed that voltage, you might damage (and short out) the SSR. Also, if you try to use an AC SSR in a DC application, you may find that it won't shut off. Many AC SSRs depend upon the voltage reversing to shut off the relay.

At low voltages, *minimum* current rating can be important for electromechanical relays. Many relays depend upon some arcing or contact burning to clear away the natural oxide layer which forms on the contacts. There are special low current relays for these applications.

As for automotive components being conservatively rated, I find that idea rather humourous. If anything, the ratings are rather optimistic. There is very intense cost pressure on automotive suppliers from the auto makers, so they cut every little bit they can from their designs (and then often cut a bit more). An industrial relay will typically last much longer (often an order of magnitude or more) than an automotive relay of the same rating. It will also be much larger and of course cost much more money.

 

Thank you very much for that. It pretty much covers what I was curious about.