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I want to use this automotive relay to switch 60VAC, the current max. is 10A. But all its parameters are for DC usage. Can I use it safely to switch AC? It states its "Max. switching voltage" is 50VDC, then what about AC? How can I infer the AC parameters?

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  • \$\begingroup\$ I'd like to see someone answer this. I know from experience that AC relays can usually only handle 1/4 to 1/8 of their AC rating as DC, but I've never found a good explanation with all the factors that go into calculating that. The cop out answer would be to not do this. There are DC and AC relays that just plain won't work with the wrong voltage type without modification. Usually best to use AC switch for AC. I don't know enough to describe the relationship though \$\endgroup\$ – I. Wolfe Mar 13 '15 at 4:20
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Given that AC voltage is usually measured in RMS, that means the peak voltage you expect to see on your AC line is 60 * sqrt(2) ~= 84.85 V(peak). This is quite a bit higher than the rated 50V(dc) (peak is the same as given voltage for DC), and you could possibly damage the device and the rest of your circuit.

EDIT: I had a another look at the data sheet, and the issue you may run into is that if you select one of the options with a diode (called a freewheel diode, used to prevent reverse voltage spikes when the contacts are opened), it will cause issues as the diode will conduct when the AC voltage goes negative (even with the contacts open).

Also, I'm assuming you are using a DC control current with a AC load current

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    \$\begingroup\$ It's not as simple as that. For AC relays the DC voltage rating is usually much lower than AC, I've seen up to 8 times lower. The main issue using DC relay with AC is the contacts might not be spaced enough and you can see arcing. It's that the relay isn't made for AC so it can't handle it. So you're right in that AC can damage a DC relay, but your reasoning is not quite correct. \$\endgroup\$ – I. Wolfe Mar 13 '15 at 4:16
  • \$\begingroup\$ Can you provide a data sheet with differing AC and DC voltage rating? The possiblilty of extra arcing with AC could be due to capaictance, which could be a problem depending on what frequency the AC is operating at. \$\endgroup\$ – Joel Gibson Mar 14 '15 at 6:16
  • \$\begingroup\$ @I.Wolfe: I think you have it backwards, in that the spacing between the open AC rated contacts of a relay is usually very much smaller than that for equally rated DC contacts. The reason is that the arc which is formed when the contacts open, under load, will self-quench at an AC zero-crossing but - for the same contact spacing - will persist for DC since there is no zero-crossing. \$\endgroup\$ – EM Fields Mar 14 '15 at 6:53
  • \$\begingroup\$ @EMFields - yeah you are right. I got that backwards. Joel's edit solves my issue with his answer anyway. \$\endgroup\$ – I. Wolfe Mar 14 '15 at 15:22
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    \$\begingroup\$ @JoelGibson: For the data sheet with different AC and DC votlage rating, please refer to this: www3.panasonic.biz/ac/e_download/control/relay/safety/catalog/…, it's Panasonic's SF series. It's rated 250V AC but 30V DC. I'm not sure if it's only applied to this type of relay. But the DC rating is indeed less than 1/8 of the AC rating. \$\endgroup\$ – diverger Apr 9 '15 at 4:37
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I've used them in a pinch in lighting scenarios and never had trouble. Lasted longer than my solid state relay for a tiny fraction of the price. It's the size of the load I worry about. I've never went over than a few amps.

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  • \$\begingroup\$ Please be more specific which relay you used and what was the rating, and how much load. \$\endgroup\$ – AKR Oct 30 '17 at 6:32

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