Is it safe to run AC power through an automotive relay?

I want to use a microcontroller to trigger an automotive relay to control AC power. Is this a safe thing to do?

Are Automotive relays designed to handle DC power only?


I want to use an Arduino to control a fog machine and need it ready by Saturday. Maybe you guys can recommend something I could find at a regular store like RadioShack.

5 Answers

  • 9 years ago
    Favorite Answer

    Using an Arduino system to control the relay coil suggests a DC output to the relay coil. As long as this is at the 12 Volt level, the coil will work to close or open the relay contacts. If you are supplying an AC signal to the operating coil, the coil may or may not operate the relay because the coil will present a different impedance to the AC system that may be so high that the current necessary to create enough magnetic force to close the coil will be to small.

    I am assuming that you want to use the relay to open/close an AC circuit. If this is the case, the contacts do not care if they are used in a DC or AC circuit as long as they voltage is not so high that the relay flashes over (shorts to ground). If the voltage applied exceeds the relays insulation level it may short out. Second, most relay contacts have a current rating that you must not exceed as this will cause the contacts to burn open or melt.

    Hope this helps,


  • mark p
    Lv 5
    9 years ago

    It would probably be safe. AC is actually much easier for a relay to handle than DC is. In any case, if the relay can't handle it, what will happen is that it won't manage to turn off (it will arc across the contacts when it tries to turn off) I imagine that the fog machine draws relatively little current.

    This is of course, for the contacts. For the coil you would need to use some DC voltage (presumably 12V since this is an automotive relay). It is more important to make sure that the Arduino can drive the relay coil and not get fried; when you try to turn off the coil it "kicks" and can develop a high voltage; you want to make sure there is a snubber diode across the relay coil, and that the Arduino is protected and can handle a little kickback.

  • 9 years ago

    No, not safe. the relays are usually rated for DC only. But read the ratings on the relay.

    Why would you use an auto relay, there are plenty of relays around that have 12 VDC or 6 VDC coils and can handle AC power.

  • 9 years ago

    You've failed to mention what the AC voltage is.

    If we assume 12VAC, provided by some step-down transformer, then I'd say *maybe*, with qualification.

    If we assume 120VAC, directly, then certainly not across the coil. Possibly across the switch contacts, but you'd need to verify this with the datasheet for the specific coil.

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  • 9 years ago

    Absolutely safe to use it to control AC power but keep in mind,never excess its voltage rate.

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