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Basic relay question — Parallax Forums

Basic relay question

wolffwolff Posts: 43
edited 2006-05-06 08:12 in General Discussion
I have a relay that is rated for 30Amps @ 120VAC. (It opperates at 12VDC but the contacts are rated @30 amp 120vac)
I would like to use it to drive a 12 AMP 20VDC load. Would that work OK?
Thanks for any insight!!!

Post Edited (wolff) : 5/4/2006 9:53:17 PM GMT

Comments

  • GadgetmanGadgetman Posts: 2,436
    edited 2006-05-04 21:44
    The ratings are maximum ratings, so 12A @ 20V DC should work just fine.

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  • wolffwolff Posts: 43
    edited 2006-05-04 21:51
    What about the difference between AC and DC?
  • GadgetmanGadgetman Posts: 2,436
    edited 2006-05-04 21:56
    In a normal relay I doubt it matters.

    If it is a solid-state, on the other hand...

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  • MikeKMikeK Posts: 118
    edited 2006-05-04 22:24
    It matters. It also matters what type of load you're switching (inductive, resistive). Sorry I can't offer more details. A search on "relay contact ratings" yielded this info.
    Mike
  • stamptrolstamptrol Posts: 1,731
    edited 2006-05-04 23:42
    What MikeK is alluding to is that an AC arc on a contact that is opening will extinguish much more easily because the current is following the power supply sinewave and actually going out 60 times per second.

    A DC arc has to be extinguished simply by the speed and distance the contacts separate as they open. A resistive load arc ( light bulb, heater, etc) is very easy to extinguish. A highly inductive load ( coil, motor, etc) is hard to extinguish so it is common to derate contacts depending on the type of load.

    Having said all that, a 12A load on a 30A contact and at a lower than rated voltage will probably be OK. If your relay has multiple contacts, put two in series to increase the arc extinguishing capability.
  • BullwinkleBullwinkle Posts: 101
    edited 2006-05-06 08:12
    I used relays (albeit low spec) for my first prototype irrigation controller. Switching 24vac into inductive loads (solenoids). The kit lasted one season before the first relay blew. My choice was then to either move up to overspeced relays, like the ones you describe, or competely change the design. In the end I switched to optoisolators and triacs. They're cheaper than big bolshy relays but still overspeced enough to take the nasty transient voltage spikes involved with punching AC through an inductive load.

    At least, I haven't blown one up yet... tongue.gif
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