Using AC rated switches on DC

Discussion in 'General Electronics Chat' started by electricnewb, Nov 10, 2013.

  1. electricnewb

    Thread Starter New Member

    Nov 9, 2013
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    I have a 12v DC load (probably more like 14v on a fully charged car battery) and my max circuit will be around 15 amps. Can I use an AC rated switch to power this load and if so what ratings do I need to be looking for on the switch? Specifically I am looking at AC rated wind up timer switches.
     
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    technically NO..

    DC is harder to switch and as such requires better contact design/materials,etc... Buy a dC rated switch..
     
  3. #12

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    Nov 30, 2010
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  4. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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    An AC only rated switch won't work very long as the contacts will arc badly when opened on a inductive load. At 15A even on a resistive load it would fail quickly. I would use the AC switch to power a 12vdc auto relay. A 30 to 40 amp relay is only a few dollars.

    [​IMG]
     
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  5. #12

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    That's a way! Use the timer to control a DC relay which uses only a little current for its coil.
     
  6. MaxHeadRoom

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    A good source of DC relays is if you have an auto wrecker near you, you can usually get a handful of relays for a couple of $.
    Look for the larger ones not the mini relay size, there are usually two sized used in a modern auto.
    The one near me you can remove the whole relay centre complete with fuses for $5.00!
    Max.
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    A good source of DC relays is EBay, where they come with sockets pre-wired with 6 inch colored wires. Dirt cheap if you have a few weeks for the free shipping to work.
     
  8. electricnewb

    Thread Starter New Member

    Nov 9, 2013
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    So if I understand correctly there will still be DC running through the switch that will in turn be switching the DC relay but the switch will be fine since he load being switched at that point will be very nominal (just the coil from the relay) Do I understand that correctly?
     
  9. MaxHeadRoom

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    Correct, the switch will just operate on a few Ma.
    Incidentally I don't think you mentioned if it was inductive or resistive final load, makes quite a bit of difference.
    Max.
     
  10. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Yes, an AC switch should be fine to control a few milliamperes of a relay coil.

    The relay is designed to switch high DC currents.
     
  11. electricnewb

    Thread Starter New Member

    Nov 9, 2013
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    It is a resistive load in this case (lights)

    Thanks all. The relay approach will work perfectly!
     
  12. MaxHeadRoom

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    Resistive load? you probably would have got away with just the switch!
    Max.
     
  13. #12

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    I don't think so. "Lights" probably means tungsten.
    The links I provided specifically down-rate the timer to 7 amps for tungsten compared to 20 amps resistive.
     
  14. nsaspook

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    It might work for a while but the internal mechanics of a AC switch will cause the contacts to pit and develop high resistance causing heating if there is a large on-rush current. A true DC switch is designed to wipe while quickly snapping from a contact landing zone to a contact parking zone to keep the internal resistance low. A regular AC switch just does a slow make or break on one spot.
     
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  15. MaxHeadRoom

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    IMO Very little difference for most switches out there switching a DC resistive load.
    Max.
     
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