Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

SPST Relay Ratings and Operation Confusion

  1. Apr 26, 2013 #1


    User Avatar

    I'm having trouble understanding the ratings for an SPST Relay. I need one with the following specs:

    Coil Rating: 12Vdc, Contact Rating: 20Amps @ 15Vdc

    If I use a relay with a 24Vdc Coil rating, does that mean that it would require 24Vdc to operate, or is that the maximum amount of voltage it can handle? Will it work with lower than 24Vdc?

    Also, I have found one with the following:

    Coil Rating: 12Vdc, Contact Rating: 50Amps @ 14Vdc

    I was contemplating using this one because it is 50Amps @ 14Vdc, not 15Vdc. Will I be okay using this relay?
  2. jcsd
  3. Apr 26, 2013 #2


    User Avatar
    Gold Member

    I'm pretty sure "yes" because it's such a minor difference and your main concern is the amperage anyhow. Wait for someone with a real knowledge of electricity to answer before doing anything, though.
  4. Apr 26, 2013 #3
    THe second one should be OK - depends on what the load is - lights, no problem. The contacts are rated for the worst case - typically a pure inductive load like a solenoid or motor.
    You should find one with ~20A or more and 24V. The 15V is an odd rating.
  5. Apr 26, 2013 #4


    User Avatar
    Gold Member

    I thought so too, but in the old days Radio Shack carried more of those than anything else. I always figured that it was to give leeway in a 12V circuit.
  6. Apr 26, 2013 #5


    User Avatar
    Science Advisor

    That's normal for a automobile relay as the voltage during charge could be up to 15vdc for a nominal 12vdc electrical system.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook