Low current relay switch

Discussion in 'The Projects Forum' started by Newlink, Aug 21, 2015.

  1. Newlink

    Thread Starter New Member

    Aug 21, 2015
    2
    0
    Hi,

    In need of a little help....
    I'm getting an amplifier that has a 12V DC trigger out on the back but the Max current is 100mA i want to use this with this relay circuit- http://www.ebay.co.uk/itm/131386633300?_trksid=p2060353.m1438.l2649&ssPageName=STRK:MEBIDX:IT
    to turn on my Sub-woofer & Secondary amplifier on but from the relay spec i believe it draws 77mA.

    Is there a circuit i can put in the middle so it draws 50mA or less? I don't want to damage the amp etc...

    Thanks
    Newlink.
     
  2. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    You have a relay that is rated to operate safely (not damage the amplifier) with the power available, and you don't like that.
    1) choose a different 12V coil relay that uses the amount of power you do like. Not likely, I just checked.
    2) remove the LED so the circuit board uses less current.
    3) add a smaller 12 volt relay to power the 30 amp relay from a different 12 volt source of power.
    4) You can use a Solid State relay for $54.60 plus shipping.
    5) choose a 12 volt relay with less ability to conduct current on the output side.
     
  3. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,546
    1,252
    The 100 mA rating almost certainly has some margin, so using it to operate a 77 mA relay should be perfectly reliable. BE sure to add a diode across the relay coil to prevent back-EMF from upsetting the 12 V signal source.

    ak
     
  4. Newlink

    Thread Starter New Member

    Aug 21, 2015
    2
    0
    #12, its more of just a worry that it might be too much current. Don't want the amp to die after just getting it.

    But if you guys think it will be safe to use the circuits together then i don't think i'm going to need anything in the middle.
    I only need 5-6Amps most out of the 30A it can supposedly supply.

    AnalogKid, what diode would you recommend?
     
  5. #12

    Expert

    Nov 30, 2010
    16,355
    6,852
    Anything in the 1N4000 series will work. 1N4001, 1N4002, up to 1N4007.
    The point is that the relay coil uses way less than the 1 amp which the diode is rated for. When the relay coil suddenly gets switched off, its current needs to escape as the magnetic field collapses. The energy from the collapsing magnetic field can escape through the 1N400x diode without hurting anything.

    As for the "worry"? The amplifier says it can supply 100 ma, you need 77 ma, what's the problem?
    When you lowered the contact current requirement, I can find 25 relays under 50 ma.

    http://www.mouser.com/Electromechan...0x20aZ1z0x366Z1z0x34eZ1z0x3cgZ1yzu87pZ1yzu82k
     
    Last edited: Aug 22, 2015
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