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12V relay surging

Discussion in 'Electronic Basics' started by [email protected], May 19, 2007.

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  1. Guest

    One problem here hopefully I can get this to work..
    I wired a an automotive relay to drive a 12VDC RV water pump.

    from a power supply,
    12V+ going to 30 and 87 output to pump.
    split same 12V going to coil +

    ground going to the pump directly
    split same ground going to the pump and also the coil -, I put a
    switch inline with this wire to turn the relay on and off.

    I am not sure which one 85 or 86 for the coil, it should not matter?

    When the switch is on, coil is energized and supposed to connect 30
    and 87.. but the problem is that the relay clicks on and off
    repeatedly.

    The relay does say negative spike suppression, is this the problem or
    do I need some kind of isolation, I have not wired up a relay sharing
    the same 12V before...
    Another thing may be the pump has a strong startup surge and dropping
    the power supply voltage, but I am not sure.
     
  2. Chris

    Chris Guest

    Hi, Jeremy. Using automotive relays can be chancy. They're made to
    be produced by the tens or hundreds of thousands, and are made
    specifically to the manufacturers' requirements. That can mean
    internal connections between the coil and contact, chassis connection
    to coil and/or contact, or whatever.

    Since you didn't mention specifically which relay you're using, it's
    not possible for even a seasoned automotive pro to tell you if you've
    got a wiring problem. And this is the wrong group for seasoned
    automotive technicians, anyway.

    Coil dropout from voltage sag is possible, but not likely. Once a
    typical 12V relay coil pulls the armature in, the voltage across the
    coil will have to drop below 4V for it to open. It's your call. If
    you don't have a meter, put a small 12V bulb across the power supply,
    and watch the light intensity when the motor turns on. At 4V, the
    lamp should really dim big time.

    One thing you might want to do is pop off the relay cover, and take a
    look at the wiring.

    I'd recommend springing for a new relay, though.

    Good luck
    Chris
     
  3. Sorry, I am not familiar with this designation.
    I think this last sentence is the answer. The same thing
    happens with the starter relay when a car battery gets
    really low. The moment the load sucks amps, the voltage
    sags and the relay drops out. But then, there is not so
    much load and the voltage springs back up, and the relay
    pulls in again, etc.

    Try the circuit powered from a 12 volt battery, instead of
    the supply, or add a large capacitor across the supply, to
    help with the pump start surge. Have you tried running the
    pump directly from the supply, to make sure the supply is
    capable of driving that load?
     
  4. Jamie

    Jamie Guest

    Well, you just answered your own question.
    your 12 V is getting sucked down.
    this is what you do that may correct it.
    Feed the Relay coil with a diode, a simple 1N4001 and equal will do.
    the line side goes to coil side for the + side of the voltage.
    put a capacitor (larger one) like 1000 uf across the coil..
    what this will do, is hold a charge to keep the relay closed long
    enough to allow the motor to recover from the inrush of current.
    The Diode is to prevent the draining of current to not pull it from
    the cap.
    P.S.
    May I suggest that you use a much higher amp supply and larger
    wire.
     
  5. Circa 18 May 2007 23:24:40 -0700 recorded as
    I get the impression that the "automotive relay" in question just might be
    a turn signal flasher. In which case, it is working just fine.
     
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