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Help needed with very simple circuit!

Discussion in 'Automotive Electronics' started by finlay, Jan 17, 2016.

  1. finlay

    finlay New Member

    Jan 17, 2016
    Hi there
    i am trying to wire in an hour meter and a small electric water pump on a small work boat.
    I need some form of circuit that when fed a 12v signal ( i tapped into the signal that controls the crank relay) it feeds the hour meter and water pump until the ignition is switched off.

    My first idea was a latching relay but that requires a second momentary signal to reset it once it has latched,
    so then i moved on to a Silicone Controlled Rectifier, in theory its perfect, latching on until you remove the power then "resets" itself.
    I have however managed to fry 2 SCR's! so i was looking for help wiring it up properly with resistors to protect it, and also how would i get the SCR to trigger a bog standard relay? when i was messing around i couldnt get the SCR to latch with a relay or an LED but when i used a normal headlight bulb it did latch as i understand the SCR has a Holding current?

    can anyone help an idiot like me out?!!
  2. Les Jones

    Les Jones Well-Known Member

    May 15, 2015
    Lancashire UK
    Why do you need a latching circuit ? Can't you just connect the pump and hour meter to the output of the ignition switch. If you do need a latching circuit and us an SCR to latch without using a relay (Or power transistor.) then the pump and hour meter will have to be connected between the SCR anode and the + ve supply from the ignition switch. If you are going to use a relay then that can be made to latch. (This is assuming that the negative of the pump and hour meter are connected permanently to the -Ve - ground.) Connect the relay coil in parallel with the pump and hour meter via a diode on the positive connection. The diode positive to the relay coil and the diode negative to the positive of the pump and hour meter. Connect one side of the relay contacts to the positive of the pump and hour meter. Connect the other side of the relay contacts to the ignition switched supply. Connect a second diode with the positive to the relay coil positive and the negative to crank relay. The diodes only need to be rated to carry the current drawn by the relay coil.
    Last edited: Jan 18, 2016
  3. b.james

    b.james Member

    Feb 4, 2014
    Sounds like you only want the circuits on when you switch on so just connect them directly to a circuit that is only live when you want them on . Don't see the problem or the need for any fancy electrics?

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