Need Help

Discussion in 'General Electronics Chat' started by Lanz, Feb 4, 2010.

  1. Lanz

    Thread Starter Active Member

    Oct 30, 2008
    153
    0
    I need to switch ON a circuit electronically.I hook up this circuit to my car.I want the circuit to be ON when i turn off the ignition key and OFF when i turn on the ignition.I had tried using relay but relay consume power from the battery.I just want to know wether there are any solution for this using transistor or something else.
     
  2. thyristor

    Active Member

    Dec 27, 2009
    94
    0
    You can use a relay wired to the ignition switch such that when the ignition is OFF, you are using the normally closed relay contacts to power your circuit. That way, only the circuit is drawing power, and not the relay, with the ignition OFF.

    With the ignition ON, the relay is activated and the contacts open, removing power from your circuit.
     
  3. Lanz

    Thread Starter Active Member

    Oct 30, 2008
    153
    0
    Thanks for the info thyristor.
    Can i accomplish this using any PNP transistor?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    A BJT transistor will require constant current through the base to keep it turned on.

    Relays are pretty robust, and are readily available at automotive supply stores. You will need a SPDT type automotive relay.

    You could use a P-channel power MOSFET, but it would be more vulnerable to electrical system fluctuations, so the wiring would be more complicated.
     
  5. Lanz

    Thread Starter Active Member

    Oct 30, 2008
    153
    0
    So the best thing for this kind of situation is RELAY?
     
  6. Bychon

    Member

    Mar 12, 2010
    469
    41
    Yep. Just use the normally closed contacts and reverse the way you think about relays. They can have closed contacts with no power applied.
     
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