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Relay to turn off power instead of on.

Discussion in 'General Electronics Discussion' started by john2k, Jul 10, 2017.

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  1. john2k

    john2k

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    2
    Jun 13, 2012
    I have a 12v supply coming from the cigarette/accessory socket from my car going to some LED lights. This turns on as soon as my car is turned on. However, I want this 12V source to turn off when my sidelight +12v line becomes live. I was thinking of a relay but that wouldn't work because a relay turns on when given power but what i'm actually looking for is for it to turn off when power is detected on the sidelight. Any ideas how I can do this?
     
  2. kellys_eye

    kellys_eye

    4,276
    1,146
    Jun 25, 2010
    The relay can turn things off as well as on when powered - it depends on the contacts you use.

    If you get a SPST (single pole, single throw) relay it has a common connection (c), a normally-open (n.o) set and a normally-closed (n.c) set of contacts.

    Connect +12V to the common connection and your LEDs to the normally-closed connection. The LEDs will be ON until you apply power to the relay coil (taken from the sidelight supply line) when the relay will activate and the LEDs go off.
     
    john2k likes this.
  3. john2k

    john2k

    164
    2
    Jun 13, 2012
    Awesome, i'll try that. Thanks.
     
  4. kellys_eye

    kellys_eye

    4,276
    1,146
    Jun 25, 2010
    I should have said SPDT - SIngle Pole Double Throw (also known as a change-over contact). Sorry about that.
     
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