antiparallel diode with relay coil

Discussion in 'The Projects Forum' started by alkos, Jun 24, 2011.

  1. alkos

    Thread Starter New Member

    Jun 24, 2011
    hi i have a gsm alarm that has outputs it says. to wire a antiparallel diode with relay coil to them.
    it says when switched to high it is 5v and low is 0v when i checked the volt's on high it was only 4.7v
    all i want to do is use it to switch on a 12v light
  2. Wendy


    Mar 24, 2008
    That diode is called a swamping diode. It is meant to keep the flyback (aka, kick back) from the coil from damaging the rest of the sensitive solid state electronics. Without a schematic it is hard to be possitive, but that sounds what it is like.
  3. Jaguarjoe

    Active Member

    Apr 7, 2010
    Did the alarm specification tell you how much current it can switch at 5 volts? You must have that information if you want to use a simple 5 volt relay to energize your 12 volt lamp. This current spec must be higher than the relay's coil current.

    Sounds like when they specified the 5 volt output value they forgot to mention 'nominal'.

    If you can use a relay to turn your 12v light on, make certain that the relay contacts are rated for 4 times the normal operating current of the lamp. This is the inrush current of a lamp when first turned on. If you fuse this circuit, use a slow blow fuse.

    Use a common 1N4001 diode across the relay coil.
  4. wayneh


    Sep 9, 2010
    I think Jaguarjoe is referring to TWO specifications: 1) The ability of the alarm circuit to power the coil of the relay, and 2) The capacity of the relay, how big a load it can safely switch. Failing to consider either of these could lead to a problem.