Advice on low trigger relay

Discussion in 'Electronics & Electrics' started by Clacker, Sep 12, 2018.

  1. Clacker

    Clacker Member

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    I have a 12v low trigger relay module (like this one on ebay ) that is working correctly in an alarm panel. Aux input on the alarm board activates to ground and the relay operates.

    I currently bridge the ground and trigger pins so the aux input both powers and triggers the relay module.

    If I remove the wire bridge so the relay module is just powered (but not triggered) and use my multi meter (as ohm meter) to bridge the ground and trigger together, the relay activates.

    The datasheet says 5ma trigger current but a multi meter resistance is in Mohms isn't it? How could enough current flow?

    The aim is to use an Arduino low output to also activate the relay but even though I show Mohms between the output and ground, the relay instantly activates.

    I've tried two different multi meters.

    Thanks for any insight.
     
  2. mAJORD

    mAJORD Member

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    Not sure what you're trying to measure - but no, not on Ohms setting - you're applying a small voltage , with a 'source' impedance of the internal battery+ whatever current limiting in multimeter. since this voltage is most likely much lower , current will flow to ground through this impedance.

    try the same on volt's range of multimeter and see what happens. On this range the input impedance should be very high, as you say , and not trigger

    also if it's a 5mA trigger rating - it'll likely still trigger below that, maybe 2 or so mA could be enough
     
  3. merlin13

    merlin13 Member

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    If I'm reading your issue there right, most if not all alarm sensor loops (ie in your case trigger input to ground) nowadays will have "an impedance" on/across them, to be able to detect a hard open circuit/short circuit condition that used to be able to 'disable' that particular monitored sector.

    ie cut the wires or short them out, a la the Bad Old Days of Quiet Break 'n Enter.

    When that impedance changes the alarm will think "Aha! someone's dicking about with one of my sectors, I'l start screaming for Mummy...".

    Could be the megohm impedance and/or the Ohm Mode voltage is actually enough to stooge your alarm there. Try it with a fixed resistor of a megohm or so.

    Wouldn't think though that it's voltage triggering it - long enough wire loop in an alarm sector and a thunderstorm then generating an induced voltage spike on it would need to be compensated for. Which it probably is, so again try a fixed resistance...
     
  4. OP
    OP
    Clacker

    Clacker Member

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    Yes that was my confusion. Volt meter has the high resistance.

    This is on the auxiliary output, not one of the alarm inputs.

    I'll try with the fixed resistor.

    Thanks guys
     
  5. merlin13

    merlin13 Member

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    Relay attached to an output?

    Check if the relay has a back EMF protection diode across the internal relay coil pins - you're prob have to check this with the specs on the make/model number 'coz Ohms or Diode Test Mode on your multimeter will still see the actual coil winding resitance if/when you connect your meter in both orientations across the coil.

    After all that, make make 100% sure you're not connecting the relay coil the wrong way 'round on your output there.
     

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