110 volt magnetic door switches not opening

Discussion in 'The Projects Forum' started by chs111, Apr 8, 2010.

  1. chs111

    Thread Starter New Member

    Apr 8, 2010
    2
    0
    Hello:
    We have a piece of equipment that is protected by magnetic door switches. They are 110 volt reed switches. The problem is that they do not always work. We are switching an Allen Bradley 700 series relay. I believe that the current is to high for the switches. I tried using a resistor in series (recommended by the manufacturer of the switches) but this made it worse. I'm looking for suggestions before I rewire for a 24 volt circuit.

    Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Reed switches won't stand much current before their contacts burn up. Do the A-B 700 series relays have transorbs/MOVs/other snubbers on the coils? If not, you'll get lots of arcing/pitting of the reed switch contacts when the circuit is broken.

    You might consider using the reed switches to control the gate of a DIAC/thyristor, or to operate a relay with a very low coil current to switch the A-B 700 relay.
     
  3. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    The correct way to use reed switches on long wires it to fit a ten ohm (or higher) resistor directly against the switch. The value should be selected so an instantaneous short circuit of the supply voltage would give well under the rated current of the switch.

    This prevents any excess current surge when the switch closes and shorts the connecting cable, which is acting as a capacitor.

    Something like 47 Ohms would probably be reasonable on a 110V supply.

    To prevent damage due to arcing when the contacts open, fit a snubber network across the load the switch is controlling. A standard 0.1uF + 100 Ohm type should work fine.

    Once a switch has started acting up, all you can do is replace it. Make sure you put the protective components in place before fitting a new switch.
     
  4. chs111

    Thread Starter New Member

    Apr 8, 2010
    2
    0
    Thanks for your help!!

    Chuck
     
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