Long 12VDC cable run protection components

Discussion in 'The Projects Forum' started by Netwaves, Mar 22, 2015.

  1. Netwaves

    Thread Starter Member

    Feb 9, 2015
    44
    1
    I'm building a security system that has 32 long cable runs, approx 100' to 1000'. It's 12VDC for the reed relay switches/sensors. What component(s)/circuit(s) could I use for protecting the optoisolators on the board side from noise and spikes generated along the sensor cable runs?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,546
    2,370
    Make sure you use individually shielded twisted pairs with overall braid shield cable, also you may need a voltage source with remote sense to correct for volt drop.
    Max.
     
  3. ronv

    AAC Fanatic!

    Nov 12, 2008
    3,290
    1,255
    I think I would do it like this:
    The diode would protect the opto from positive transients and any negative ones would just turn it on which if you do the logic right is a safe condition for the alarm.
    Twisted pair is a good idea,
     
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  4. AnalogKid

    Distinguished Member

    Aug 1, 2013
    4,539
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    To protect from large transients, I'd use a zener diode in ronv's schematic. LED's do not make good self-clampers. If you are protecting from pulse transients like lightning or spot welders, think about a tranzorb rather than a standard zener. These look like zeners but have extra-low junction capacitance for very fast response time.

    ak
     
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