Long 12VDC cable run protection components

Thread Starter

Netwaves

Joined Feb 9, 2015
44
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?
 

MaxHeadRoom

Joined Jul 18, 2013
21,644
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.
 

ronv

Joined Nov 12, 2008
3,770
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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AnalogKid

Joined Aug 1, 2013
8,708
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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