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resistor across relay contacts

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hemnath

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unknown element across relay contacts

I have a circuit which is attached below. Please check it.

It is named as loop fault monitoring. There is a element which is connected across the relay contacts. What could be that element? How does the circuit works. Any idea.
 

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KlausST

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Hi,

without any other connection the circuit is useless.

If there are other connections, then you should show us.
The resistor has no value?

Klaus
 

betwixt

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From the name I would guess something monitors the resistance (or current) between 'C' and one of 'NO' or 'NC' and operating the relay causes 'R' to be inserted in series to change the loop current.

A similar thing is used in some intruder alarms, for example the ones that detect whether a window or door has been opened. A reed switch is operated by a magnet but it has a resistor across the contacts. If the door/window opens the resistor is shorted and the increased loop current trips the alarm. It works better than a simple series switch because it can detect a cut cable by no current flowing and if someone shorts the cable to bypass the detector is also triggers the alarm.

Brian.
 
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