Relay causes circuit interference?

[belias]

Hello,

I'm working on a fire/burglary alarm panel and I need to add an external relay that closes a circuit whenever a fire alarm is detected (i.e. no voltage, just circuit closure). The alarm panel has one internal relay that activates for fire, and one that activates for burglary; each of these internal relays have the following terminals:

Common (hard wired to +12 V)
N/O
N/C (connected to the siren driver, activating the appropriate sound)


I thought I could use a simple relay connected as shown here:

http://dl.dropbox.com/u/2382226/alarm%20system.jpg

However, when I connect the relay the alarm panel experiences interference causing various issues. As soon as I disconnect the relay - the interference goes away. The relay does NOT have to be energized for the interference to be a problem.

So here's the question: How can this happen? I had thought that the relay should completely isolate both sides of the coil (the only portions connected to the panel) when off. Any suggestions on how to solve this would be great!


Thanks,

- Brian

 

[schip666!]

I'm not sure what "steady" is on the alarm panel, but your relay may be drawing too much current from it. You could experiment with different resistors in place of the relay to see how much you can get w/o disturbing the rest of the functions.

 

[Voltux]

Hey Belias,

I have to agree with schip666! here, your relay could be drawing too much current. If your circuits are getting damaged or quit working it could also be Back-EMF from your relay which can be solved with a simple diode. You could also use a Solid State relay or even a MOSFET in place of a mechanical Relay.

Hope this helps, and best of luck on getting your alarm system setup! :)