ECN Forum Forums General Topics of the Trade General Discussion Area Low voltage door contacts

The alarm job I talked about earlier is kind of giving me an education. I endeavored to upgrade an existing alarm type system, which had a 120 volt a/c to 15 vdc converter, that fed normally opened door contacts, then fed back to an alarm. The door contacts were broke and I replaced them, plus added other door contacts throughout the facility, and also added 2 more 12 vdc sirens and 1 more internal alarm. Since this didn't go through a control panel, the full load of the alarms went through the contacts. I didn't realize how much of a load was going through the contacts, until I mounted one of the last ones before fininshing the job, or so I thought The last door contact welded itself together when I tested the alarm again. Hopefully I can remedy this situation, by going through the door switches, then letting the return wire energize a 12 vdc relay, and breaking the main 12 vdc through the contacts on the relay, thus allowing the load to flow through the contacts on the relay, instead of the contacts on the door. The contacts on the relay are rated 7.5 amps, and I will only be pulling 4 amps on 15vdc. The coil itself only pulls 1.5 watts verses the 60 watts that the contacts were loaded to. Is there anyone with any other suggestions, to my dilema here. Wonder if I'm missing something about this set up I'm thinking about doing? Any thoughts would be appreciated. Thanks.. Steve

That would be a very unusual set-up with the contacts just operating the alarm directly. The norm would be for a relay (or these days an equivalent electronic circuit) which serves a dual purpose: 1. Keeps the current in the contacts down, as you describe, and 2. Provides a method for the alarm to latch on.

With the contacts operating the alarm directly, anyone opening a door and hearing the alarm start only has to close it again to stop it.

If you just want a basic latching relay circuit, use a relay with double-pole normally-open contacts. Wire one pair in parallel with the door contacts and use the other pair to operate the alarms. Then when the alarm is triggered, the relay latches on via its own contacts and the alarm will continue until turned off.

The more common arrangement is for door/window contacts to be normally closed and wired in series so that any break in the circuit triggers the alarm.