Electronics 2000

Hi Guys

I'm trying to hook up a GSM Alarm system that monitors various inputs, and I'm wanting to detect mains (AC 240V) failure.

With my limited knowledge I was thinking of using a general purpose relay (sourced from Maplins, a Beta BMY-2C) to do this. It's a 5A 240VAC coil DPDT. I was going to connect the AC main from a distribution panel (on a boat) to the coil, and then connect the alarm 12V DC input wire to the switch of the relay. Thus in the event of mains failure, the switch will throw and then the alarm trip.

Is it that easy? Am I missing something???

Also, is the BMY-2C the best relay to use; I'm looking for something as small as possible and one that consumes as less power as possible? Or should I use something like this:
http://uk.farnell.com/tyco-electronics/ ... /dp/270362

Cheers
Kenny

Hi,

What you are suggesting sounds fine - make sure you fuse the mains (coil) side of the relay though (100mA anti-surge should be sufficient) and insulate all mains connections appropriately.

Your choice of relay is partly governed by your mounting method - the Farnell one seems sensible as it has tab connections and doesn't need mounting on a circuit board. An alternative would be plug-in type and suitable base. I haven't looked up the Maplin one, as I find there website about as painful as the dentist!

Note that 240V AC relay coils tend to hum - I ended up replacing one in a heating control with a small transformer, rectifier and DC 12V relay as the noise was annoying. Probably not an issue for your application though.