Undervoltage Relays!

Leccy1976 · Mar 20, 2009

Hi there.
Just wondering whether undervoltage relays exist?
I am hoping to use one in a circuit which when it senses an undervoltage will switch, so i can then power a circuit from a battery back-up?
In other words, when the power is lost, the relay will detect this and then switch which then allows another supply to power the circuit?

Any help would be appreciated.
Thanks Leccy

hughesy · Mar 20, 2009

dont know too much about them but sure you could get a 4way contactor with 2 normally open switches and 2 normally closed so when the coil was pulled in the mains supply would run and when the supply is dropped releasing the coil to change the switch to normally open the main supply couldnt run but the batterry on the normally open switch would then close letting the battery take over . what are you hoping to use it on .

Leccy1976 · Mar 20, 2009

I was thinking of using a contactor, so might give that a go.
I am using it on a door, well an electromagnetic plate lock. Its a battery back-up incase the power is lost, to make sure the door is kept locked.
I might give the contactor idea a go, because when the power is lost, and the coil is de-energised, the normally open contacts will then close, which can then feed the circuit from the batteries, that sound right?
Thanks mate

Guest123 · Mar 20, 2009

Hey Leccy.

Just a thought but is it not the norm to only need a power supply to unlock a door?? i.e energise the coil to drag in the solenoid and unlock??

johnnyb · Mar 20, 2009

That means that on mains failure the relay switches over onto the battery feed, thus keeping the door energised and locked, yes /no well i hope this door when locked is no means of escape or anything or the people inside are in the dark and locked in, could this not be dangerous situation arising here or not!.

hughesy · Mar 21, 2009

Leccy1976 said:
I was thinking of using a contactor, so might give that a go.
I am using it on a door, well an electromagnetic plate lock. Its a battery back-up incase the power is lost, to make sure the door is kept locked.
I might give the contactor idea a go, because when the power is lost, and the coil is de-energised, the normally open contacts will then close, which can then feed the circuit from the batteries, that sound right?
Thanks mate

yes thats right can i have for more details on the door system i assumme the electromagnetic plate lock only keeps the door locked when you energise it and for safety reasons there will be a emergency door release as in the feed from the mains and the battery will run through the emergency door release there for breaking both supplys if pressed .

Leccy1976 · Mar 21, 2009

johnnyb said:
That means that on mains failure the relay switches over onto the battery feed, thus keeping the door energised and locked, yes /no well i hope this door when locked is no means of escape or anything or the people inside are in the dark and locked in, could this not be dangerous situation arising here or not!.

Haha, nah, its nothing like that.
Its for a storage room.
At certain times of the day (usually around 6pm when everyone goes home) the door will be automatically locked. There are override switches in place (inside and outside) to unlock the door.
I think a contactor will do the job. N/C is the mains supply, N/O is the back-up supply, so when mains power is lost, the N/O will close and therefore keep the door locked.