Mains OFF senseing

[walid]

I need a circuit that composed from a 220 to say 12 volts transformer then I rectify this to DC and then connectes to some transistor citcuit. this transistor citcuit must sense the existing or not of the 220vAC.
if there is no 220volts the circuit o/p is high and vice versa.
thank you for your help.

[Shahriar]

Dear walid

It is a little bit confusing. when there is no Mains power your cicuit will go off.
maybe you use a battery, when there is power, your citcuit wil use the power from 12V rail, and also charges the battery. and when ther is no 220V, your circuit will use the Battery power.

HTH -Shahriar

[walid]

yes exactly as in an emergency nion lamp

[MP]

If you are only looking for a momentary alarm so that someone knows when the power was lost immediately, the circuit could get power from a large capacitor. This would sound an alarm until the capacitor discharged.

MP

[Guest Alun]

Yes one of those super capacitors powering an astable TS555 with a piezo buzzer would probably be a good idea and it would last for a long time if the duty cycle was low.

[Shahriar]

Dear tnk2k

When there is no power, The relay can not do anything.  8)

Shahriar

[Guest Alun]

That's the whole point, tnk2k was suggesting using a NORMALLY CLOSED RELAY. When the power's on the relay contacts are open as the coil is powered, this disconnects the load (lamp buzzer etc.) from the supply. If the power is interrupted  the contacts will close and connect the load to the capacitor. You'll need to design the circuit carefully though to stop the relay being powered from the capcaitor. ;D

I personally wouldn't use a relay, I'd use a cheaper componant like a comparator or transistors.

[Shahriar]

Dear Alun

I know what tnk2k  means, but obviously when power is disconnected and the relay or transistor was disconnected, he wants to turn on or off a siren or any indicator? then how you provide power for that circuit.

shahriar

[Guest Alun]

With a battery or a large capacitor as explained earlier.