Power failure alarm using transformer and relay
This power failure alarm circuit is very simple and is very useful when used with electric ovens, a refrigeration systems or to prevent a thief from cutting off the electrical power because it immediately warns of the problem. It also serves you when the fuses have blown.
At the output of the circuit (relay) you can connect any type of device to attracts attention, such as a light, a siren or both. See the diagram below.
Power Failure Alarm Operation
The primary winding of the transformer (T) is connected to the power lines, either 120 or 240 VAC. Transformer secondary have 12 VAC. The transformer secondary sine wave voltage is half-wave rectified by the diode (D) and then flattened by means of an electrolytic capacitor (C).
The result is a pulsating voltage of approximately 18 VDC. This voltage will constantly feed the relay and keep it active. Relay should be a 1 pole 2 throw device. This means that the armature move between two possible contacts. The diagram below shows the diagram of a 1-pole, 2-shot relay.
- When there is electrical power, relay is activated and the armature is connected to one throw. (contacts D and C are active)
- When there is no electrical power, relay is deactivated and the armature changes to the other throw and allows the activation of devices that give the alarm signal. (contacts C and E are active).
These devices can be powered by another source of energy, be it DC or AC. A DC source can be a 9V battery. See the first diagram.
List of circuit components
- 1 miniature 120/240 VAC transformer at 12 VAC (T)
- 1 rectifier diode 1N4001 or similar (D)
- 1 electrolytic capacitor of 470 uF of 25 volts or more (C)
- 1 12 /18-volt high-impedance relay