Voltage drop detection circuit + relay

Thread Starter


Joined Feb 8, 2019
Hello all
I'm in need of a device/circuit that can be connected to a 277 volt motion activated lighting system that will detect voltage drop that occurs when one of the 32 watt fluorescent lights comes on or when power to the whole system gets shut off. So this would work kind of like a hard wired 277 volt alarm circuit. I want this device/circuit to activate/close a relay that will power up a seperate 120 volt circuit.
Please note:
I don't want the 277 volt system powering the 120 volt system. Their totally seperate systems being powered seperate. I want this device to detect change in the 277 volt system and activate a relay only. This 277 volt motion activated light system is on a 20 amp breaker using about 400 feet of 12 gauge wire and powers (16) of these (32 watt) fluorescent lights so when one of these lights clicks on their should be enough voltage drop to be easily detected. I'm pretty sure seen devices like this advertised but I'm drawing a blank on where I saw them. Does anybody have any idea where I could get this or make it if need be. Please let me know.
Thanks Matt


Joined Jan 29, 2010
hi mat,
Welcome to AAC.
I am assuming the quoted voltages are AC rms values.?
The loss of the 277Vac could be detected by using it to power a 230Vac relay coil, via a suitable voltage dropper.
The contacts on the 277Vac relay would switch On the 120Vac when the power to the power to the 277Vac relay is lost.

A 32Watt light at 277Vac is only a current change of 32W/277Vac = 115mA rms.
There are Current Transformers, with associated circuitry, that could detect that change in current.