Emergency back up light

Thread Starter


Joined Sep 20, 2008
IF ac power goes off I want a car 12v battery to power a 32w dc bulb thru a programable timer. The timer can come from a programable thermostat but how do i handle sensing the loss off AC power? thnx.


Joined Aug 9, 2008
It sounds like you need a 120VAC relay that is NC. As long as the power is on the contacts are open. At the loss of 120V the contact will close and your other circuit will be switched on.


Joined Jul 22, 2007
A relay might work, but don't know if its okay to keep the coil energized for long periods of time. When you lose AC, relay is de-energized, the N.C. contacts are made...


Joined Jul 17, 2007
If your application is to be used indoors, you must use a SLA (Sealed Lead-Acid), AGM or gel-cell type battery. Automotive batteries will emit hydrogen and oxygen gasses when being charged, which will present an explosion hazard.

Additionally, automotive batteries will have a very short lifespan when used in such an application. They are designed for brief, heavy loads (starter motor) and then must be immediately recharged. SLA, AGM and gel cells are designed for deep cycle use. Even then, you should not discharge your battery below 50% (about 11.7v) or it's life will be significantly reduced.

Consider using super-bright white LEDs rather than incandescent lamps; they are far more efficient.