9volt battery backup??

[mrehder]

I need to add a 9-volt battery as backup power. I have a door alarm/transmitter that alarms and pages when a door is opened, it operates on 12vdc. If the AC power is out temporarily, I need to keep this device working for a short period of time.............How do I incorporate a 9-volt battery (like in a smoke detector) to only supply power when the normal 12volts is gone. I do not want to provide any charging circuit........Thx

 

[EN0]

I need to add a 9-volt battery as backup power. I have a door alarm/transmitter that alarms and pages when a door is opened, it operates on 12vdc. If the AC power is out temporarily, I need to keep this device working for a short period of time.............How do I incorporate a 9-volt battery (like in a smoke detector) to only supply power when the normal 12volts is gone. I do not want to provide any charging circuit........Thx

For your door alarm/transmitter, how much current does it consume? A standard 9V battery can provide up to a maximum of 25mA; but not for very long. How short of a period do you want it to run on emergency power? If it's extremely short you might consider a capacitor.

 

[mrehder]

My transmitter requires 150mA, guess I need a better battery source.........How do I wire this in?

 

[crutschow]

You could use 8 AA batteries in series to provide the 12V. Radio Shack (8 “AA” Battery Holder - RadioShack.com) and others sell battery holders for this purpose.

Use a diode in series with the batteries (anode to plus battery terminal) to isolate the battery from the 12V supply.