Help with 12v battery backup circuit

You could use a relay that is operated whenever external power is available. The wiper of the relay connects to the output power to the radio. The contact that is closed when external power is available is connected to the external power source. The contack that is closed when no external power is available is connected to the battery pack.

Do you know where I can pick up a small 12v relay I would asume15amp max? I found a few bigger ones but a smaller one would be good.

If the voltage of the external power source will be higher than the battery voltage, then: (view in Courier)

Get a closed-circuit jack. When there's nothing plugged into the jack, the battery provides power to the output through a third contact. When you plug in the wall-wart or car adapter, the plug displaces that third contact, taking the battery out of the circuit.

Good Luck! Rich

On 09/02/2006 the venerable Bob etched in runes:

to build a

all memory is lost.

pack to plug in to the

Can anyone help me

just need help with

Two 1N4001 diodes.

want to build a

all memory is lost.

pack to plug in to the

Can anyone help me

just need help with

Try

There is no way you will need 15A contacts. it should say somewhere on the radio what is required. unlikely it is more than an amp.

The two 1N4001 diodes arranged in the 'wired OR' configuration is the easiest and cheapest solution, I have implemented this circuit in commercial applications and works a treat.

Regards Alan