Efficient AC Adapter Circuit for Battery-Powered Devices

[Perfection]

I'd like to stick an ac adapter onto a circuit such that when the adapter is plugged in it runs on the ac adapter, but when it isn't plugged in it runs on batteries. Anyone know of a quick elegant solution?

I'm sure these circuits get made all the time, but I can't seem to find any!

I'm sure I could come up with something, but I was wondering if there was a standard way of doing it.

 

[vk6kro]

Yes, you can get small power sockets that have a switch in them.

This switch makes contact unless the AC adaptor is plugged in.

So, you have the battery positive connected through this switch and then to the centre input pin of the socket and then to the circuit.

When the AC plug is inserted, the switch opens and the battery is disconnected. Power just comes from the AC adaptor. This has to provide filtered DC, of course.

The negative leads are all joined together.

 

[Perfection]

Unfortunately, I can't use that option. The specified adapter uses a USB mini plug.

 

[schip666!]

You might be able to use a diode in series with each power source going to whatever the load is. If the voltage from the AC adapter is slightly higher than the batteries the adapter's diode will conduct in "preference" to the batteries'.

 

[MATLABdude]

There are ICs called 'supervisors' (specifically, supervisors with battery switchover) which allow you to switch between two power supplies. They usually have a little hysteresis so they don't switch-over as soon as one voltage dips below the other.

I think they're also called a 'power mux' by some manufacturers, at least according to a TI datasheet I found on a quick Google search:
http://focus.ti.com/docs/prod/folders/print/tps2110.html