Author Options:

How to build circuit that would offer power by way of either a "wall-wart", or through batteries. Answered

How is the circuit built that would allow a device to be operated by either a power pack ("wall wart") or through batteries. Like in so many toys it would be either/ or. It would automatically switch between one or the other, like, say you're out of batteries and still need this device to work, you can find either the power pack, or even direct ac. I have a radio for example, which I'm sure is like so many other devices of this nature, that can be plugged in (120 volt ac), or will work on the batteries. As soon as I unplug, it automatically switches to the batteries. Is there a circuit internally that changes the 120 volt ac to "wall wart" / dc voltage? Does anyone have a schematic for such a circuit. I don't have any specifics, other than the batteries being maybe 4 D cell batteries at 1.5 volts each (6 v). Thank you.


The forums are retiring in 2021 and are now closed for new topics and comments.

6 years ago

One of the beauties of those 2.1mm (and other sized) power jacks (ie, receptacles) found on a myriad of devices is that they already provide that switching ability.

When a source is plugged in, the jack routes power from point A to B, when disconnected A is connected to C...

In electrical parlance it's

when disconnected: Common to Normally Closed, or N.C.

when connected: Common to Normally Open, or N.O.

find one you can scavenge (for the purposes of a quick experiment), grab your DMM, and test it for yourself.

In fact, there are many different configurations that can be quite helpful for particular projects., but this basic one covers a good share of what one usually encounters in the wild (this and audio of course, where an audio amp in is automatically connected to ground on open condition to prevent it being a noise pickup)

Hint complete


Answer 6 years ago

PS by "audio amp in", I meant to say "an audio amp's input".