I'm building a circuit to run off of a 7V AC-DC adapter regulated down to 6V for my devices. I also want to be able to alternately run the system off a 6V (non-rechargeable) battery.
What's the best way to isolate the battery from getting charged by the supply and preferably to favor the AC adapter if both are connected?
Three ways:
get a DC power jack with a switch that will disconnect the battery when it is plugged in
connect the DC jack and the battery to the input regulator with a pair of diodes. So long as the DC input voltage is larger than the battery voltage, power will be drawn almost exclusively from the DC input
put a series PMOS transistor in the battery connection and wire it up so that it is turned off when the DC power supply is connected but turned on when it is not. You will need to watch the orientation of the body diode - it may need to be installed backwards for this to work properly. You'll need a diode in series with the DC power supply connection. Then you connect the DC power supply input before the diode to the gate of the PMOS and add a pull-down resistor to ground. When the DC power supply is turned off, the PMOS gate will be pulled to GND and the PMOS will turn on. When the power supply is connected, the PMOS gate will be pulled up to the power rail and the PMOS will turn off.
By far the preferred method is simply using diodes, if you can afford the diode drop. This is guaranteed to work very reliably with minimal cost and complexity. Not much can go wrong.
If diodes are not an acceptable way of doing things, you will have to make some kind of active switching circuit. One way to do this, is to have P-channel MOSFETs on both power entries controlled by a small microcontroller or dedicated supervisor chip (search farnell/digikey/mouser for 'supervisors' or 'monitors' and you will see what these chips do), switching between the alternative power supplies as you require.
