Isolating battery input from dc jack input on a PCB

[sean87]

I want to design a PCB which gets powered from 6xAA batteries or a 9v wall adapter. I am wondering how to isolate these 2 supplu inputs from eachother, so that only one should work? I was thinking about some diodes but how? 

 

[Ian.M]

The usual approach is to simply connect the battery negative via the switched contact on the jack.  When the DC jack plug is inserted the batteries are automatically disconnected.

[Buriedcode]

Several options.  The most common for battery/DC jack powered devices is to use the DC socket's normally closed switch to route the ground of the battery to the ground of the circuit.  When a plug is plugged in, this is disconnected and the outer connection of the DC plug is connected to circuits ground - disconnecting the battery.

Other options are, use a diode on each power input with their cathodes connected - the higher voltage supply with be the one that powers the circuit.  The downside with that is, you'll lose ~0.6V (~0.2-0.4V for schottky diodes) of the battery voltage, which for portable applications just reduces battery life/efficiency.

https://tangentsoft.net/audio/cmoy/bitmaps/diode-or-bridge.png


More complicated (but still straightforward) options are to use a P-channel MOSFET on the battery input, with its gate pulled down to ground with a resistor.  The gate can be pulled 'high' (so the FET is off) when power is applied to the DC jack, using transistors to invert the signal.



Like i said, many ways to do this, its a question of efficiency, cost, but by far the most common is using a DC socket with switch:

[Brumby]

The usual approach is to simply connect the battery negative via the switched contact on the jack.  When the DC jack plug is inserted the batteries are automatically disconnected.

Several options.  The most common for battery/DC jack powered devices is to use the DC socket's normally closed switch to route the ground of the battery to the ground of the circuit.  When a plug is plugged in, this is disconnected and the outer connection of the DC plug is connected to circuits ground - disconnecting the battery.

... ...

Like i said, many ways to do this, its a question of efficiency, cost, but by far the most common is using a DC socket with switch:

The switched socket has been around for decades.  Easy solution with effective isolation of the batteries when the adapter plug is inserted.  Not fancy, just effective.

[johnkenyon]

The switched socket has been around for decades.  Easy solution with effective isolation of the batteries when the adapter plug is inserted.  Not fancy, just effective.

With the added advantage that for many power connectors you get the functionality for free, or for no more than pennies compared to an unswitched connector.