switch between wall and battery power

[icydash]

I'm trying to make a (low voltage) circuit that runs off wall power when it's plugged in (a 9v DC power adapter), otherwise it runs off the attached 9v battery. I'm not sure how to make the circuit detect if it's plugged into the wall or not, though. Is there some IC that is commonly used for this or something?

 

[IanP]

The simplest solution will be to build an OR-SWITCH using two diodes, as shown on the attached picture ...
Also see:
https://saeedsolutions.blogspot.com.../12/how-to-switch-between-power-supplies.html
:wink:
IanP

 

[icydash]

The simplest solution will be to build an OR-SWITCH using two diodes, as shown on the attached picture ...
Also see:
https://saeedsolutions.blogspot.com.../12/how-to-switch-between-power-supplies.html
:wink:
IanP

Thanks for your reply. The diodes seems like a good solution. So when the wall power is plugged in, if it has a higher voltage than the battery, the wall will power the circuit. If the battery has a higher voltage (even when the wall power source is plugged in) the battery powers the circuit. If no wall power is connected, the battery powers the circuit. Is that correct?

What if the battery and wall power have the same voltage (i.e the wall adapter is plugged in and the battery starts off with a higher voltage than the wall power, but drains over time until they both have the same voltage in some instant)?

My concern is that people using this circuit will often install fresh 9v batteries, which may have equal (or greater) voltage than the 9v DC wall power supply.

 

[IanP]

Thanks for your reply. The diodes seems like a good solution. So when the wall power is plugged in, if it has a higher voltage than the battery, the wall will power the circuit. If the battery has a higher voltage (even when the wall power source is plugged in) the battery powers the circuit. If no wall power is connected, the battery powers the circuit. Is that correct?

What if the battery and wall power have the same voltage (i.e the wall adapter is plugged in and the battery starts off with a higher voltage than the wall power, but drains over time until they both have the same voltage in some instant)?

My concern is that people using this circuit will often install fresh 9v batteries, which may have equal (or greater) voltage than the 9v DC wall power supply.

Any circuit that senses a voltage also drains some micro-power from that source ..

You can use P-channel mosfet in series with the battery diode.
The gate of this mosfet is connected to the PS node, as shown on the attached picture (conceptual diagram) …

:wink:
IanP

 

[icydash]

What exactly does that do?