What's a simpler way to switch between power sources?

Question

I want to make a device that allows the user to switch between two different power sources (a wall mount and batteries).

I could perform this circuit using two DPDT switches, but I would need to switch the two switches each time I want to change sources. Is there a simpler way to perform this function without using relays?

Here's a schematic of my device as a reference:

Answer 1

The easiest solution (Initially suggested by @PeterJ) Would be to simply switch only the power connection:

^{simulate this circuit – Schematic created using CircuitLab}

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Alternatively, you can probably get away with just using some schottky diodes:

^{simulate this circuit}

^{_{Wow, Circuitlab is a clumsy tool. It doesn't have entry/exit ports? Or Schottky Diodes? Really?}}

But if you're really loading your RPi or it's USB ports, the drop in the diodes could be large enough that you get brownouts, which would be a real pain in the ass to diagnose.

The best solution, I think, would be to put the diodes before the voltage regulator.
This would mean you would have to have a higher power supply voltage (>7V), but it would solve the problem with the diode V_f described above.

Answer 2

The highest performance (most power efficient/coolest) method is to use a FET OR-ing setup. Their primary advantage is a near-zero voltage drop, limited only by the R_DS(on) of the FET and current sense resistor (10 mΩ total resistance is fairly easy, but 1-2 mΩ if you really need).

Controllers for said systems typically use a low-value sense resistor and appropriately-sized FET to connect the power supply to the circuit. It measures the voltage across the sense resistor to ensure current is flowing in to the device from its supply, rather than being siphoned off from the other supply, then the FET does the switching. Some controllers don't use a sense resistor and just measure across the FET.

Here's an article on Digi-Key's site about ORing controllers.

Furthermore, if your system uses a mid- to low-current (<5 A), there are even controllers with integrated FETs, so your component count can be super low. But if you're in the low-current range and if you can get away with the voltage drop of some ORing diodes (0.3 V for some Schottkys), you're not going to beat them on price.

Anyways, several suppliers make these controllers (search for "power supply oring controller" on Google/DK/Mouser/etc.), to name a few: TI, Linear, Micrel, IR, Maxim, and Vicor.