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Hello all,

I have a GPS device (resistor in diagram) that I want to be powered from either of two 3.3 V sources. If both voltage sources are on, it doesn't matter which one is used. However, if one is not turned on, I want to prevent reverse current flowing back into the other side of the circuit.

I know that I could use a switch, but I want this to be automated without user selection. Would two diodes in series with each of the voltage sources work? From my googling, it seems that Schottky diodes would be best, is that right, and what voltage rating should they be? Would the voltage drop be of a concern (GPS module needs 3.3-5 V input, current at 45-70 mA)?

Thanks!

Here another solution, with the advantage to deliver almost all the voltage to the load.

Note that if BOTH sources are present, the circuit is OFF (The circuit acts like a XOR logical gate).

You MAY have to add a large resistance between the gate and the ground for each transistor in order to discharge the capacitor of the PMOS when removing the source tied to the grid. The pull down resistor consumes uselessly energy, that is why it should be of a large resistance in order to minimize the lost.

The simulator does not need it, since the source forces the voltage to be 0, which is subtly different than simply disconnecting the source.

Thank you Vanderghast for the idea. I do need the electronics to be on if both voltage sources are present, so I won't be able to use the PMOS as designed there. I hadn't thought of using those, so I'll play around and see if there is a logic that works for that. Alternatively, I could get power off a 5 V regulator and not worry about the voltage drop across the diode.