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This question already has an answer here:

I have two in voltage about equal power sources, a built in 12V and an external solar+battery backed up stable 12V source.

I looking for a simple solution to switch immediately to the other one because my circuit contains a raspberry pi 2 as well what I would not like to reboot.

There can be only 2 situations for my circuit:

1, Solar power source comes on (the main is force switched off)

2, Solar power goes out (the circuit is switching back to main)

Also I would like to completely turn off the main adapter with a relay when the device operates from solar not to waste power.

I realize this is possible by using a simple 12V relay in default close position connected to the solar input only and using 2 reverse current protections on both lines but would a big cap on the supply end be sufficient enough to keep the device operation for a second when the power switching happens?

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marked as duplicate by Marcus Müller, Enric Blanco, Wesley Lee, Dmitry Grigoryev, laptop2d May 17 '17 at 17:47

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The normal method is to use two diodes to combine the power supplies but that only works if 1) at least one supply is on all the time and 2) the preferred supply is higher voltage.

If you have two sources at the same voltage and you want to completely shut backup power off when the solar is available then things get a little more tricky.

You can a relay to shut off the backup power, wire it so that the coil is energized by the solar supply and the backup supply is on when the relay is powered down. The issue is that when solar fails you'll get a brownout while the main supply comes up.

Either you need to know that the solar supply is going to die just before it does (either the voltage dips below a threshold or some other warning signal) or you need a 3rd short term power source to keep things running, either a battery or very large capacitor.

Assuming your system can manage with a lower voltage supply this can then be diode or'ed in with the other supplies. Since it'll only be used for ~1/2 a second when failing over from solar to line power it doesn't have to be very large.

e.g.

schematic

simulate this circuit – Schematic created using CircuitLab

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