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I am planning on having a backup power supply for my ESP32 circuit and, since I am dealing with power supply, I thought that having an active circuit would be a downside (otherwise, who will feed the opamp?).

I thought of a simple circuit with diodes and I wonder what problems would it cause or if there is another way.

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My main idea is using an adjustable voltage regulator to my main supply (V1), setting it to 5.6 V. My secondary supply (V2) will be set to 5 V.

When V1 is normal, the current will flow from it and, since the voltage on the second diode is less than 0.6 V, V2 won't supply.

After V1 drops below 5 V, the D2 voltage will become sufficient for it to conduct, supplying my ESP32 until V1 is back to normal.

Is there any problem with the circuit or things I should be aware of?

V1 won't normally drop to 5 V, it will either be 5.6 V or 0V so "splitting" the current when V1=5 V won't be a problem.

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You idea will work perfectly, and has worked perfectly in a number of space missions where I built the power supplies with battery backups.

The only possible problems are related to the behavior of the diodes. They drop a certain voltage, which varies with current. So your "regulated" 5.6 volts will be around 5.1 volts (for Schottky diodes), and the backup supply will produce something like 4.5 volts. Additionally, depending on the currents involved, there can be significant power dissipation in the diodes.

In my systems, the diode drop wasn't an issue, because the diodes fed DC-DC converters, which have a wide voltage input range.

In my systems, the input voltages were about 28 volts, so losing about 0.5 volts was an efficiency loss of less than 2%, which nobody worried about. YMMV, of course. In my case, the simplicity and smooth operation of the circuit drove my choice.

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  • \$\begingroup\$ Thanks a bunch! Unfortunately this is my first comment in this stackexchange, so I can't give you an upvote! \$\endgroup\$ Oct 13, 2023 at 16:57

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