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With the help of a couple of diodes wired as an OR gate, 2 different power sources can be connected to the same load, but Isolated from each other. The power source with the higher voltage will provide all of the power to the load.
One of the power sources is typically connected to the grid so that it is almost an unlimited source. That is, of course, until it fails for some reason. It needs to have the higher voltage, so that it provides all the current whenever it is able to.
The backup power source is usually some kind of battery. Batteries have a limited amount of stored charge, which is ready to power the load in this circuit when there is no higher voltage powering it. The battery can have a separate system keeping it charged, or even the power supply that it is backup up, as long as their voltages work out well together.
A 5V USB power bank can be used as the backup supply if you are OK with it’s limitations.
- Actual USB portable power bank voltage is probably 5.1V to 5.2V. Maybe even higher. So the main power supply voltage should be 5.3V or higher. Since this circuit uses diodes to form an OR gate, each supply voltage has one of those diodes in series with it before they both diodes connect to the load.
- USB power banks tend to turn off when current is low. In the video below, My power bank stayed on while it was powering a 1 watt LED at approx. 0.25A (250mA). Whereas it turned off in about a minute while it powered an indicator LED at only about 0.015A (15mA).
Video goes live at 6PM CST 5 May 2023
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