Basic low backup power demo circuit

With a main power source, you can also parallel a supercapacitor or battery to a load for instant backup power during supply voltage interruptions. Make sure the power supply, and supercapacitor/battery can handle to power demands involved.

Basic backup power using supercap with internal resistance demo circuit schematic
Basic backup power using supercap with internal resistance demo circuit schematic

It’s generally assumed that backup power will be needed due to problems with the main supply voltage instead of it being completely cut off from the circuit like an open switch. Therefore, you need to make sure that the backup power doesn’t send power back to the main source of power.

A diode will let the power supply power the load and charge a capacitor/battery, with the loss (diode drop) of some of the supply voltage. If the supply voltage is disrupted, then the diode will make sure that none of the stored charged is returned towards the power supply. A Schottky diode will drop less of that voltage than a purely silicon based rectifier diode.

Most supercapacitors can’t be instantly charged and discharged, I probably shouldn’t even do so with the ones I have. They should have some series resistance or other means of limiting current. But from testing, mine appear to have about 10Ω of internal resistance, which limits current during a short circuit. And will drop the voltage a slight amount when it takes over for the main power supply. More resistance added in series with it, will drop the voltage even more.

I appeared to have damaged a supercapacitor like the one used in the video below, by shorting it  multiple times while charging and discharging it in the previous video. I talk about that in the video below.

This circuit is just a demonstration that we can keep an LED lit at about the same brightness for a while when the power is suddenly cut off. It’s just a starting point for learning about, or designing, more complex backup power circuits.

Video below goes live 6pm CST on 2 May 2023.

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