Battery Backup

Backup - Main Power Circuit

This battery backup circuit switches over to battery power when the main power fails, and I have used this simple circuit for some time and it works well. The main power enters through D1 and the current goes out to power a circuit. When the current comes in through D1 it cannot go through D2 because that diode is in reverse bias mode.

D1 and D2 have to be power diodes capable of carrying the amount of current that the circuit requires. For example, the 1N4001 through 1N4007 series have a maximum output current rating of 1 ampere, hence if you have a circuit that will draw around 300 mA then these diodes will work fine.


Backup Power

When the main power through D1 is no longer present, the circuit draws current through the battery, and it flows through D2, which is in forward bias mode. This current cannot flow into D1 because it is in reverse bias mode. The 1 µF capacitor should have a voltage rating that is the same as the voltage source. Its purpose is to provide the voltage for a fraction of a second for the circuit to change over to battery power.


Recharge Circuit

If you are using Ni-Cad rechargeable batteries, you may want to have a current limiting resistor R that provides a very small trickle charge to the battery. This ensures that the batteries will always remain in a charged condition. The charge carrying capacity C of the battery determines the value of the resistor R. Databooks recommend the ideal current to be equivalent to the C/10 rating of the battery.

If you had a 300 mAh battery, then C/10 = 30 mA.