A simple and cheap UPS
by sv3ora

This UPS can be used whenever a battery backup is needed for an electronic device that operates from a jack wall transformer or other PSU. The schematic of the UPS is shown below.
D2 Vout

D1 and D2 are two identical diodes and they have to withstand the voltage and current drawn by your device. Vin is the voltage from the power supply and Vout the voltage output to the device. Vbat is the voltage of the backup battery. As long as Vin is slightly higher than Vbat, D1 will conduct and D2 will be cut off. When Vin is lower than Vbat, D2 will conduct and D1 will be cut off. Thus a useful battery backup circuit is formed, where the switching between the battery and the PSU is automatic and fast.

Note that this circuit intentionally does not include a battery charger. A single resistor, used as a battery charger (as seen in most simple circuits), is not a good idea, because the battery will be overcharged.

I use this circuit to battery backup an Alix-1D computer (power consumption 5W), used as a server (shown below).

Eight AAA alkaline batteries in series provide about 12v battery backup to the computer. The voltage drop of the diodes (~0.6v) is identical for both diodes and does not affect the operation of the computer.

Alkaline batteries have been used because of their very low self-discharge. A lead-acid or Ni-MH rechargables are not good for this circuit, as these batteries have high self-discharge and need charging after a while, even if they have not been used previously to power any circuit. An exception to this, are the Eneloop Ni-MH batteries (that must be previously charged with an external charger) that have very low self-discharge, so they are suitable for the circuit.

The circuit is very simple but it is very useful and works extremely well! No more shut down failures, due to short mains black out.

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