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