Description This circuit is a simple form of the
commercial UPS, the circuit provides a constant regulated 5 Volt output and an
unregulated 12 Volt supply. In the event of electrical supply line failure the
battery takes over, with no spikes on the regulated supply.
Notes: This circuit can be adapted for other
regulated and unregulated voltages by using different regulators and batteries.
For a 15 Volt regulated supply use two 12 Volt batteries in series and a 7815
regulator. There is a lot of flexibility in this circuit. TR1 has a primary
matched to the local electrical supply which is 240 Volts in the UK. The
secondary winding should be rated at least 12 Volts at 2 amp, but can be higher,
for example 15 Volts. FS1 is a slow blow type and protects against short
circuits on the output, or indeed a faulty cell in a rechargeable battery. LED 1
will light ONLY when the electricity supply is present, with a power failure the
LED will go out and output voltage is maintained by the battery. The circuit
below simulates a working circuit with mains power applied:
Between terminals VP1 and VP3 the nominal
unregulated supply is available and a 5 Volt regulated supply between VP1 and
VP2. Resistor R1 and D1 are the charging path for battery B1. D1 and D3 prevent
LED1 being illuminated under power fail conditions. The battery is designed to
be trickle charged, charging current defined as :-
(VP5 - 0.6 ) / R1 where VP5 is the unregulated DC power
supply voltage.
D2 must be included in the circuit, without D2 the
battery would charge from the full supply voltage without current limit, which
would cause damage and overheating of some rechargeable batteries. An electrical
power outage is simulated below:
Note that in
all cases the 5 Volt regulated supply is maintained constantly, whilst the
unregulated supply will vary a few volts.
Standby
Capacity The ability to maintain the regulated supply with no
electrical supply depends on the load taken from the UPS and also the Ampere
hour capacity of the battery. If you were using a 7A/h 12 Volt battery and load
from the 5 Volt regulator was 0.5 Amp (and no load from the unregulated supply)
then the regulated supply would be maintained for around 14 hours. Greater A/h
capacity batteries would provide a longer standby time, and vice versa.
