This
circuit provides an uninterrupted power supply (UPS) to operate 12V, 9V
and 5V DC-powered instruments at up to 1A current. The backup battery
takes up the load without spikes or delay when the mains power gets
interrupted. It can also be used as a work bench power supply that
provides 12V, 9V and 5V operating voltages. The circuit immediately
disconnects the load when the battery voltage reduces to 10.5V to
prevent deep discharge of the battery. LED1 indication is provided to
show the full charge voltage level of the battery. Miniature white LEDs
(LED2 and LED3) are used as emergency lamps during power failure at
night.
A standard step-down transformer provides 12V of AC, which
is rectified by diodes D1 and D2. Capacitor C1 provides ripple-free DC
to charge the battery and to the remaining circuit. When the mains power
is on, diode D3 gets forward biased to charge the battery. Resistor R1
limits the charging current. Potentiometer VR1 (10k) with transistor T1
acts as the voltage comparator to indicate the voltage level. VR1 is so
adjusted that LED1 is in the 'off' mode. when the battery is fully
charged, LED1 glows indicating a full voltage level of 12V.
When
the mains power fails, diode D3 gets reverse biased and D4 gets forward
biased so that the battery can automatically take up the load without
any delay. When the battery voltage or input voltage falls below 10.5V, a
cut-off circuit is used to prevent deep discharging of the battery.
Resistor R3, zener diode ZD1 (10.5V) and transistor T2 form the cut-off
circuit. When the voltage level is above 10.5V, transistor T2 conducts
and its base becomes negative (as set by R3, VR2 and ZD1). But when the
voltage reduces below 10.5V, the zener diode stops conduction and the
base voltage of transistor T2 becomes positive. It goes into the
'cut-off' mode and prevents the current in the output stage. Preset VR2
(22k) adjusts the voltage below 0.6V to make T2 work if the voltage is
above 10.5V.
When power from the mains is available, all output voltages-12V,
9V and 5V-are ready to run the load. On the other hand, when the mains
power is down, output voltages can run the load only when the battery is
fully charged (as indicated by LED1). For the partially charged
battery, only 9V and 5V are available. Also, no output is available when
the voltage goes below 10.5V. If battery voltage varies between 10.5V
and 13V, output at terminal A may also vary between 10.5V and 12V, when
the UPS system is in battery mode.
Outputs at points B and C
provide 9V and 5V, respectively, through regulator ICs (IC1 and IC2),
while output A provides 12V through the zener diode. The emergency lamp
uses two ultra-bright white LEDs (LED2 and LED3) with current limiting
resistors R5 and R6. The lamp can be manually switched 'on 'and 'off'by
S1.the circuit is assembled on a general-purpose PCB. There is adequate
space between the components to avoid overlapping. heat sinks for
transistor T2 and regulator ICs (7809 and 7805) to dissipate heat are
used.
The positive and negative rails should be strong enough to
handle high current. Before connecting the circuit to the battery and
transformer, connect it to a variable power supply. Provide 12V DC and
adjust VR1 till LED1 glows. After setting the high voltage level, reduce
the voltage to 10.5V and adjust VR2 till the output trips off. After
the settings are complete, remove the variable power supply and connect a
fully-charged battery to the terminals and see that LED1 is on. After
making all the adjustments connect the circuit to the battery and
transformer. The battery used in the circuit is a 12V, 4.5Ah UPS
battery.
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