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This circuit provides an uninterruptedpower supply (UPS)to operate 12V, 9V and 5VDC-powered instruments at up to 1Acurrent. The backup battery takes upthe load without spikes or delay whenthe mains power gets interrupted. Itcan also be used as a workbench powersupply that provides 12V, 9V and 5Voperating voltages. The circuit immediatelydisconnects the load whenthe battery voltage reduces to 10.5V toprevent deep discharge of the battery.LED1 indication is provided to showthe full charge voltage level of the battery.miniature white LEDs (LED2 andLED3) are used as emergency lampsduring power failure at night.A standard step-down transformerprovides 12V of AC, whichis rectified by diodes D1 and D2. CapacitorC1 provides ripple-free DC tocharge the battery and to the remainingcircuit. When the mains power ison, diode D3 gets forward biased tocharge the battery. Resistor R1 limitsthe charging current. PotentiometerVR1 (10k) with transistor T1 acts asthe voltage comparator to indicatethe voltage level. VR1 is so adjustedthat LED1 is in the ‘off’ mode. whenthe battery is fully charged, LED1glows indicating a full voltage levelWhen the mains power fails,diode D3 gets reverse biased andD4 gets forward biased so that thebattery can automatically take upthe load without any delay. Whenthe battery voltage or input voltagefalls below 10.5V, a cut-off circuit isused to prevent deep discharging ofthe battery. Resistor R3, zener diodeZD1 (10.5V) and transistor T2 formthe cut-off circuit. When the voltagelevel is above 10.5V, transistorT2 conducts and its base becomesnegative (as set by R3, VR2 and ZD1).But when the voltage reduces below10.5V, the zener diode stops conductionand the base voltage of transistorT2 becomes positive. It goes intothe ‘cut-off’ mode and prevents thecurrent in the output stage. PresetVR2 (22k) adjusts the voltage below0.6V to make T2 work if the voltageis above 10.5V.When power from the mains isavailable, all output voltages—12V,9V and 5V—are ready to run theload. On the other hand, when themains power is down, output voltagescan run the load only when thebattery is fully charged (as indicatedby LED1). For the partially chargedbattery, only 9V and 5V are available.Also, no output is available when thevoltage goes below 10.5V. If batteryvoltage varies between 10.5V and13V, output at terminal A may alsovary between 10.5V and 12V, whenthe UPS system is in battery mode.Outputs at points B and C provide9V and 5V, respectively, through regulatorICs (IC1 and IC2), while outputA provides 12V through the zenerdiode. The emergency lamp uses twoultra-bright white LEDs (LED2 andLED3) with current limiting resistorsR5 and R6. The lamp can be manuallyswitched ‘on’ and ‘off’ by S1.the circuit is assembled on a general-purpose PCB. There is adequatespace between the components toavoid overlapping. heat sinks for transistorT2 and regulator ICs (7809 and7805) to dissipate heat are used.The positive and negative railsshould be strong enough to handlehigh current. Before connecting thecircuit to the battery and transformer,connect it to a variable power supply.Provide 12V DC and adjust VR1 tillLED1 glows. After setting the highvoltage level, reduce the voltage to10.5V and adjust VR2 till the outputtrips off. After the settings are complete,remove the variable power supplyand connect a fully-charged batteryto the terminals and see that LED1 ison. After making all the adjustmentsconnect the circuit to the battery andtransformer. The battery used in thecircuit is a 12V, 4.5Ah UPS battery.

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Mini UPS System


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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 workbench 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.