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.
Apparatus Required for this are:
- Variable transformer
- Diode
- SCR or transistor
- Battery
- Zener diode
- ON/OFF switch
- Resistance & Capacitor
- IC: IC7809 ,IC7805
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 limit 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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