07-04-2012, 12:55 PM
Mini UPS System
CI-02_Nov09.pdf (Size: 205.61 KB / Downloads: 88)
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.
CI-02_Nov09.pdf (Size: 205.61 KB / Downloads: 88)
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.