Specifications

CONSTRUCTION
inverting terminal voltage of the compara-
tor IC5(a) above and below the reference
voltage of 5.1V. Similarly, non-inverting
input of the comparator IC5(b) can also
be varied above and below the 5.1V refer-
ence voltage applied to the inverting in-
put of IC5(b), using preset PR3.
Preset PR2 is adjusted such that when
AC mains voltage goes below 170 volts,
the voltage at inverting input of compara-
tor IC5(a) goes below 5.1 volts, so that its
output goes high. As a result, transistor
T3 conducts and its collector voltage (con-
nected to the gate of triac TR) drops to
0.7 volt, and hence the triac cuts off. This
causes relay RLY to de-energise and the
system changes over to back-up mode of
operation. Glowing of LED3 indicates the
under-voltage condition.
Similarly, preset PR3 is adjusted such
that when mains voltage goes above 270V
AC, the voltage at non-inverting input of
the conparator IC5(b) goes above 5.1 volts,
so that its output goes high to eventually
cut-off the triac, and the system again
operates in the backup mode. The over-
voltage indication is shown by glowing of
LED4.
This means that as long as the mains
voltage is within the range of 170V AC to
270 V AC, the voltage at the collector of
transistor T3 is 12 volts, and hence triac
TR conducts fully and relay RLY activates.
As a result, the system remains on mains
mode. Diodes D6 and D7 act as 2-input
wired-OR gate for combining the outputs
from the two comparators and prevent
the output of one comparator going into
the output terminal of other comparator.
Zener diode ZD6 is used to limit the gate
voltage of the traic to 5.1 volts.
Low-battery indicator. This circuit
is wired around op-amp µA741 (IC6),
which functions as a comparator here.
Battery voltage is applied across pins 7
and 4. Voltage at non-inverting input of
IC6 is maintained constant at 5.1 volts
by zener diode ZD5 and series resistor
R17. Voltage at the inverting input of IC6
can be varied above and below 5.1 volts
using preset PR4. Preset PR4 is adjusted
in such a way that if battery voltage goes
below 10V, the voltage at inverting input
goes below 5.1 volts, so that output volt-
age at pin 6 of IC6 goes high (about 10
V). Hence, LED5 glows and produces in-
termittent sound from the buzzer, indi-
cating low-battery status.
At the beginning of the indication, the
output voltage of inverter would be around
225V AC. This enables the user to take
timely action such as saving data (in case
load comprises a computer).
Battery deep-discharge cut-out. If
the UPS system keeps operating in the
inverter mode, the battery voltage will
drop eventually to prohibitively low level
(say, 5 volts). If such condition occurs fre-
quently, the life of the battery will be con-
siderably reduced. To remove this draw-
back, it is necessary to use battery deep-
discharge cut-out circuit. If battery volt-
age goes below 9.5 volts, this circuit will
cause the UPS to shut down, which pre-
vents the battery from further discharge.
This circuit is also built around op-
amp µA741 (IC7) working as a compara-
tor. Voltage at non-inverting input of IC7
is 5.1 volts, which is kept constant by ze-
ner diode ZD6 and resistor R20. Preset
PR5 is adjusted in such a way that if bat-
tery voltage goes below 9.5 volts, IC7 out-
put would go high to turn on SCR. Once
SCR conducts, the supply voltage for con-
trol circuit drops to near 0V. As a result,
the control circuit is unable to produce gate
drive pulses for the two MOSFET banks
and the inverter stops producing AC out-
put.
Suppose the mains supply is not avail-
able and you want to switch on the UPS
on load (say, computer). If battery deep-
discharge cut-out is set for a battery volt-
age of 9.5 volts, this means that you want
to ‘cold’ start the UPS. On initial switch-
ing ‘on’ of the UPS, the starting current
requirement from the battery is quite high
to cause a drop in battery voltage, due to
which battery deep-discharge cut-out cir-
cuit would be activated and inverter is
not switched ‘on’. To overcome this prob-
lem, 100µF capacitor C8 is connected
across gate-source terminals of SCR. It
provides necessary delay for the battery
current/voltage to settle down to its stable
value after switching on.
Reverse battery protection. A 16A
to 20A diode (D8) in conjunction with fuse
F2 provides reverse battery protection, in
case battery is connected with reverse po-
larity. In case of reverse polarity, fuse F2
will blow and battery supply to the cir-
cuit will be immediately switched off.
Protection against no-load. An op-
tional circuit for ‘no load‘ condition, dur-
ing which the output voltage may shoot
up to 290V AC or more, is shown in Fig. 2
(within dotted lines). The rectifier and fil-
ter used are identical to that of under-
voltage or over-voltage protection circuit,
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