Specifications
CONSTRUCTION
and capacitor C2 on the secondary side
act as filter/resonant circuit (at 50 Hz) to
produce a waveform approaching a
sinewave. LED1, when ‘on’, indicates that
the system is on back up.
Charger circuit. This circuit com-
prises step-down transformer X2, followed
by rectifier, regulator, and double-
changeover 12V relay RLY. Mains supply
of 230V AC is applied across the primary
of the transformer through triac BT136.
The gate of the triac is connected to the
output of over-/under-voltage cut-off cir-
cuit. As long as the mains voltage is be-
tween 170 and 270 volts, +5V is provided
to the gate of the triac and hence it con-
ducts. If AC mains voltage goes out of the
above-mentioned limits, the gate voltage
falls to 0.7 volt and the triac does not
conduct.
When traic conducts, 16-0-
16V AC voltage is developed
across the secondary of X2. It
is converted into DC voltage by
the diodes D3 and D4, and the
rectified output is given to the
input of the 12V regulator 7812
(IC3). The output of the regu-
lator is connected across the
relay coil through series resis-
tor R7, which ensures that the
relay just operates when AC
mains is at 170 volts (or more).
When mains voltage is
within the range of 170-270
volts, relay activates. In this
mode, mains voltage is directly
routed to the load through
N/O contacts (lower) of relay
RLY and 4-amp rated contact
breaker (CB). LED2 indicates
that the system is on mains.
At the same time, the rectified
voltage from diodes D3' and D4'
is made available through N/O
contacts (upper) of relay RLY
for charging the battery via
charging resistance R
C
. If the
mains supply fails or goes out
of the range of 170-270 volts,
relay de-energises and the bat-
tery supply of 12V is connected
to the inverter circuit through
N/C contacts (upper). The volt-
age developed by the inverter
goes to the output socket of
UPS through the N/C contacts
(lower) via 4-amp CB.
Under-/over-voltage cut-
out. The 230V AC mains is
stepped down to 12V AC, us-
ing transformer X3. It is recti-
fied by the bridge rectifier and
filtered by two Pi (F) section
filters to reduce the level of
ripple voltage. The filtered DC
voltage is applied to dual op-
amp IC5 (used as dual com-
parator). The reference voltage
for the comparators is devel-
oped across zener diode ZD4, which is con-
nected to the filtered DC positive rail via
resistor R9. Even if the AC mains voltage
varies between 170V and 270V, the volt-
age across zener ZD4 remains constant
at 5.1 volts. The cathode of zener diode
ZD4 is connected to the inverting input
of the comparator IC5(b) and non-invert-
ing input of the comparator IC5(a).
Preset PR2 can be used to vary the
Fig. 6: Actual-size solder-side track layout for the PCB.
160