Specifications

CONSTRUCTION
while the comparator circuit is identical
to over-voltage comparator. And hence,
no separate explanation is required to be
included. The output of the circuit is con-
nected to the gate of SCR1 in Fig 2.
Spike suppression. Since triac TR is
connected in series with the primary of
charging transformer X2 and gate volt-
age is obtained from the under-/over-volt-
age cut-out, a spike is treated on par with
the over-voltage (>270V) condition. If
mains voltage spike goes above 270V AC,
gate voltage of triac TR becomes 0.7 volt,
and hence triac does not conduct. As volt-
age across the coil of relay RLY is zero,
the relay is de-energised and system
changes over to back-up mode during volt-
age spike period. Thus the load is pro-
tected from the voltage spikes in the
mains.
Fig. 7: Component layout for the PCB
Back-up time. Using a
single battery of 12V, 7Ah with
a load (100 to 120W) compris-
ing computer along with colour
monitor, the back-up time is 10
to 15 minutes with squarewave
output (half with sinewave out-
put). With a battery of 12V,
180Ah, the back-up time is 4 to
5 hours with squarewave out-
put (2 to 2.5 hours with
sinewave output).
Charging resistance. For
12V/7Ah battery, charging re-
sistance R
C
should be 10 ohms/
20 watts so that the battery will
not be heated during charging.
Similarly, for 12V/90Ah bat-
tery, charging resistance Rc
should be 4.7 ohms/25 watts,
and for 12V/180Ah battery, 3.3
ohms/30 watts.
Square/sinewave output
selection. The selection of
sinewave or squarewave output
is done using slide switch SW2.
In squarewave position, capaci-
tor C2 is directly shunted
across 230V terminal of the sec-
ondary of transformer X1, while
in sinewave position, coil L1
(extension of 230V secondary,
marked 600V) is added in se-
ries with capacitor C2 to reso-
nate at 50 Hz.The power con-
sumption from the battery in-
creases in sinewave output po-
sition of switch SW2. The
graphs of supply (battery) cur-
rent versus the load for each
position of switch SW2, indicat-
ing the comparative values, are
shown in Figs 3 and 4. (Note.
The secondary winding current
rating for 230V section for
200W output may be chosen as
1 amp, and that for 600V ex-
tension forming inductor L1,
the current rating of the wind-
ing may be chosen as 300 mA.)
Output power. Using six MOSFETs
(three per bank) with proper heat sinks
with inverter transformer of 16-ampere
primary rating, the UPS-cum-EPS pro-
vides power up to 250 watts. With this
power, two computers with B&W moni-
tors, or one computer with colour moni-
tor and a small printer can be driven.
Using the same circuit, if you use ten
MOSFETs (five per bank) and inverter
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