Specifications

CONSTRUCTION
S.C. DWIVEDI
MOSFET-BASED 50Hz
SINEWAVE UPS-CUM-EPS
interruption in operation of a computer
or continuity of the play mode of a VCR
and TV is caused. The complete schematic
diagram of the circuit is shown in Fig. 2.
When mains is present and is within
the specified limits, the same is fed to the
load. At the same time, battery is charged.
If mains voltage goes below 170 volts (or
mains power fails) or above 270 volts, sys-
tem changes over from mains to back-up
mode. In the back-up mode, battery volt-
age of 12V DC is converted into 230V AC
and applied to the load within 1 milli-
second.
However, if battery voltage drops be-
low 10V DC, or output voltage goes below
225V AC, there will be a visual and au-
dible indication of low-battery state. Dur-
ing this warning period, one can save the
data and switch off the computer safely.
But during the low-battery indication, if
the computer or load is not switched off,
it remains on back-up mode. After the
end of back-up time, system switches off
automatically, due to activation of
battery’s deep discharge cut-out circuit,
which reduces the power consumption
from the battery to a negligible value (only
90 mA).
Inverter control circuit. It uses the
basic squarewave (astable multivibrator)
oscillator employing IC 555, with 5.1V
supply voltage derived from 12V battery
by using 5.1V zener ZD3 in series with a
resistance. Astable multivibrator is de-
signed for a frequency of 100 Hz, which
can be varied above or below 100 Hz us-
ing preset PR1. The frequency ‘f’ of astable
multivibrator is given by the relation-
ship:
where RB = In-circuit resistance of pre-
set PR1.
If R
A
=220 ohms and R
B
=15 kilo-
ohms, then f=100 Hz. Due to the
tolarance of the component values, ob-
served frequency may not be exactly
equal to 100 Hz, and therefore preset PR1
may need to be suitably adjusted.
The output of the astable
multivibrator is given to pin 5 of the
bistable multivibrator wired around IC
Fig. 1: Proposed front and rear panal layouts
R.V. DHEKALE AND S.D. PHADKE
M
ost of the UPS (uninterrupted
power supplies) available in
the market internally use a fre-
quency ranging from 100 Hz to 50 kHz.
The regulation of output voltage is done
using the pulse width modulation tech-
nique, which produces a quasi-square
waveform output from the inverter trans-
former. Such an output waveform produces
lots of noise, which is not desirable for a
computer and other sensitive equipment.
This voltage waveform can drive a com-
puter, but not the tubelight, fan, EPBAX,
TV, VCR, etc properly. The advantage of-
fered by a UPS is that its changeover pe-
riod is quite low, so that the computer or
any other sensitive load is not interrupted
during the mains failure.
EPS (emergency power supply) of vari-
ous brands, providing 50Hz squarewave
output, can drive the computer, tubelight,
TV, VCR, fan, etc, but considerable noise
is produced from the EPS or the load.
Another drawback of an EPS is that its
changeover period is relatively high, so
the computer may get reset or continuity
of the play mode of the VCR may get
interrupted on mains failure.
The circuit
A 50Hz sinewave offline UPS-cum-EPS
circuit is presented here which produces
a sinewave output with very low noise
level. It drives the equipment/load (< 250
watts), which normally operates on 230V,
50Hz AC. Changeover period of this sys-
tem is less than 1 millisecond so that no
PARTS LIST
Semiconductors:
IC1 - NE555 timer
IC2 - 7473 dual JK-flip-flop
IC3 - 7812 regulator, 12V
IC4 - DB107 bridge rectifier, 1A
IC5 - TL062 dual op-amp
IC6, IC7 - µA741 op-amp
D1,D2, D5,
D6, D7 - 1N4148 switching diode
D3, D3',
D4, D4’ - 1N5408 rectifier diode, 3A
D8 - Rectifier diode, 16A–(TO3)
ZD1-ZD7 - 5.1V, 1W zener diode
T1-T3 - BEL187 npn transistor
TR - BT136 triac
SCR1 - BT169 SCR
M1-M6 - IRF250 MOSFET
LED1-LED6 - 3mm LED
Resistors (all ¼-watt, ±5% carbon, unless
stated otherwise):
R
A
- 220-ohm
R
C
- 4.7-ohm, W/W 20W
R1, R3-R6, R8
R12-R15, R18
R20, R21 - 1-kilo-ohm
R2 - 100-ohm, 1W
- 4.7-kilo-ohm
R7 - 4.7-ohm
R9 - 220-ohm
R10, R11 - 68-kilo-ohm
R16 - 100-ohm
R17 - 470-ohm
R19, R23 - 1.2-kilo-ohm
R22 - 39-kilo-ohm
PR1 (R
B
)
PR2, PR3, - 22-kilo-ohm preset
PR6 - 10-kilo-ohm preset
PR4, PR5 - 1-Meg-ohm preset
Capacitors:
C1 - 0.01µF ceramic disk
C2 - 3 x 0.47µF, 600V polyester
C3, C13 - 0.1µF ceramic disk
C4-C7 - 1000µF, 16V electrolytic
C8 - 100µF, 25V electrolytic
C9 - 2200µF, 40V electrolytic
C10 - 0.47µF, 25V electrolytic
C11, C12 - 0.22µF polyester
Miscellaneous:
F1 - 5A cartridge fuse
F2 - 16A cartrige (slow-blow) fuse
X1 - Primary 9-0-9V/20A
Sec. 230V section (1A),
Sec. 600V section (300mA
used as L1) transformer
X2 - 230V AC primary to 16V-0-
16V, 3A secondary trans-
former.
X3 - 230V AC primary to 0-12V,
500mA secondary transformer
SW1 - DPDT switch, 5A
SW2 - SPDT slide switch
CB - MCCB 4A
BZ1 - Piezo buzzer
L1 - 1 Henry (part of X1, 600V tap-
ping)
L2, L3 - 100 µH (20T, 22SWG, air-core,
8mm dia.)
RLY - 57DP-12-2C2 O/E/N 12V, 150-
ohm (2-changeover) relay
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