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This
over/under voltage cut-out will save your costly electrical and
electronic appliances from the adverse effects of very high and
very low mains voltages. The circuit features auto reset and
utilises easily available components. It makes use of the
comparators available inside 555 timer ICs. Supply is tapped from
different points of the power supply circuit for relay and control
circuit operation to achieve reliability. The circuit utilises
comparator 2 for control while comparator 1 output (connected to
reset pin R) is kept low by shorting pins 5 and 6 of 555 IC. The
positive input pin of comparator 2 is at 1/3rd of Vcc voltage .
Thus as long as negative input pin 2 is less positive than 1/3 Vcc,
comparator 2 output is high and the internal flip-flop is set,
i.e. its Q output (pin 3) is high. At the same time pin 7 is in
high impedance state and LED connected to pin 7 is therefore off.
The output (at pin 3) reverses (goes low) when pin 2 is taken more
positive than 1/3 Vcc. At the same time pin 7 goes low (as Q
ouptput* of internal flip- flop is high) and the LED connected to
pin 7 is lit. Both timers (IC1 and IC2) are configured to function
in the same fashion. Preset VR1 is adjusted for under voltage (say
160 volts) cut-out by observing that LED1 just lights up when
mains voltage is slightly greater than 160V AC. At this setting
the output at pin 3 of IC1 is low and transistor T1 is in cut-off
state. As a result RESET* pin 4 of IC2 is held high since it is
connected to Vcc via 100 kilo-ohm resistor R4. Preset VR2 is
adjusted for over voltage (say 270V AC) cut-out by observing that
LED2 just extinguishes when the mains voltage is slightly less
than 270V AC. With RESET* pin 4 of IC2 high, the output pin 3 is
also high. As a result transistor T2 conducts and energises relay
RL1, connecting load to power supply via its N/O contacts. This is
the situation as long as mains voltage is greater than 160V AC but
less than 270V AC. When mains voltage goes beyond 270V AC, it
causes output pin 3 of IC2 to go low and cut-off transistor T2 and
de-energise relay RL1, in spite of RESET* pin 4 still being high.
When mains voltage goes below 160V AC, IC1’s pin 3 goes high and
LED1 is extinguished. The high output at pin 3 results in
conduction of transistor T1. As a result collector of transistor
T1 as also RESET* pin 4 of IC2 are pulled low. Thus output of IC2
goes low and transistor T2 does not conduct. As a result relay RL1
is de-energised, which causes load to be disconnected from the
supply. When mains voltage again goes beyond 160V AC (but less
than 270V AC) the relay again energises to connect the load to
power supply
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