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.
