High Low Voltage Cutout Without Timer

This inexpensive circuit
can be connected to an air-conditioner/fridge or to any other
sophisticated electrical appliance for its protection. Generally, costly
voltage stabilizers are used with such appliances for maintaining
constant AC voltage. However, due to fluctuations in AC mains supply, a
regular ‘click’ sound in the relays is heard. The frequent
energisation/de-energisation of the relays leads to electrical noise and
shortening of the life of electrical appliances and the
relay/stabilizer itself. The costly yet fault-prone stabiliser may be
replaced by this inexpensive high-low cutout circuit with timer.

The circuit is so designed that relay RL1 gets energised when the
mains voltage is above 270V. This causes resistor R8 to be inserted in
series with the load and thereby dropping most of the voltage across it
and limiting the current through the appliance to a very low value. If
the input AC mains is less than 180 volts or so, the low-voltage cut-off
circuit interrupts the supply to the electrical appliance due to
energisation of relay RL2. After a preset time delay of one minute
(adjustable), it automatically tries again. If the input AC mains supply
is still low, the power to the appliance is again interrupted for
another one minute, and so on, until the mains supply comes within
limits (>180V AC).


Circuit diagram

The AC mains supply is resumed to appliance only when it is above
the lower limit. When the input AC mains increases beyond 270 volts,
preset VR1 is adjusted such that transistor T1 conducts and relay RL1
energises and resistance R8 gets connected in series with the electrical
appliance. This 10-kilo-ohm, 20W resistor produces a voltage drop of
approximately 200V, with the fridge as load. The value and wattage of
resistor R8 may be suitably chosen according to the electrical appliance
to be used. It is practically observed that after continuous use, the
value of resistor R8 changes with time, due to heating. So adjustment of
preset VR1 is needed two to three times in the beginning.

But once it attains a constant value, no further adjustment is
required. This is the only adjustment required in the beginning, which
is done using a variac. Further, the base voltage of transistor T2 is
adjusted with the help of preset VR2 so that it conducts up to the lower
limit of the input supply and cuts off when the input supply is less
than this limit (say, 180V). As a result, transistor T3 remains cut off
(with its collector remaining high) until the mains supply falls below
the lower limit, causing its collector voltage to fall. The collector of
transistor T3 is connected to the trigger point (pin 2) of IC1. When
the input is more than the lower limit, pin 2 of IC1 is nearly at +Vcc.

In this condition the output of IC1 is low, relay RL2 is
de-energised and power is supplied to the appliance through the N/C
terminals of relay RL2. If the mains supply is less than the lower
limit, pin 2 of IC1 becomes momentarily low (nearly ground potential)
and thus the output of IC1 changes state from ‘low’ to ‘high’, resulting
in energisation of relay RL2. As a result, power to the load/appliance
is cut off. Now, capacitor C2 starts charging through resistor R6 and
preset VR3. When the capacitor charges to (2/3)Vcc, IC1 changes state
from ‘high’ to ‘low’. The value of preset VR3 may be so adjusted that it
takes about one minute (or as desired) to charge capacitor C1 to

Relay is now de-energised and the power is supplied to the appliance
if the mains supply voltage has risen above the lower cut-off limit,
otherwise the next cycle repeats automatically. One additional advantage
of this circuit is that both relays are de-energised when the input AC
mains voltage lies within the specified limit and the normal supply is
extended to the appliance via the N/C contacts of both relays.

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