10-04-2012, 03:29 PM
OVER-/UNDER-VOLTAGE PROTECTION OF ELECTRICAL APPLIANCES
OVER-UNDER-VOLTAGE PROTECTION OF ELECTRICAL APPLIANCES.pdf (Size: 54.11 KB / Downloads: 128)
This circuit protects refrigerators as
well as other appliances from overand
under-voltage. Operational amplifier
IC LM324 (IC2) is used here as a
comparator. IC LM324 consists of four operational
amplifiers, of which only two operational
amplifiers (N1 and N2) are used
in the circuit.
The unregulated power supply is connected
to the series combination of resistors
R1 and R2 and potmeter VR1. The
same supply is also connected to a 6.8V
zener diode (ZD1) through resistor R3.
Preset VR1 is adjusted such that for
the normal supply of 180V to 240V, the
voltage at the non-inverting terminal (pin
3) of operational amplifier N1 is less than
6.8V. Hence the output of the operational
amplifier is zero and transistor T1 remains
off. The relay, which is connected to the
collector of transistor T1, also remains deenergised.
As the AC supply to the electrical
appliances is given through the normally
closed (N/C) terminal of the relay,
the supply is not disconnected during normal
operation.
When the AC voltage increases beyond
240V, the voltage at the non-inverting terminal
(pin 3) of operational amplifier N1
increases. The voltage at the inverting terminal
is still 6.8V because of the zener
diode. Thus now if the voltage at pin 3 of
the operational amplifier is higher than
6.8V, the output of the operational amplifier
goes high to drive transistor T1 and
hence energise relay RL. Consequently, the
AC supply is disconnected and electrical
appliances turn off. Thus the appliances
are protected against over-voltage.
Now let’s consider the under-voltage
condition. When the line voltage is below
180V, the voltage at the inverting terminal
(pin 6) of operational amplifier N2 is less
than the voltage at the non-inverting terminal
(6V). Thus the output of operational
amplifier N2 goes high and it
energises the relay through transistor T1.
The AC supply is disconnected and electrical
appliances turn off. Thus the appliances
are protected against under-voltage.
IC1 is wired for a regulated 12V supply.
Thus the relay energises in two conditions:
first, if the voltage at pin 3 of IC2 is
above 6.8V, and second, if the voltage at
pin 6 of IC2 is below 6V. Over-voltage and
under-voltage levels can be adjusted using
presets VR1 and VR2, respectively.
This circuit costs around Rs 110.