High & Low voltage cut-off with delay& alarm
Description.
This
straight forward circuit will protect electrical appliances from over
voltage as well as under voltage. The circuit also produces an alarm
when the power supply comes back. An ideal circuit for home to protect
your valuable equipments from voltage fluctuations. The same circuit
with some modifications can be used to make a automatic voltage
stabilizer.
When
the mains voltage is in the normal level, the voltage at the negative
terminal of zener diode D4 will be less than 5.6 Volts. At this
condition transistor T1 will not conduct. The same time voltage at the
negative terminal of zener diode D5 will be greater than 5.6 and so the
transistor T2 will be conducting. The relay will be activated and the
green LED will be glowing.
When
the mains voltage is higher than the set limit the transistor T1
becomes conducting since the voltage at the negative terminal of D4 is
greater than 5.6 V. At the same time transistor T2 will be non
conducting which results in the deactivation of relay to cut the mains
supply from load. When the mains voltage is less than the set limit
transistors T1 & T2 becomes non conducting making the relay to
de-activate and cut the load from mains.
The
timer NE555 is wired as a monostable multivibrator with a pulse width
of 10ms.When the power comes back after a cut off a negative voltage is
obtained at the trigger pin which triggers the IC NE555. The transistor
T3 gets forward biased and it drives the buzzer to produce a beep as an
indication of power resumption. Also the transistor T1 is made on which
in turn makes T2 off. As a result the relay will remain de- activate for
10ms and this provides the sufficient delay and the equipment is
protected from surge voltages.
Circuit diagram with Parts list.Click to enlarge.
Notes.
- To calibrate the circuit a autotransformer is needed. Connect the output of autotransformer to the transformer primary.
- Set the voltage to 260V and adjust VR1 to make the relay deactivated.
- Now set the autotransformer to 160V and adjust VR2 so that the relay is de-energized.
- VR3 can be used to vary the volume of buzzer.
