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Voltage
variations and power cuts adversely affect various equip- ment
such as TVs, VCRs, music systems and refrigerators. This simple
circuit will protect the costly equipment from high as well as low
voltages and the voltage surges (when power resumes). It also
gives a melodious tune when mains power resumes. When mains
voltage is normal, the DC voltage at the cathode of zener diode D4
is less then 5.6V. As a result transistor T1 is in ‘off’ state.
The DC voltage at the cathode of zener diode D5 is greater than
5.6V and as a result transistor T2 is in ‘on’ state. Consequently,
relay RL1 gets energised, which is indicated by lighting up of
green LED. Under high mains voltage condition, transistor T1
switches to ‘on’ state because the voltage at cathode of zener
diode D4 becomes greater than 5.6V. Consequently, transistor T2
switches to ‘off’ state, making the relay to de-energise Under low
mains voltage condition, transistor T1 switches to ‘off’ state and
as a result transistor T2 also switches to ‘off’ state, making the
relay to de-energise.
Timer IC 555 in the circuit is configured to operate in a
monostable mode. The pulse width is about 10 seconds with the
timing component values used in the circuit. When the power
resumes after a break, pin 2 of IC 555 goes low briefly and this
triggers it. Its output makes music IC UM66 to operate through
transistor T3. Simultaneously, transistor T1 also gets forward
biased as the monostable IC1 output is connected to its base via
diode D8 and resistor R7. As a result, transistor T1 conducts and
biases transistor T2 to cut off. Thus relay RL1 remains de-energised
for the duration of mono pulse and the load is protected against
the voltage surges.
To adjust presets VR1 and VR2, you may use a manually variable
auto-transformer. Set the output of auto-transformer to 270V AC
and connect it to the primary of transformer X1. Adjust preset VR1
such that relay RL1 just de-energises. Next set the output of
auto-transformer to 170V AC. Now adjust preset VR2 such that relay
RL1 again de-energises. Volume control VR3 may be adjusted for the
desired output volume of the tune generated by IC UM66
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