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Power-Supply
Failure Alarm
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| Most
of the power-supply failure indicator circuits need a
separate power-supply for themselves. But the alarm circuit
presented here needs no additional supply source. It employs
an electrolytic capacitor to store adequate charge, to
feed power to the alarm circuit which sounds an alarm
for a reasonable duration when the mains supply fails.
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| During
the presence of mains power supply, the rectified mains
voltage is stepped down to a required low level. A zener
is used to limit the filtered voltage to 15-volt level.
Mains presence is indicated by an LED. The low-level DC
is used for charging capacitor C3 and reverse biasing
switching transistor T1.
Thus, transistor T1 remains cut-off as long as
the mains supply is present. As soon as the mains power
fails, the charge stored in the capacitor acts as a power-supply
source for transistor T1. Since, in the absence of mains
supply, the base of transistor is pulled ‘low’ via resistor
R8, it conducts and sounds the buzzer (alarm) to give
a warning of the power-failure. |
| With
the value of C3 as shown, a good-quality buzzer would
sound for about a minute. By increasing or decreasing
the value of capacitor C3, this time can be altered to
serve one’s need. |
| Assembly
is quite easy. The values of the components are not critical.
If the alarm circuit is powered from any external DC power-supply
source, the mains-supply section up to points ‘P’ and
‘M’ can be omitted from the circuit. Following points
may be noted: |
| 1.)
At a higher DC
voltage level, transistor T1 (BC558) may pass some collector-to-emitter
leakage current, causing a continuous murmuring sound
from the buzzer. In that case, replace it with some low-gain
transistor. |
| 2.)
Piezo buzzer must be a continuous tone version, with built-in
oscillator. |
| To
save space, one may use five small-sized 1000µF capacitors
(in parallel) in place of bulky high-value capacitor C3. |
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