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Blown
Fuse Indicator
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| Generally,
when an equipment indicates no power, the cause may be
just a blown fuse. Here is a circuit that shows the condition
of fuse through LEDs. This compact circuit is very useful
and reliable. It uses very few components, which makes
it inexpensive too. |
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Under
normal conditions (when fuse is alright), voltage drop
in first arm is 2V + (2 x 0.7V) = 3.4V, whereas in second
arm it is only 2V. So current flows through the second
arm, i.e. through the green LED, causing it to glow;
whereas the red LED remains off.
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| When
the fuse blows off, the supply to green LED gets blocked,
and because only one LED is in the circuit, the red LED
glows. In case of power failure, both LEDs remain ‘off’. |
| This
circuit can be easily modified to produce a siren in fuse-blown
condition (see Fig. 2). An optocoupler is used to trigger
the siren. When the fuse blows, red LED glows. Simultaneously
it switches ‘on’ the siren. |
| In
place of a bicolour LED, two LEDs of red and green colour
can be used. Similarly, only one diode in place of D1
and D2 may be used. Two diodes are used to increase the
voltage drop, since the two LEDs may produce different
voltage drops. |
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