This is indicator circuit to indicate that there is blown fuse, it will show the condition of fuse through LEDs. This circuit works for fuse 220V mains. This compact circuit is very useful and reliable. It is a low cost circuit which use very few components. Generally, when an electronic device indicates no power, the potential cause may be just a blown fuse.
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.
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”.
- RED LED : the fuse blows
- GREEN LED : normal condition, the fuse is OK
- Both LED Off : power failure, the mains is off or the circuit is broken.
This circuit can be easily modified by adding alarm/buzzer circuit 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.