Safety fuses or fusible links see wide use in modern electronic equipment to protect the load and the power supply – especially batteries – against short circuits and excessive load current. Fuses are inexpensive and simple, and a wide range of parts is available. However, you must replace them when they blow, and, when they do, you need an indicating circuit that warns you about its failure, especially when the fuse body is ceramic or sand-filled for improved protection against arcing.
The circuit in Figure 1 signals that a fuse has blown. Input voltage ranges from 4 to 30 V dc. The input range of the 78L05 voltage regulator determines the high limit; the lower one is less than the input range of the voltage regulator, but 4 V dc is sufficient for the indicator to operate.
 |
|
| Figure 1. | When fuse F1 blows, the transistor biases on, sounding the buzzer and powering D2. |
When fuse F1 is in good order, diode D1 is forward-biased, but its forward voltage is insufficient to bias forward-flashing diode D2 and the Q1’s base-emitter junction. The self-driven HCM1206X buzzer is off, and the flashing diode does not flash. So, the alarm circuit is in standby mode. When F1 blows, it no longer bridges the base-emitter-flashing-LED network. The 1-kΩ resistor forward biases D2 and Q1’s base-emitter junction, forcing the buzzer to sound at a low frequency equal to the flashing frequency of D2. During circuit operation, the 0.1-µF capacitor eliminates the buzzer’s “tinkling” when the flashing LED is in the off state.
