Simple blown-fuse indicator sounds an alarm
A simple blown-fuse-indicator circuit sounds a buzzer and flashes an LED when a fuse blows.
Vladimir Oleynik, Moscow, Russia; Edited by Martin Rowe and Fran Granville -- EDN, July 10, 2008
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 30V 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 4V dc is sufficient for the indicator to operate.
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.
-
Would it be possible to use this with a ptc or polyfuse? I wonder what effect it would have when tripped.
Chris - 2012-8-2 06:29:37 PST -
Good point about the fuse being in the negative, but it is trivial to invert the polarity of the components (NPN > PNP, 78L05 > 79L05 etc) and use the circuit with a fuse in the positive supply.
Alex Hiley - 2008-16-7 06:05:00 PDT -
Blown fuse indicator looks good, BUT since when is a fuse in the negative line. (Normally grounded)
Wouldn't the fuse be bypassed when other equipment is connected as a load?
Robert DeGowin - 2008-11-7 00:50:00 PDT
