Isolated Fuse Fail Indicator

This circuit uses standard components and shows a method of indicating the fuse status of mains powered equipment while providing electrical isolation from the mains supply. A standard miniature low power mains transformer (e.g. with an output of around 6 V at 1.5 VA) is used as a ‘sense’ transformer with its primary winding (230 V) connected across the equipment’s input fuse so that when the fuse blows, mains voltage is applied to the transformer and a 6 V ac output voltage appears at the secondary winding. The 1N4148 diode rectifies this voltage and the LED lights to indicate that the fuse has failed.

The rectified voltage is now connected to an RC low-pass filter formed by the 10 kΩ resistor and 100 nF capacitor. The resulting positive signal can now be used as an input to an A/D converter or as a digital input to a microcontroller (make sure that the signal level is within the microcontroller input voltage level specification). The 1 MΩ resistor is used to discharge the capacitor if the input impedance of the connected equipment is very high. As long as the fuse remains intact it will short out the primary winding of the ‘sense’ transformer so that its secondary output is zero.