AC-powered devices are prevalent in residential, industrial, and commercial settings. Do you want a DIY-solution for determining when a fuse is blown in an AC-powered device? Check out Hans-Norbert Gerbig’s blown fuse indicator circuit from 2014.
 
Blown fuse indicator

The Circuit and Its Operation

The six-component blown fuse indicator circuit flashes an LED when a fuse in an AC-powered device has blown. It operates as follows: With the AC fuse intact, the circuit will be short-circuited and inactive. When the fuse blows, power is supplied to the circuit and the 22-μF capacitor begins to charge via the 470-kΩ resistor and the 1N4006 diode. All the time the capacitor voltage is below the DIAC trigger voltage the DIAC remains non-conducting.

"When the rising capacitor voltage reaches the DIAC’s trigger point, it conducts and discharges the capacitor through the LED and the 680-Ω series resistor which limits the LED current to a safe value. When the capacitor voltage drops below the DIAC trigger level it stops conducting, the LED extinguishes and the capacitor begins to charge up again and so the cycle repeats."

The entire blown fuse indicator circuit can be assembled on a small prototyping board. Once completed, the module can be connected in parallel with the AC fuse being monitored. When the equipment is powered on and the fuse is intact, the circuit remains inactive as it is effectively short-circuited. However, if the fuse blows, the DIAC flash circuit activates, drawing current and causing the LED to flash. Note that the circuit is not galvanically isolated from the AC supply, so take care to prevent any accidental contact with the circuit during operation.

The Original Blown Fuse Indicator Project

The article, “Blown Fuse Indicator,” appeared in Elektor July/August 2014. You can download the project article for free during the two weeks following the publication of this post. Let us know if you start a similar project!
Editor's Note: The article first appeared in a 2024 edition of ElektorMag. Some of the components, products, PCBs, and/or links might not be available anymore. Still, we feel this educational content is valuable, and we think it will inspire you to start new projects of your own.

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