Fuse box front
Fuse box front with LEDs and fuses.

This fuse box module used to be installed in my rack box project. It had a total of 4 channels, or outputs, each with fuses on both polarities. All fuses were monitored, meaning that if a fuse broke a failure LED would turn on and a fuse failed output activate. In addition to the internal fuses, this module could also monitor an external fuse.

Table of content


Each pair of fuses powered an optocoupler, which again were connected to an inverter. So if any of the fuses broke the optocoupler would loose power, meaning that the inverter input would go LOW, and the inverter output HIGH. Each inverter output was connected to a failure LED and a OR gate, which was used to alert another monitoring module of a failed fuse. So the failure LED would indicate which circuit was broken, and the fuse failed output signal just meant that one or more circuits were broken.

So, in summary; this is what happened when a fuse broke:

  1. Fuse breaks
  2. Optocoupler looses power
  3. Signal to inverter is lost
  4. Inverter output turns on
  5. Failure LED for that channel lights up
  6. Fuse failure output is activated, alerting other monitoring modules

Fuse failure LED

The left amber LED would blink in the case of a fuse failure, regardless of where that failure was. The LED was controlled by another monitoring module, and shared between multiple modules.


The solution worked just fine for loads like light bulbs, LEDs etc. But turned out to be a bit strange if electronics were connected, like a microcontroller. The current would find ways to leak through the connected module and back into the output of the fuse box. Which meant that the alarm would turn off. One way to remedy this is to place a diode in series with the outputs from the fuse box, but this introduces a voltage drop.

In my setup I had multiple voltage levels, from multiple power supplies. All their ground terminals where connected together, so I had one common ground. By having fuses on the ground supply I introduced a way that this common ground could break. And that produced some pretty strange results, like microcontrollers powered themselves from low inputs.

Lastly; it’s a complex and time consuming solution to solve a pretty simple task.

I ended up removing this module from the rack box project, and instead put small fuses inside the modules I was building.

D-Sub 25-pin

  1. 5V fuse 1 +
  2. 5V fuse 1 -
  3. 5V fuse 2 +
  4. 5V fuse 2 -
  5. 5V fuse 3 +
  6. 5V fuse 3 -
  7. 5V fuse 4 +
  8. 5V fuse 4 -
  9. Signal from external fuse +
  10. Fuse failure alarm output
  11. Fuse failure LED input

Schematic drawing

Fuse box 2 circuit
Schematic drawing for fuse box with monitoring #2 module.

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Parts list

Project: Fuse box with 4 monitored channels by Thomas Jensen is licensed under CC BY-SA 4.0 with attribution required.

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  •   Created May 06, 2006
  •   Last modified 5 days ago
  • Project status: Completed