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Fuse box with 4 monitored channels

From CaveLab
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This module was part of the rack box project — which was terminated when I moved out of my first apartment.
FB2 installed in the Rack box

A monitored fuse box that used to be installed in my rack box project, with 4 channels and 8 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.

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:

  • Fuse breaks
  • Optocoupler looses power
  • Signal to inverter is lost
  • Inverter output turns on
  • Failure LED for that channel lights up
  • 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. From LED power supply
  10. Alarm signal to Main monitoring unit
  11. Signal from external fuse


Schematic drawing


Parts list

Qty. Part
1 × Capacitor, aluminium electrolytic, 10 µF, 25V
1 × Capacitor, ceramic, 1 nF, 100V
1 × D-sub soldering cups, 25 pin female
2 × DIL socket, 16-pin, 7.62mm
1 × DIL socket, 6-pin, 7.62mm
8 × Diode, rectifier, 1 A, 400V, 1N4004
5 × Diode, small signal, 1N4148/Ph
1 × Enclosure, plastic (1591), 120x80x59mm
1 × Fuse 5x20 mm, 315 mA, time-delay
9 × Fuse holder, open, PCB, 5x20mm
8 × Fuse holder, open, PCB, Protective cover
1 × Hex inverter buffer, 4049, 6 channels, DIL16
1 × LED 5mm coloured clear, Green, 2.1V, 20mA, 30mcd, 10°
1 × LED 5mm, Orange, 2.0V, 20mA
1 × LED 5mm, Red, 2.0V, 20mA
4 × LED 5mm, Yellow, 2.0V, 25mA
2 × Mounting bracket, DIN rail, Plastic
1 × Optocoupler, 4-channels, PC817
1 × Optocoupler, single, CNY17F-3, DIL-6
32 cm2 PCB, stripboard, 100x160mm, 160cm2
15 × Resistor, carbon film, 0.25W, 330 Ω, 5%
5 × Resistor, carbon film, 0.25W, 4.7 kΩ, 5%
6 × Resistor, carbon film, 0.25W, 10 kΩ, 5%
2 × Resistor, metal film, 0.6W, 1 kΩ, 1%
2 × Terminal block, screw, 2.5 mm
5 × Transistor, NPN, 100 mA, 45V, 0.5W, BC547B