Make An Indicating Fuse Box!

3d printed fuse box with leds that show if a fuse blew

3D Printed Indicating Fuse Box

BEHOLD! The indicating fuse box! Gone are the days that you must ask yourself, “Did the Earth stop spinning? Or did I just blow a fuse?!?” For eons, mankind has been stuck with boring fuse connections, BUT NO MORE!!! If a fuse goes out, the light next to it does as well. Easy peasy.

Actually, indicating fuse blocks are nothing new. In my day job, I spec and use them frequently in industrial control cabinets at chemical and gas plants. The idea behind them is simple: If a fuse blows, either the LED will turn ON or turn OFF depending on how you wire them (or buy them as the case might be).

I created this simple mini-project for one of my many 3D printers. A good chunk of printer control boards (for example, RAMPS 1.4) typically use things called “polyfuses” in the place of normal fuses. These are great in certain applications because they “blow” when the current gets too high and then automatically “reset” after power is removed. The issue I had with them is that they don’t actually blow when a certain current is reached, they blow when the polyfuse itself gets too warm. When you are pulling 15-20 amps through one of these for a heated bed, they tend to get warm….

Long story short, ACTUAL fuses don’t typically have the same problems as polyfuses. A typical fuse is rated to blow within 1-hour at continuous rated current (ex. 1 Amp through a 1 Amp fuse). As long as you derate your fuses by 20% or so, you are golden.

The beauty of actual fuses is that if you pick a rating that is slightly too low, it isn’t difficult to replace it with a higher rated fuse. In other words, if you find a “10A” fuse keeps blowing repeatedly (and your wiring can handle it), you can easily just pop in a “15A” fuse in its place. No soldering, no fans to keep a polyfuse cool, just pop in a new fuse.

What You’ll Need

Here’s what you will need for this project, complete with links for your convenience. With the possible exception of the fuse holders themselves, you can mix and match just about anything.

Electrical Bits:

  • 4x ATC Fuse Holders
    • You will need these specifically if you plan on using my 3D printed enclosure. I haven’t tested other makes, so I can’t guarantee other fuse holders will fit.
  • 4x ATC Fuses
    • Any will do as long as they fit. You don’t need waterproof ones. This set has several different sizes.
  • 4x 3mm Diffused LEDs
    • Any color will do as long as they are diffused (not perfectly clear)
  • 4x 470Ohm Resistors
    • Use 470Ohm for 12V systems, 1kOhm for 24V systems
  • 8x Female Spade Connectors
    • Used to attach wires to the fuse holders.
    • You could solder these connections, but be warned that with high current draw comes high heat. Soldered connections may fail.
  • Wire
    • The size you use depends on how much current you are pulling. Google “AWG ampacity chart” to see what you need. I used 14AWG here which is likely overkill, but I’d rather the fuse blow out before the insulation melts off an undersized wire. How much you’ll need also depends on your situation.
  • Solder and Flux
    • Use what you have, but I very highly suggest getting lead free solder and some flux paste for this.

Hardware:

  • 1x 3D printed Enclosure
    • You could similarly use a bit o’ tupperware or a project box if you’d like
  • 8x M3-12mm Machine Screws
    • These hold the fuse holders on.
    • Link is for a 160pc variety pack – it costs roughly the same as 100ct of just 12mm screws and we only need eight. Might as well stock up for other projects 😉
  • 4x M3-35mm Machine Screws
    • Really, anything over 20mm-45mm will work here. The holes in the 3D printed enclosure go all the way through the chassis.
    • Same deal with the link as before.
  • 8x M3 Nylock Nuts
    • We want these specifically so that we don’t have a nut work its way off and accidentally short something.

Tools:

  • Crimping Tool
    • If you have strong hands, you might be able to get away with pliers.
    • The link here is namely to give you an idea of what you’d need. You don’t need a $50 or $200 crimper here.
  • Soldering Iron
    • Not going to link here. There are a hundred different irons. I’d recommend getting a higher wattage one since we will be soldering largish-contacts.
  • Wire Strippers
    • Again, not linking. I’d recommend a nicer set simply because they are easier to use and last longer.

Circuit

The circuit we are going to be using will light an LED as long as the fuse is good. There is a variation of this where the LED (or a neon indicator) comes on when the fuse blows, but it doesn’t tend to work well with the DC voltage applications most hobby electronics revolve around.

Our circuit is fairly simple. On one side of each fuse is the incoming power (shown here as 12V). On the other side is the outgoing power and our little LED circuit. As long as the fuse hasn’t blown, current can flow from 12V, through the resistor and LED, and down to 0V.

The actual assembly and wiring is pretty simple. My assembly pics got deleted by my parrot chewing on my phone, so text only for now. I may make another fusebox soon and will update this post.

  1. Bolt each of the four fuse holders onto the lid using the 12mm M3 screws and the nylock nuts.
    • There will be a bit of slack to allow you some wiggle room – make sure everything is nicely aligned before tightening the nuts down completely.
  2. Place each LED into their corresponding holes with the long lead facing towards the fuse. Feel free to use a bit of glue if you’d like to keep them in place.

LED Indicator Wiring

  1. Solder the long (positive) lead of the LED to the closest fuse holder terminal. This can be done by simply bending the positive lead at a 90 degree angle to the LED.
    • You want to do this as close as possible to the base and not towards the end sticking out (we want to be able to push a spade connector on later).
    • Use plenty of flux. Chances are, the fuse holder terminal has oxidized from sitting on the shelf for a while.
    • Use only a small amount of solder. Too much, and the spade connector won’t slide on easily.
    • Trim any excess length of the positive lead
  2. Trim all but a centimeter or so from the negative lead of the LED and solder on the resistor.
  3. If you have it, slide a bit of heat shrink over any exposed metal between the resistor and the LED.
  4. After repeating steps 3-5 for each fuse holder, solder each of the remaining resistor ends together and to a wire.
    • This wire will need to go all the way to the 0V/GND of your power supply, so make sure it is long enough to reach.

Running Power

  1. On four separate wires, crimp the spade connector to one end of each. Slide the spade connector of each wire onto a different fuse holder on the side you soldered the LED circuit (the “right” side of the fuse on our schematic).
    • These wires will go to the positive power terminal on each of our devices, so make sure they are long enough to reach.
  2. From here, you have two options: You can either run four separate 12V wires for each power circuit or daisy chain all four together with one wire.
    • Either option will work for low currents. For higher currents, I would avoid daisy chaining.
    • If you choose to go with four separate wires, simply crimp a spade connector onto four separate wires and connect them to the open side of each fuse holder (ie, the “left” side of each fuse on our schematic).
    • If you choose to go with the daisy chained method:
      1. Cut one long length of wire that will extend from the fuse block to your power supply and three lengths of wire about 4cm long.
      2. Strip one end off the long wire and both ends off each shorter wires.
      3. Place one end of the long wire and one end of a shorter wire into the same spade connector and crimp them together. Stretched out, you should now have a long piece of wire and a short piece of wire held together with a connector.
      4. Repeat step 3 with the two other wires and connectors until you are left with one open end and one connector. Crimp the last connector on the end of the wire.
      5. After all is said and done, your assembly should look like this: long-connector-short-connector-short-connector-short-connector.

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