Thermal fuses and 3D Printers

Preamble

I had the idea of employing thermal fuses in my 3D printer designs, after reading Build a 3D printer workhorse, not an amazing disappointment machine. Coincidently, I had been replacing thermal fuses, when repairing fans and rice cookers in Bangkok, and so I was familiar with them.

The question that came to mind, however, was “Where would you place a thermal fuse, and would you use more than one?”. Would you use them at the power supply, on the heat bed, on the hot end, for each of the stepper motors?

From Build a 3D printer workhorse, not an amazing disappointment machine:

It’s rare, but 3D printers can catch fire. Use the safety features provided by the firmware, but don’t solely rely on them. Both plain MOSFETs and solid state relays typically fail in their conducting state, which can result in runaway heating with disastrous outcomes. Thermal cutoff fuses are $1 components, but they are well able to prevent a runway heated bed from turning your workshop into a crater.

Note: A thermal fuse is not the same as a thermo fuse, which is a regular type of fuse that goes open circuit, when excessive current is pulled. A thermal fuse goes open circuit when the temperature reaches a certain point.

Animation of the Operation of the Elcut Brand Thermal Fuse
Animation of the Operation of the Elcut Brand Thermal Fuse

They, generally, require replacement once they have been triggered. From Elcut Brand Thermal Fuse:

How a thermal fuse works
How a thermal fuse works

Thermal fuses come in a variety of forms, axial and radial:

Various thermal fuses
Various thermal fuses

There are two main types of thermal fuse that I had been using.

In fans:

In rice cookers:

50-Pcs-Fuji-Microtemp-Thermal-Fuse-115℃-TF

Links

Some links, which were thrown up by a google search: where to place a thermal fuse on 3d printer:

1) 240°C is a really the highest you can get for thermal fuse (not easily), which might be ok for our hotend but barely. You are right appliances do use cutoffs, but lets say your dryer, probably about 160°C fuse
2) Where to put it? There is not a good location for it, we considered placing it near the end effector board, with lower cutoff, but realistically by time that blows, its too late.

We are working on coming up with solutions that would protect against thermal runaway that dont involve a thermal fuse.

  1. I’m adding thermal fuses (170C?) on my heatsinks. Last night my hotend was on without me even sending it a file to print or pulling up the control software. The controller was not regulating the temperature or turning on fans. So, the heatsink was probably just about as hot as the heater block. This should never happen and if the heatsink does get this hot then something is seriously wrong. A controller based solution to this problem is not sufficient.
  2. Thermal fuses on heated beds. This was always planned but I’m listing it here for completeness.
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