Is it a good idea to include thermal fuses in a DIY 3D printer design?

Question

3D printing should be relatively safe, however, the inherent nature of 3D printers, with all of the heated parts, constitutes a fire risk. A well designed 3D printer should be designed to be as safe as possible, especially one used in the home... Yes, the recommendation is, when printing, to watch the 3D printer at all times and never leave a print unattended. However, with some print times lasting hours and days, this is not always feasible, nor practical. So, some inbuilt safety features should be included, to at least mitigate the risk of fire, to some extent.

• Is the use of thermal fuses^1,2 a good idea³?

• Would you use more than one?

• Where should one place a thermal fuse? Next to a particular component, or free standing, in the air, to get an average, rather than highly localised temperature?

• Against which components should a thermal fuse be placed? There are a number of places to choose from, such as next to:

  • The hotend?
  • The heated bed?
  • The extruder?
  • Each of the stepper motors?
  • The power supply?
  • The RAMPS stepper motor drivers?

• Of lesser import, which type should one use⁴, radial or axial?

Has anyone added thermal fuses to their 3D printers? Or has anyone examined where the thermal fuses are placed in commercial 3D printer designs, if used at all?

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Background

I have recently found myself having to repair rice cookers and fans in Thailand. In those, it is very often the thermal fuse (axial thermal fuses for the rice cookers and the square "radial" types for fans) that requires replacing, as they have blown before the device got hot enough to start a fire. This got me thinking about their use in a 3D printer.

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Footnotes

¹ We are not talking about the standard, replaceable, thermo-fuse,or fuse, which blow upon a current surge, short-circuit, etc. These are thermal fuses that contain metal connector within them that melts (permanently) at a specific temperature (typically ~135°C), thereby breaking the circuit.

² Nor am I referring to resettable fuses (AKA PPTC, multifuse, polyfuse or polyswitch)

³ Would a thermal fuse be preferable to thermal cut offs, in the case of fire?

⁴ The thermal fuses used in rice cookers are the axial type, and in the motors of fans are the radial type.

Answer 1

Whether you should use a thermal fuse or not depends on what other safety measures you've taken. You can't look at the safety features of a printer in isolation, you need to look at what other measures are in place.

The main fire hazard in printers is unfortunately (still) the fact that some manufacturers use underrated connectors on their boards, and that some users put bare wires in screw terminals or use inadequate torque when tightening terminals. As the wire works itself loose, it starts arcing and burning the connector. A thermal fuse does not help in this situation (unless you place thermal fuses near all of the connectors, which is impractical). Instead:

• Properly tighten screw terminals, check them, and consider using proper wire termination (crimp lugs).

• Use strain relief on wires. Make sure wires don't rub against anything, and guide them so they do not bend in a tight radius. Since the extruder (or print bed) is constantly moving, those wires are subject to fatigue.

• Make sure connectors (especially those for the heated bed) are rated for the current running through them, and solder wires directly to the board if necessary.

Using a regular fuse may protect against wires shorting against each other should their insulation be damaged. Fuses are usually already integrated into the main board.

Most firmwares include some variant of thermal runaway protection, a feature that monitors the heaters and shuts the printer down if it notices something gone wrong. This protects against:

• The thermistor coming loose/reading incorrect values/etc...

but not against:

• Bugs in the firmware itself

• Failure of the MOSFET

Most printers use MOSFETs to switch power to the heating element. Unfortunately, when MOSFETs fail, they usually fail closed (i.e. conducting). This means that, even if the firmware detects something has gone wrong, it won't be able to do anything about it. Solid State Relays (TRIACs) can fail in the same way.

To protect against this, mounting a thermal fuse (or resettable bimetallic switch*) on the heated bed may be a good idea. However, thermal fuses with ratings up to the operating temperature of a hotend do not appear to be available so this is not an option.

Attaching the fuse physically to the part it is monitoring is the most reliable, but for instance with the hotend (if you wanted to protect it all) this might not be feasible to the high temperatures involved so you'd have to settle with monitoring the air temperature close by.

Also consider thermal balancing. A thermal fuse is unnecessary if the component can not overheat to begin with. For instance, most MK2 heated beds struggle getting up to even 100C, so even with a shorted MOSFET they present no danger. However, if you have a powerful high wattage (mains-powered) heated bed, you should definitely install thermal protection.

E3D supplies their hot ends with 25W, 30W and 40W heaters. The 25W heater is the safer choice, since it limits the maximum temperature the hot end can get to, while with the 40W heater you can reach higher temperatures (and reach them faster). Barring a very unlikely scenario in which simultaneously (1) the power supply fails and starts supplying excessive voltage and (2) the MOSFET and/or firmware fails, a heater that is sized appropriately to the load it is driving can never pose any danger.

I don't think it's common to install thermal fuses on steppers, stepper drives or the power supply (which should have its own protection). For every possible location to place a thermal fuse, you can probably think up a failure mode in which that fuse would save the day, but at a certain point it just becomes overkill. The stepper drivers would likely burn out well before the steppers would get hot enough to pose a threat, and overheating of the stepper driver would probably (violently) destroy it but afterwards it should not pose any threat.

Axial v.s. radial does not matter, just use whatever is convenient for your situation.

^{* Note that some bimetallic switches short one of the leads to the (metal) case when tripped, which poses a danger, especially with mains-powered heaters.}

Answer 2

Yes, you should use a thermal fuse. But only to protect the board itself from an overheating condition. PTC fuses however; is a no.

The PTC resettable fuse after it trips the first time, doesn't return to it's former low impedence state for a long time. Sometimes hours, sometimes days, sometimes years. As it maintains its higher resistance even though the system is not in an over heated state, the fuse will trip faster the next time. This could lead to false positives during print or other operation.

The resetting will often not take place even if the fault alone has been removed with the power still flowing as the operating current may be above the holding current of the PPTC. The device may not return to its original resistance value; it will most likely stabilize at a significantly higher resistance (up to 4 times initial value). It could take hours, days, weeks or even years for the device to return to a resistance value similar to its original value, if at all.

https://en.wikipedia.org/wiki/Resettable_fuse

Currently, I am investigating the use of a hall effect current sensor and some circuitry. Based on the duration and magnitude of the current, a small capacitor is charged. When a threshold is reached (as determined by a voltage comparitor), the power is withdrawn to let the capacitor discharge (and the heating element cool). This should keep the heating element from reaching a temp above a configurable maximum.

Answer 3

Yes, measures which decrease the chance of a fire are a good idea.

In a DIY project, such a fuse is indeed a good idea. I use a printer/software/firmware package which monitors the temperature and fails open (removing current) if readings are out of range; however, there is also hardware protection (a thermal snap switch) just in case the other precautions fail.

Answer 4

IMHO - if you seek hands-off unattended fire safety you'd be looking at something a bit more specific to that job than measuring or limiting the temperature of printer components. That's great, as far as it goes, when it works, but it's not anything like a complete solution. Cutting off the electrical supply won't put out a fire that has progressed to burning plastic, etc. Reasonable over-current protection for a start. Contain the printer inside a metal cabinet or cementboard-lined closet (a non-flammable enclosure), add a heat and/or smoke detector in the top of that enclosure, and rig that to not only cut off power to the printer, but (given things have gone far enough wrong to set that off) also set off an extinguishing means (the effectiveness of which will be enhanced by an enclosure.)

A little bit of looking finds (for predictably high prices, sorry) that there are, indeed, fire extinguishers with a "sprinker-head" valve attached for this type of "spot suppression" job that would cover the "set off an extinguisher" part of the job without need of any power. You'd still want a heat detector that would shut off the electrical supply to the printer in the event of abnormally high temperatures in the enclosure, and you might want a smoke detector doing the same.

This is probably representative of about 0% of actual installations at present, but it would be a good way to reduce the risk in the event of a serious malfunction. Presumably the current distribution of risk looks a lot like the maker stating:

"the recommendation is, when printing, to watch the 3D printer at all times and never leave a print unattended."

And then the user choosing not to do so, putting the responsibility for any bad result on the user as not following the maker's recommendation, however impractical.

Answer 5

I have a Thing-O-Matic which included a thermal fuse above the heat break.

For my home-brew printer (a large delta machine) I have a smoke alarm and a fire alarm mounted under the top of the print volume. These are wired together and give me a NO and NC dry contact. My plan is to connect this to a power kill circuit. I haven't found a good way to flood the entire machine, including the electronics, with CO2 gas or another extinguisher when the alarm sounds.

I've added some additional information on parts purchasable on DigiKey: The first is a 10 A, 152 C thermal fuse rated for line voltage. This would be appropriate for bonding to a heated bed. The second is rated to trip at 260 C. Both of these are less than $1.50 each.

Answer 6

I haven't found a good way to flood the entire machine, including the electronics, with CO2 gas or another extinguisher when the alarm sounds.

A servo or some other actuator attached to a fire extinguisher, and then attach it to a thermocouple or some other sensor. Maybe a knockoff Arduino, which the small ones are 2 bucks a pop. Since Arduinos are open source, you aren't pirating anything, so clear conscience.

Answer 7

I'm going to set this up for my printer: 3D Printer Safety Shutdown - Smoke Detector

Looks like a great solution for preventing fires.