Category Archives: Safety equipment

Cheap Parts Aren’t Worth the Money

In the previous post I talked about a fire safety circuit I’m building for a 3D printer.  One essential piece of that design was the inclusion of a thermal fuse.  The idea being that if things get too hot, the fuse will break the circuit and we can avoid starting any unintended fires.

I was getting ready to assemble part of this project and before I put everything together, I thought maybe I should test the fuses to see if they actually behave as expected.  Since you’ve read this far, you can probably guess the answer to that question.

Thermal Fuses?  Or solid wire?  In this case, there is no difference.

A good thermal fuse will blow at the design temperature, plus or minus a little bit.  A faulty one can blow early when the temperature is still below the trigger point.  A REALLY faulty one will blow at some temperature above the trigger point.  An INCREDIBLY faulty one won’t blow even though other parts of the circuit are already melting.

Care to hazard a guess as to which one I got?  That’s right… I ended up with a bag of INCREDIBLY faulty thermal fuses.  By spec, these fuses are rated to blow at 100 degrees Celsius.  I figured that if my circuit were hot enough to boil water, then it was too hot.

As a test, I setup a circuit with a temperature sensor and one of these thermal fuses side-by-side.  Then I pointed a heat gun at them and watched the temperature rise. I was expecting the fuse to blow somewhere between 95 and 105 degrees but much to my surprise, it was still going strong at a blazing

206 degrees Celsius!

For those of you that don’t like metric, 206 Celsius is 402 degrees Fahrenheit! In other words, I could put this thing in the oven next to my frozen pizza and it would happily continue to pass current while the cheese melted and the crust browned.

It’s a bummer that I can’t build the project the way I originally envisioned but on the other hand, I guess I saved myself some design and assembly time.

What is the takeaway from this experience?  The ability to order any kind of electrical component off of Amazon and have it show up at your house a week or two later is great but don’t assume that what you receive is what you want.  Test it and make sure.

3D Printer Fire Safety

I have a sneaking suspicion that Santa is bringing me a 3D printer for Christmas. (I sat next to Teri while she ordered the right one, so it isn’t a real stretch to think that it might happen. 🙂

Until next week when I can actually play with it er… investigate the capabilities of this new tool, I’m trying to get everything ready.  During this preparation, I read a few items about printer malfunctions that either caused a fire or nearly did so.  3D printers have high-temperature components and hardware problems can result in fire, though this is admittedly rare.

My doctor says I’m allergic to fire and should try to avoid it so I’m putting together a fire detection circuit that will cut power to the printer if it detects fire or even an overheated environment.  I’ll post more when it is done but check out these cool little bits I’m using.

First up is an AC cord that has a built-in relay that can be controlled with a low-voltage signal.  The printer will plug into this and can only be turned on if my safety circuit sends a signal to close the relay.




Next is a Mini Flame Sensor that can be read from your favorite micro-controller or embedded computer.  In my case, I’m using a Raspberry Pi.  This boards pulls a digital signal low if it detects light in the same spectral range as fire.




This temperature sensor will give you the current temperature over I2C.  I’m using several of them at various places around the printer.

When the detected temperature exceeds some pre-determined limit (say 60 degrees Celsius), or when the flame sensor detects fire, the Raspberry Pi will turn off the control signal to the relay, thereby cutting power to the printer.

Software solutions are fine and dandy but sometimes software or the computer it is running on don’t behave as intended. That’s why you should always have a hardware-only layer of protection.

This is a replacement thermal fuse for a clothes dryer.  I plan to put this inline with the mains supply inside the control unit of the 3D printer. If the ambient temperature gets too high, it will break the circuit.



These thermal fuses are another layer that operates independently of the software.  I’ll put these at various points on the printer, along with the temperature sensors.  But these won’t be used as input to the Raspberry Pi so a software problem won’t keep them from working.  Instead, they will be in series in the control signal going to the relay.  If one of these blows, the relay will lose its signal and cut off power to the printer.



So what is my total investment in fire safety?  Disregarding the Raspberry Pi because I was including that anyway so it can provide a web interface to the printer, the components add up as follows:

  • PowerSwitch – $26
  • Flame sensor – $5
  • 5 temperature sensors – $25
  • Thermal fuse for dryer – $12
  • Inline thermal fuses – $8

That adds up to $70.  Throw in shipping on those items I didn’t get from Amazon Prime and it is closer to $80. Is that worth it?  Lots of people (most) operate 3D printers without any sort of fire detection circuit.  Is this degree of safety unnecessary on my part?  Obviously I don’t think so.  Anyway, it was fun sourcing the parts and putting it together and since the whole purpose of this hobby is to have fun, it is a win-win.

Comments?  What would you have done differently?