Monthly Archives: January 2017

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.