Shattered Thermistor Bead

Hey all,
So I just got my first printer working last night, a Prusa Mendel V2 from makerfarm using RAMPS 1.4 and a J-Head hot end (Mk IV I believe). After a couple successful prints, I found that I wasn't getting a reading from my hot end thermistor. Turns out that the glass bead is broken: the top half of the head is gone, exposing the metal inside. I have another thermistor to replace it, but I want to find out the reason the first one broke before frying another. It's very possible there was a short or exposed wire, but I don't know if it would cause the glass to shatter like that. Thanks for the help!

Here's a photo of the broken thermistor if it helps: [i.imgur.com]

The only likely way to break a thermistor is mechanical. You likely can connect it directly to the 12V supply and it won't be bothered enough to break. The most likely causes would be that you broke it when you installed it or that it was defective and broke when warmed up.

Thanks for the reply. It's been working fine for the past few weeks, but maybe you're right and it's defective. I'll make sure I relieve mechanical stress from the next one as well.

I've probably gone through a few million glass bead thermistors over the years. I can't think of the last one that cracked without being bashed. That's not to say it can't happen.

It's a long shot but if it was a tight fit in the hole could the expansion of the metal crushed it? I know the expansion would be small. I'm sure someone will tell me it's not possible, but just an idea.

Holes in metal typically expand with heat.

The hole should be over sized, and a dab of thermal compound should be packed into the hole to help transfer the heat.