Nichrome wire resistance has shot up?

Upon playing around with my heater's temperature, merely hitting the 200C mark, I noticed large plumes of smoke coming from the cemented monstrosity that was my hot end. So I duely backed off the temperature to say 160C. The smoke went away, but so did the heat, now it only stays at room temperature. So, figuring I has blown yet another MOSFET I checked with my multimeter, but no, it was still working fine, say 11.6V. But when i unplugged the heater wires and tested the resistance, I was getting about 130R! Checking the meter was set right, yep, 130R... Now Im pretty sure I don't have enough nichrome around here to even get that much resistance, so I'm sure is wound it to ~8R.

So my question to you all is, how and what did I just do? 0.o

Ollie

The exact same thing happened to me a month ago, seemingly spontaneously. I ended up having to replace the heatcore entirely to salvage the hot end. Never did figure out what caused it.

this was the first heat up of this hot end as well :/ oh well, ordered s'more wire so should be here soon to destroy another one

Ollie, did you solder the nichrome to your heater wires? I suspect you melted the solder. I now use bootlaces ferrules: run the nichrome thru and then wrap it back around, insert the heater wire, and crimp the heck out of it. You won't NEEd to solder the connection then, altho I still do, very lightly.

Huh, that would sorta make sense, the solder i use melts ~230, but how would that effect resistance? Surely its attached on not? Although im probably not the best person to quote on that...

Oh and Mkouri, i ran out of crimps so started using solder xD but im not anywhere near a diy shop till the weekend. Are there any alternatives? Or cant some homemade ones do till then?

Ollie

I have ran into the solder problem as well, it tends to oxidize when at high temps for a long period of time and just stops making a good connection after a while.

I ended up making a FR-4 heat sheild type thing with through hole screws to connect the heating element to the wires that run to the board.

Haven't had a problem since!

Ollie,
If you have any extra pins or sockets for molex connectors, or even for RS-232 connectors, you can cut them down and use the "wings" for crimp connectors. That may even be easier to wrap the nichrome around, in between the wing that would normally hold the insulation on wire and the wing that would normally be crimped to the bare strands. Then lightly solder it and wrap the whole thing in kapton tape.

Edited 1 time(s). Last edit at 04/05/2012 12:38AM by Mkouri.

Huh, I didn't think of those two ideas thanks but I think I've got some spare molex connectors round here so I'll try that as a temp fix.

Cheers Guys,

Ollie