Question about Nichrome Heated Bed issues.

I just purchased some 22 gauge Nichrome wire and I put it on my bed and it worked. 3 ohms @ 24v but the wire gets red hot in spots and other spots it doesn't and the spots that got red hot burnt clean through my Kapton tape and charcoaled it.

Anyone who made a bed out of this have any wisdom you can shed as to what is happening and how to fix it?

Thanks.

Edited 1 time(s). Last edit at 03/30/2016 05:05PM by Dark Alchemist.

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I await Skynet and my last vision will be of a RepRap self replicating the robots that is destroying the human race.

Sharp bends or mechanically stressed spots in the wire will cause it to heat up more than the rest of the wire. Cheap / poor quality wire that has carbon/ inclusions will also cause hot spots. I gave up on nichrome because of twists in the wire when spooling and de-spooling and the mechanical stresses caused by handling. I have been "playing" with kanthal wire not that it is any better in those regards but because it develops an "anodized" low resistance surface so there is less chance of developing a contact short. I really don't know what to tell you other than to examine your wire very closely and to handle it carefully.

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SumDude
Sharp bends or mechanically stressed spots in the wire will cause it to heat up more than the rest of the wire. Cheap / poor quality wire that has carbon/ inclusions will also cause hot spots. I gave up on nichrome because of twists in the wire when spooling and de-spooling and the mechanical stresses caused by handling. I have been "playing" with kanthal wire not that it is any better in those regards but because it develops an "anodized" low resistance surface so there is less chance of developing a contact short. I really don't know what to tell you other than to examine your wire very closely and to handle it carefully.

All of these hot spots were on the long runs not a single one, remarkably, was near a turn but I use rounded edges as any sharp bends will create a resistive point I know from past wire experiences. I wanted to make my own heated bed because I don't want to have to run out and buy another silicone mat when I replace the bed sometime in the future since they use 3m tape they are a one time use dealio.

edit: What I think is happening is that the 22 awg is just too stiff and is pulling away from the aluminum plate and that will cause heat spots.

Edited 1 time(s). Last edit at 03/30/2016 11:41PM by Dark Alchemist.

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I await Skynet and my last vision will be of a RepRap self replicating the robots that is destroying the human race.

Yep, I smashed the wire to see better and that is exactly what is happening. I can't burn this Kapton with my 40w soldering iron but this wire it is easy to do and the smell of burning Kapton is horrible.

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I await Skynet and my last vision will be of a RepRap self replicating the robots that is destroying the human race.