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Electrically insulating hotend resistors

Posted by billyzelsnack 
Electrically insulating hotend resistors
December 11, 2011 01:36AM
My hotend uses axial vitreous enamel resistors and I'm having issues with electrical insulation between the resistors and the heater block. These particular resistors (I use two in series) have about 150 hours or so of print time on them and still perform well. I finally got around to mounting my electronics and if I ground the electronics to my metal frame the electronics are not happy while the hotend is plugged in.

Both resistors in series lead to lead: 14.9
Resistor A lead to series junction: 7.7
Resistor B lead to series junction: 7.9
Resistor A lead to hotend: 14.7
Resistor B lead to hotend: 0.9
Series junction to hotend 7.9

What are people doing to electrically insulate the resistors without thermally insulating them?
Re: Electrically insulating hotend resistors
December 11, 2011 02:16AM
I would check to see if your resistor B is cracked. It looks from your measurements that it is shorted to the hotend at your resistor B lead end.

Edit: changed "grounded" to "shorted to the hotend".

Edited 1 time(s). Last edit at 12/11/2011 02:17AM by C. Cecil.

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Re: Electrically insulating hotend resistors
December 11, 2011 03:50AM
This happened with my previous hotend as well. It seems like the constant abuse degrades the enamel coating over time. The resistor has a cap on it under the coating, similar looking to a fuse, and I think that's what is touching the hotend. I'd like to do some sort of preventative measure the next time I build a hotend. I could wrap the resistor in kapton, but I dunno if I want to do that. Or.. I'm the only one having this issue and it's from just drilling the holes in the hotend too small or something.
Re: Electrically insulating hotend resistors
December 11, 2011 05:24AM
You can't just insert the resistor in the hole because it has a non-uniform exterior. The hole has to have a bit of clearance, otherwise it will scrape off the enamel if it is a tight fit. Then you need something thermally conductive to bridge the gaps, otherwise it will overheat.

I have used two techniques successfully: one is to wrap the resistor in aluminium foil with just enough turns to make it a push fit. It takes a bit of trial and error. The other is to glue it in with glass rope adhesive. With both techniques the resistors last forever.

I also have some from reprap-fab that appear to be glued in with silicone. So far they have worked but I haven't run them for as long.

Re: Electrically insulating hotend resistors
December 11, 2011 02:18PM
My earlier hotend experiments used aluminum foil, but I used it only to help with thermal conductivity in an effort to just make it work better. Later versions I found I did not actually need it for a working hotend and stopped using it. What I was missing was the long term benefit of increasing the thermal conductivity.. To protect the resistor from over-heating damage over time! Thanks.
Re: Electrically insulating hotend resistors
December 11, 2011 04:45PM
I use heat proof silicone to hold in my resistor and I also use the same to hold my thermistors in place and my main printer has now printed 15 sets of Prusa parts + some other odd stuff and is still going strong

Experimenting in 3D in New Zealand
Re: Electrically insulating hotend resistors
December 11, 2011 06:08PM
I also have used foil around the resistor in the hotend and use silicone that will stand 625F to fix the thermistors to the bed and hotend. I found that fire cement came loose after a time.
Re: Electrically insulating hotend resistors
December 11, 2011 09:09PM
I also used 2 resistors once, and they shorted to each other via the block. Might be same case.

1) Probably best solution is to use only one resistor.

2) If two are used, one way is to put at least 1 layer of kapton on each. Leads aswell in kapton or silicone coated or "dressing".

In any case, there should be no contact between any of the resistor leads and the block itself. Block should be at no potential. Same thing with thermistor, make proper insulation for both thermistor wires. I also notice that you must be using a psu with more than 12V with those resistors.
Re: Electrically insulating hotend resistors
December 11, 2011 10:00PM
Yeah. 19V.
Re: Electrically insulating hotend resistors
December 15, 2011 08:56PM
I had a roll of PTFE tubing from Omega kicking around that works awesome. A little goes a long ways. I just slide it over the resistor leads and cement them in.

PTFE Tubing
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