Extruder heater overpowered to death?

Ok so I know this happens often but I think I worked out why in my case and searching hasn't answered my question. My extruder heated up for the first time last night and then this morning nothing. Multimeter says open-circuit in the resistor so many google and forums searches say this is normal after a while and some can be faulty straight up (Mind you I bought a completed J-Head hotend from BilbyCNC here in Australia).

Getting a replacement is easy, even if it does take 7 days to get across the country and cost me 5 times as much in shipping But my calculations are showing that the 6.8ohm 3W resistor I was supplied is actually dissipating about 20W of power which I believe is the cause of the failure.

My power supply is a 12V ATX and the resistor is 6.8ohm so P = V^2 / R which is 12^2 / 6.8 = 21.17W.
Thats 7 times the rated power output of the resistor. I know enough about electronics to think that something isn't right here but I could be wrong. Will another resistor of the same values die early again?

Math is correct, but its how its supposed to be. Hotend resistors are over-driven, because 3w of power wont melt abs at 250C.

Measure with multimeter between the leads and heater block, it can be a short between the leads and the block, and current is simply running through the block and bypassing resistor, so there is a small chance that resistor is ok. Normally leads should not have any contact to the block they are mounted in. I use but 1 turn of kapton around resistor to insulate electrical, and i also insulate the leads.

Edited 1 time(s). Last edit at 04/21/2013 11:31AM by NoobMan.

Ok thanks. Checked for shorts and couldn't find any so I'll just replace the resistor and hopefully it was just a bung one from the factory. This time I'll be smart and order spares haha

MasterUWA Wrote:
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> Getting a replacement is easy, even if it does
> take 7 days to get across the country and cost me
> 5 times as much in shipping

Here in NZ, you can get a 5 pack of those resistors for NZ$ 2.45 with free shipping from RS. Do you not have the same supplier in OZ?

Also, when you install the resistor, make sure you seal it in with high temp silicone or similar to get a good thermal bond with the heater block- that's what allows the resistor to heat beyond its rating.

Silicone is an insulator, not a conductor. I had a resistor fixed with silicone fail but ones fixed with exhaust putty or glass rope glue never fail.

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[www.hydraraptor.blogspot.com]

Strange, I've never had a problem with it? My sealant is a car gaskets type of product.

You want a ceramic-based glue (eg: active ingredient is sodium silicate - aka rope glass) rather than silicone.

Ceramic based sealants will transfer heat, whereas silicone ones usually will not.

If the resistor is too small for the hole in the J-head block, I tack the end of a strip of aluminium foil to the body of the resistor with ceramic based glue, then wrap it around the body of the resistor to fill up as much of the remaining space as possible. I then trip off any excess foil, so the fit is snug. The alu foil will conduct heat a lot better than silicone (or even the ceramic-based glue), so use the alu foil to fill up as much space as possible.

Some options for such glue:
1. Makergear ceramic adhesive (ships from the US tho).
2. Fireup Pyrogrip - Sodium Silicate based. I'm in Victoria (Aus) and bought a small 200g bottle for ~$10-12. Supposedly rated to 1200 Deg C.

If you're searching for someone/something local, look for Sodium Silicate, high temps 800+ Deg C (we don't rely on the temp, but it means the components are suitable for the job). Most of this stuff is used for fire prevention work (eg: securing fibreglass rope seals on fire heater doors, etc) so places that specialise in that sort of stuff are a good place to start.

Note: Some sodium silicate products are slightly conductive, so protect the leads of the resistor from contact with the glue. Ideally, you don't want the glue on the ends of the resistor at all - you want a tiny amount on the device to hold it in place and that's it.

i recently tried silver past. it actually works well. you can also wrap resistor in aluminum foil . just make sure entire surface area of resistor contacts the metal. also key to get resistor to stay alive, reduce its pwm to 245 or less. this allows it to be off long enough to have the heat dissipate to outside the resistor. the resistor still will glow red, but will not die as quickly. in configuration advanced file look for power or current settings and change it from 255, to 245 or 240.