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OK to substitute larger power resistor for nozzle?

Posted by RetreadNJ 
OK to substitute larger power resistor for nozzle?
April 27, 2015 10:46PM
I am building the Prusa Mendel i2 and it's almost complete, but yesterday when I turned on the heat to the J-Head nozzle for the first time, it didn't heat at all. I measured the resistance and it is close to a short (0.3 Ohms - the spec is 5.6 Ohms or 6.8 Ohms). I ordered a new power resistor but it won't come for over a week. My year-long senior project is due, and I have to present it to the entire school, on 5/4. My local electronics supply store does not have the part - they only have power resistor in the hundreds of Ohms. Question: can I use a much larger power resistor temporarily to demonstrate my printer? Will it harm the j-head nozzle?
Re: OK to substitute larger power resistor for nozzle?
April 28, 2015 01:01AM
No, it won't work.
The 5.6 ohm resistor generates 25W to heat the J-head, the 6.8 ohm resistor generates about 20W. If you subsitute a resistor in the hundreds of ohms range, it will be lucky to generate 1W.
(the power generated is V*V/R... with V being approximately 12V).
If you can't get a resistor of 5.6 or 6.8 ohm, then you need something close. A higher value will take longer to heat (too high and it won't get hot enough.. but I don't know at what value that is).
A lower value resistor will heat up faster.. but there is a danger it could fail.
If you can get something close, give it a try. But certainly 100s of ohms won't work... in fact, I would try to avoid going much over 10 ohms.
Re: OK to substitute larger power resistor for nozzle?
April 28, 2015 01:04AM
By the way, are you sure you measured it right?
0.3 ohms definitely sounds like a short (most of that 0.3 ohms will be the leads and connectors of your multimeter). I ask just because it is unusual for a resistor to fail into a short circuit, they usually go open circuit when faulty.
Re: OK to substitute larger power resistor for nozzle?
April 28, 2015 03:36AM
Overnight mail might get you the part in time smiling smiley
Re: OK to substitute larger power resistor for nozzle?
April 28, 2015 04:01AM
Most people use cartridge heaters now, not power resistors. They are widely available on eBay. Will your Jhead accommodate one?



Large delta printer [miscsolutions.wordpress.com], Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
Re: OK to substitute larger power resistor for nozzle?
April 28, 2015 11:31AM
Thank you all very much! I'm getting a replacement of the correct part from the seller. (My nozzle does not appear to have a large enough hole for a cartridge heater.)
Re: OK to substitute larger power resistor for nozzle?
May 02, 2015 07:27PM
Update: the power resistor didn't fail short. My wiring was faulty so that the resistor leads were shorting out on the nozzle.
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