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? https://reprap.org/forum/read.php?4,502677,502677#msg-502677 Fri, 12 Jun 2026 10:30:53 -0400 Phorum 5.2.23 https://reprap.org/forum/read.php?4,502677,504471#msg-504471 https://reprap.org/forum/read.php?4,502677,504471#msg-504471 RetreadNJ Reprappers Sat, 02 May 2015 19:27:44 -0400 https://reprap.org/forum/read.php?4,502677,502863#msg-502863 https://reprap.org/forum/read.php?4,502677,502863#msg-502863 RetreadNJ Reprappers Tue, 28 Apr 2015 11:31:51 -0400 https://reprap.org/forum/read.php?4,502677,502715#msg-502715 https://reprap.org/forum/read.php?4,502677,502715#msg-502715 dc42 Reprappers Tue, 28 Apr 2015 04:01:41 -0400 https://reprap.org/forum/read.php?4,502677,502711#msg-502711 https://reprap.org/forum/read.php?4,502677,502711#msg-502711 etfrench Reprappers Tue, 28 Apr 2015 03:36:09 -0400 https://reprap.org/forum/read.php?4,502677,502698#msg-502698 https://reprap.org/forum/read.php?4,502677,502698#msg-502698 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.]]> blabbersnitch Reprappers Tue, 28 Apr 2015 01:04:23 -0400 https://reprap.org/forum/read.php?4,502677,502696#msg-502696 https://reprap.org/forum/read.php?4,502677,502696#msg-502696 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.]]> blabbersnitch Reprappers Tue, 28 Apr 2015 01:01:44 -0400 https://reprap.org/forum/read.php?4,502677,502677#msg-502677 https://reprap.org/forum/read.php?4,502677,502677#msg-502677 RetreadNJ Reprappers Mon, 27 Apr 2015 22:46:15 -0400