Sunday, 1 July 2012

Hot End Resisitor Fatigue...

After about 300m of PLA the hot end resistor gave out mid print today.

Now using the Marlin software, the machine stopped the extrusion stepper motor with messages indicating extrusion prevented due to low temperature. For a few seconds, before I saw the message I was confused as the machine kept merrily describing the print path. For one heart stopping moment I thought the stepper extruder driver or motor had died!
RepRap Pro Hot End Resistor Comparison

Having seen the message, I paused the machine and got my multimeter out to check the supply voltage and the resistance. The supply voltage was 19v D.C. whilst the resistance was in the Mega Ohms.
The poor little resistor beastie is open circuit and had to be stripped out from the hot end block. Having let everything cool and removing the hot end from the heat sink, a few taps with a small hammer and punch removed the little chap from its home.

When being fitted in Bath the resistor was very snug and then managed to wedge itself firmly into position. I spent a good while reviewing what I was going to do about the resistor and the risks entailed. This was one of the contributory factors that led to the machine not being completed before leaving. As they say measure many times and cut once, I should have filed some more I guess.

The issue is that the coating is easily cracked allowing moisture in and if the metal caps at the ends of the resistor contact the aluminium block, the resistor wire wound on the ceramic carrier is effectively shorted out by the hot end aluminium block.

So having assured my self the resistor would function as was, I elected to continue at the time. There was a lot of filing at Bath and attempting to file a round hole, even for those of us skilled in the art, is tricky over a length of 14.1 mm {the blocks aren't quite regular ;-) }.

The holes on the hot end blocks were too small and from that moment on I suspected it would fail at some point, I simply didn't know when.Well now we know.....

You can compare the two resistors both of which are RepRap Pro of the same batch supplied in Bath. Note how the coating and resistance wire on the failed device have changed.

There are two high points on each end of the resistor in the vitreous enamel over metal end cap diameters that are 4.99 mm; which is probably why the original stuck in the diameter of the 4.89 mm of the RepRap Pro hot end.

I passed a 5.00 mm drill through the hot end during the re-refurbishment, which allows the new 6.8 Ohm 5% RB59 (3 watt) resistor.

So we will order some more resistors and see how long this beastie lasts.

RepRap Pro Hot End Pack (April 2012)


  1. Is it really 6R8 and 19V supply?

    I normally use 6R8 with a 12V supply and power is V^2/R so you have 2.5 times the power. Maybe 15R would be more suitable.

    The gap needs to be filled with something conductive like AL foil or glass rope glue .

    1. Your comments on the voltage you use are very interesting.

      I have verified the voltage from the power supply on Huxley #710. My meter indicates 19.69v at PWR1/TIP (JP8) when the device is active (MOSFET on) and 18.59 across the resistor (it is thin wire).

      The hot end connector is hard wired to V+ and then the RFP30N06LE MOSFET on the Sanguinololu board seems to connect the load (e.g. hot end resistor) through to zero volts.

      The technical information on the RepRap Pro power supply supplied at Bath is: -
      Model L:PA-1121-02 and specifies the output as 19v D.C. at 6.32A

      Now you might say ... ahhh well perhaps you have the wrong power supply?
      We have a second Huxley kit; the second is having its parts printed as we find time, I have checked both power supplies. They are the same.

      What I am taking a long time to say is that RepRap Pro seem to have intended that the Huxley's operate at this power level.

      Interestingly the PCB is marked 12v and this would match with your indication.

      Perhaps the Aluminium foil, which is an excellent idea and not fitted originally, is all that is required.