Thing-O-Matic / MK5 Extruder: Resistor Wrapup

Extruder resistor wiring
Extruder resistor wiring

As nearly as I can tell, using a pair of 10 W power resistors as 30 W heating elements in the Thing-O-Matic’s MK5 Extruder Thermal Core isn’t going to work, at least if you want even minimal reliability.

The fundamental problem is that the resistor specification limits the dissipation to a few watts, tops, near 250 °C, where they must run in order to melt any of the plastic filaments.

The Thermal Core requires 20-30 W to maintain 225 °C, so each resistor must dissipate an average of 10-15 W at that temperature. That’s half of the MK5 extruder’s original design point and still nearly a factor of 10 beyond the resistor rating.

The original design runs at less than 50% duty cycle to maintain 225 °C, which agrees with my measurements:

  • 50% of 60 W = 30 W
  • 33% of 60 W = 20 W

If you want to run at lower power, it’s a drop-in replacement. Change the original 5 Ω resistors to 2.5 Ω resistors (from Digikey / Mouser / wherever), change the wiring to put them in series (not parallel!), and see how long they last. They’ll certainly fare better than at 30 W, but I wouldn’t expect more than a few hours of lifetime. The specs give them 1000 hours at rated power, which this certainly is not.

A series connection means that when one resistor fails, the heat goes off. The original parallel connection left one resistor carrying the load and, at 30 W, it can actually get the Core up to operating temperature and keep it there. Many folks have been baffled by that, but the diagnosis is simple. Measure the resistance of the parallel resistors at the Extruder Controller end of the wires:

  • 5 Ω → one resistor has failed
  • An open circuit (infinite resistance) → both are dead

The problem with the lower power dissipation, whether from a failed resistor in the original design or my suggested change, is that the extruder head has a thermal time constant of 10-11 minutes. Lower power means a longer cold-start time; 30 W should get it up to 225 °C in about 20-30 minutes depending on the insulation. That’s not really a problem if you’re printing a series of objects, but might be objectionable for quick printing sessions.

However, when a resistor fails, the heat goes off, the plastic stiffens up, the DC extruder motor stalls, and the essentially unlimited motor current kills the A3977 driver on the extruder board. My incandescent lamp workaround may alleviate that problem: when the light goes on, check for a failed resistor.

I picked up a stock of 2-to-3 Ω power resistors and will do some further experimenting with power levels, insulation, and suchlike. This is a short-term fix to get my Thing-O-Matic running, but there’s a better long-term way to go: cartridge heaters on a modified Thermal Core, which I’ll discuss shortly.

If you arrived by search engine, jump there for my earliest guesstimates, go there to the beginning of the Thing-O-Matic hardware hackage posts, then read until you get back here. The story will, perforce, continue…

6 thoughts on “Thing-O-Matic / MK5 Extruder: Resistor Wrapup

  1. How can this be possible? I understand all of your posts about this, and it seems to me that this is a very large issue and that *everyone’s* Mk5 hot end would be failing within hours?

    It is a bit surprising that MBI hasn’t chimed in with potential solutions. I have just received my Cupcake Ultimate kit, still unassembled with Gen3 electronics and a Mk5 extruder. Putting it together as is akin to building a time bomb and I don’t feel too good about it.

    I am looking forward to the cartridge solutions that you are investigating and may well take my sweet time building my machine in hopes that something with a modicum of reliability can be found.

    1. Well, in all fairness, MBI has other irons in the fire… and it seems the MK5 head did survive their production testing in reasonably good order.

      There’s no reason to avoid putting the thing together, as long as you build in the precautions I’ve explored: electrically isolate the thermocouple, ground the head, put some thermal compound under the resistors, and pay attention to the extrusions so you can catch a failure when it happens.

      That sounds like a lot of effort, but it’s worthwhile. If it were part of the kit, just like everything else, you’d never notice the added steps.

      The Thing-O-Matic is a complex beast and the MK5 Extruder is just one part of it: MBI got nearly everything else right (except, perhaps, for the H-bridge running the extruder head).

  2. Here is a thought on possible solutions, I am no electronics tech, but Digikey has this beastie:

    Basically a larger version of the resistors that come with the Mk5, but 50w of power.

    Dimensions are:
    1.938″ L x 1.156″ W

    Much bigger, but with an adapter plate made out of aluminum or copper thy should fit by my estimation.

    They are $7 CDN instead of $4.73 for the original ones.

    My question is, how would the power circuit be affected? Would a different Ohm rating work better?

    Could you take a wild guess as to how much less over driven these would be?


    1. Yup, I have a bunch of those sitting on my bench from the Hot Box Disinsector project and they’re exactly as you describe.

      However, the data sheet for all those resistors derates them to 10% of the 25 C power at 250 C: 50 W resistors must not dissipate more than 5 W. Running resistors well beyond their ratings is how we got into this mess… there’s no point in quibbling about how much you’re willing to abuse them.

      A better solution involves cartridge heaters, about which I have some doodles there.

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