Thing-O-Matic / MK5 Extruder: Resistor Autopsy

Cooked thermal compound
Cooked thermal compound

Having built cartridge heater mounting blocks, I autopsied the two aluminum-case power resistors I’d been using on the MK5 Thermal Core. They weren’t dead yet, but I have some spares in case the cartridge heaters don’t work out as expected.

First observation: the blue-tinted thermal compound I’d put under the resistors turned white! It has a 200 °C maximum rating, so it’s been cooked well beyond any reasonable limit. On the other paw, it was still soft and didn’t have any air bubbles; the resistors were pretty firmly glued in place.

Based on those thermal measurements, I had replaced the original parallel-connected 5 Ω resistors with series-connected 2 Ω resistors, thus reducing the power dissipation in each resistor from 28.8 W to 18 W. While that’s still far beyond the specification, every little bit of reduction helps.

In round numbers, the resistors ran at 50-75% duty cycle to maintain Thermal Core temperatures in the 200-230 °C range. I guesstimate I had 10-15 power-on hours on the resistors, but that may be a lowball estimate: time passes quickly when you’re having fun.

Anyhow, I slipped a brass tube around one resistor terminal, braced the other end on the drill press vise, and pressed the cores out.

Resistor elements
Resistor elements

The top core literally fell out without any urging, which means that it had shrunk and separated from the housing. That means the resistor was well on its way to failing: a loose core gets hotter and deteriorates faster.

The bottom core was still firmly attached and disintegrated as I forced it out, which means it was in good condition. Paradoxically, the crumbled resistor core in the picture came from the resistor in the best shape.

Given that I ran these resistors at 63% of the original power level, the fact that one was well on its way to heat death after only (at most) a few tens of hours suggests that you shouldn’t expect much life from the stock MK5 resistors. If you haven’t already done so, electrically isolate the thermocouple bead from the Thermal Core to protect the Extruder Controller.

I’m unwilling to sacrifice a new resistor to see if that discoloration is normal, but I suspect it’s not. The ends should be the coolest part of the resistor, which means the middle is discolored, but that picture suggests the opposite, so I really don’t know.

I’d hoped the ID of the resistor bodies would match the OD of the cartridge heaters. That didn’t work out: 0.275 vs 0.250. They’re also a bit too short. If the match was closer, I could see slipping a shim in there, but having two air gaps around the heater just doesn’t make any sense at all.

14 thoughts on “Thing-O-Matic / MK5 Extruder: Resistor Autopsy

  1. Instead of thermal compound or shims, might it make sense to mix a powdered metal into a high-temp epoxy? Then you might be able to glue the cartridge heaters into the resistor bodies and have no air gaps. You would probably want a vacuum chamber to eliminate air-bubbles from the epoxy (I surely was edified the first time I saw epoxy froth in a vacuum chamber) and an injection mechanism to ensure there are no air gaps between the cartridge and the housing.

    1. mix a powdered metal into a high-temp epoxy

      Which is pretty much what JB Industo Weld is, although its upper temperature limit wouldn’t like being snuggled up against a heater.

      The only reason I thought of putzing around with dead resistor bodies was my mistaken belief they’d be an exact fit around the cartridge heaters: a quick-and-easy drop-in replacement with minimal machining.

      Given that they aren’t, then machining a pair of close-fitting steel blocks is easier (at least for me) than re-inventing high-temperature high-conductivity ceramic cement, setting up a vacuum chamber, injection-casting everything in place, and then seeing what happens.

      Remember, the end result of this ought to be a widget that can be more-or-less easily built by someone else. There are plenty of MK5 extruder heads out there in need of a rebuild and far more people have drill presses than adhesive injectors!

      1. Instead of reinventing high-temp high-conductivity ceramic cement you could just pour some mercury in there. No worries about contact area, reasonable conductivity, zero problems with air bubbles.
        I suppose there could be some *other* problems with that idea, but I’m sure they can’t be *too* serious, right?

        1. Or just mill a sleeve from good old 63/37 eutectic solder, then put a slab in a pocket between the two blocks. Sort of like a sodium-cooled reactor, minus the radiation and fireworks-with-water.

          Hmmm. Apart from the lead, maybe that’s not such a bad idea…

  2. As for the discoloration, both your pictures and the one you link to show lighter in the middle, so I’m guessing that they actually agree, and the discoloration in this compound shows up as a lightening for some reason.

    1. Although “burned white” isn’t the condition that springs immediately to mind, the end caps around the leads are normally gray. I suppose intense heat could destroy whatever pigment they’re using.

  3. Well, one of my resistors is dead. Slow heat times, near 100 duty cycle while printing. Sigh. I am in the middle of a project and do not want to redesign the extruder now.

    In the meantime, I ordered some of these:

    Two of those in seres should give ~2 ohms for 12v/2ohms = 6 amps.

    The original MBI design was ~2.5 ohms for 12v/2.5 = 4.8 amps.

    In all cases, the resistor is seriously out of spec for power and temperature, but I figure the 1 ohm will have a fatter wire inside, and maybe last longer before self-destruction.

    In theory the transistor:

    can handle it, and I already did my PCI 12v to the extruder board mod.

    1. Two of those in series should give ~2 ohms for 12v/2ohms = 6 amps.

      Except that you’re now dissipating 72 W total = 36 W per resistor, which is even worse than the MBI setup. Inside the resistor, the winding length varies, not the wire diameter, so you’re putting more stress on less material.

      Make sure you’ve grounded the Thermal Core and insulated the thermocouple, because you’re going to test both of those safeguards!

      In theory the transistor [568-2349-2-ND] can handle it,

      True, but as with many Digikey listings, it’s out of stock. Worse, that transistor is “obsolete” and so will never return.

      The MOSFETs aren’t the weak link, anyway. I suspect most MOSFETs get killed when people short +12 V directly to the transistor’s drain with one misplaced strand of wire.

  4. > now dissipating 72 W total = 36 W per

    But that is only power when they are on, the regulated temperature is the critical parameter and in both cases is way out of spec. Still, a lower peak would be better.

    > Inside the resistor, the winding length varies, not the wire
    > diameter, so you’re putting more stress on less material.

    All right. So I agree, assuming the wire diameter in the resistors is constant, then higher resistance = better.

    I can just get to the 230C extrusion temperature on the one 5 ohm resistor, at near 100% duty cycle. That is V*V/R = 12*12/5 = 29 watts.

    With both 5 ohm resistors in place, my extruder temp warm up was way faster than my HBP, so the 29 watts * 2 was way overkill.

    So if I went for 130% of that power, or 37 watts I would need about 8 ohm resistors. Of course the only sizes available are 2,2.5,5 &10 :(

    Well, two 2-ohms in series would give 36 watts. Maybe I’ll try that. I dono. Maybe I’ll just put the regular old 5 ohm, 10W resistors back in. Sigh.

    1. But that is only power when they are on

      Well, yeah, they’ll last nearly forever if you don’t turn ’em on… [grin]

      I’ve been running 50 W from a pair of 25 W cartridge heaters with a bit over 50% duty cycle. Before that, I ran two 2 ohm resistors in series and the ensuing 36 W was fine, albeit a bit slow to heat.

      I got an assortment of those resistors to make some measurements, but there’s no real reason to vary much away from the original values. More power per resistor = much shorter life. Less power per resistor = slower heating. Tradeoffs, tradeoffs!

      Word was, MBI was going to offer a cartridge heater upgrade, but … things take time.

  5. OK, I’ll try two 2 ohm in series for the first stab.

    What voltage were those cartridge heaters? It would be nice to find a 12v solution.

    1. They’re 25 W @ 12 V: in parallel across the same wiring that used to drive the resistors. A very clean solution, indeed.

      Of course, installing them required some Quality Shop Time.

  6. I’d be up for the shop time, but unless you have a couple spares you want to part ways with, they do not look particularly available :(

    Do you have a part number or any ordering information? It looks like their work at is not really geared towards the onesies – twosies hobbyists.

    Oh well. All this is only for fun, right?

    1. Those came from MBI and are all I have left; I returned some other samples so they could up-armor their own printers.

      I’d thought of a group buy, but didn’t do that on the expectation MBI would shortly offer an upgrade. That’s surely in the works; I suspect what’s holding it up (I have no inside information) is my paranoiac insistence on a reliable thermal lockout.

      As always, eBay may be your parts locker…

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