The objective here is to determine the thermal coefficient between the resistors and the Thermal Core, with no thermal compound to fill the air gap, so we know how high the resistor temperature will get.
The Thermal Core sprouted many thermocouples:
|TOM||MK5 t-couple||Front of core|
|T2||Fluke 52||Core edge adjacent to resistor|
|CA||Craftsman A||Top of core|
|CB||Craftsman B||Bottom of core|
|MPJA||MPJA meter||Heatsink on thermal tube|
They’re positioned as shown here, with the Bottom thermocouple to the rear out of view. The ribbed black heatsink at the very top of the picture is a few millimeters below the acrylic base of the Extruder Filament Drive block.
Applying power from a bench supply produced these results, adjusted to the average value using the regression coefficients determined there. The measurements occur every ten minutes: the Core’s time constant is, mmm, languid.
The asterisk marks the spot where a clip lead shifted and dislodged the brass tube epoxied to the resistor. Of course, that’s one of the two absolutely vital temperature measurements, but so it goes. I was planning to stop at 8 W, anyway, because that’s about as much power as I wanted to apply to the resistor, as it exceeds the rated power for that temperature.
The boldified lines mark the measurements where the Core temperature has stabilized, where I defined “stabilized” to mean “hasn’t changed all that much since the last measurement”.
Some temperature differences between interesting locations on the Thermal Core, bearing in mind that the linear regression equations aren’t good for much below 1 °C, at best, so the tiny differences are mostly noise.
|Power||R – Edge||Core T-B||Edge-Bot||Top-Heatsink||R – Amb||Edge – Amb|
The Top – Heatsink column says there’s really not much temperature difference between the Core and the cute little heatsink on the Thermal Tube at the top. This is without any Core insulation, but it’s also at a a much lower Core temperature.
And now for the heart of the matter: the thermal coefficients, which are the temperature differences divided by the applied power. These are for the boldified lines above, where the temperatures have stabilized.
These are not, strictly speaking, correct, because the only interface where we know the applied power lies between the resistor and the Thermal Core. But we’ll do the best we can with what we have…
|R – Edge||Core T-B||Edge-Bot||Top-Heatsink||R – Amb||Edge – Amb|
The R – Edge column shows that the resistor-to-Core thermal coefficient hovers around 1.5 °C/W, which means dissipating 30 W in the resistor raises its temperature 45 °C above the Core. With the Core stabilized at 225 °C, the resistors run at 270 °C, far beyond their absolute maximum rating of 250 °C where the rated power drops to 1 W.
That’s why MK5 Extruder resistors fail at such a disturbing rate.
The next two columns show the relatively small temperature differences across the the Thermal Core iself: that steel block is pretty much isothermal, even with only a single resistor providing power to one side. That’s good news, of a sort: clamping the MK5 thermocouple anywhere on the Core will provide consistent results.
The Top – Heatsink coefficient declines as the power level rises, probably because of the hot air rising from the uninsulated Core.
The R – Amb and Edge – Amb columns shows that air is a pretty good insulator all by itself. If you apply 30 W to the resistor and extrapolate a 10 °C/W thermal coefficient, the resistor would reach something like 300 °C above ambient, even without insulation. Obviously, that wouldn’t work for long, but those are the numbers.
Up next: wrap some insulation around the Core…