Category: Electronics Workbench

The Thermal Core’s time constant falls neatly out of the high power measurements when they’re plotted against time:

Rule of thumb:

• Three time constants get you to 95% of the final value.

The two plateaus for the 6 W and 14 W power inputs give enough information to pull out the time constant. They’re pretty much flat after 30 minutes (which is why I turned up the power then!), so the time constant is on the order of 10 minutes.

Remember that the time constant doesn’t depend on the heater power. Higher power means the Core would stabilize at a higher temperature, but the overall curve would have the same time constant. What you’d see, though, is a faster rise to a given temperature, at which point the controller turns off the power to maintain the Core at that temperature.

In round numbers, a first-order exponential rises to 10% (really 9%) of its final value in the first 0.1 time constant. In this case, guesstimate a 10-minute time constant, apply power, measure the temperature rise after a minute, and the final temperature should be ten times that value above ambient. Various nonlinearities get in the way, but that’ll get you close to the right answer.

Just for the amusement value, I applied 22.5 W to the extruder and recorded the temperature every 10 seconds for the first 1200 seconds. I’m not going to graph it, but the salient points are:

• 21 °C — Ambient temperature
• 199 °C — Final temperature (four consecutive dupes)

So the final temperature rise is 178 °C above the 21 °C ambient. Multiply by 0.09 = 16 °C, so when the temperature passes 37 °C = (21 + 16) it’s 0.1 time constants from the start.

Squint into that table and you’ll find the temperature is 36 °C at 60 seconds and 40 °C at 70 seconds, so the time constant is pretty nearly 625 seconds… call it 10.5 minutes, which I’d say is pretty close to my original eyeballometric guesstimate from that crappy graph up at the top.

Dang, I love it when the numbers work out!

You could do it in your head: 10% is 17.8, truncate to 17, add 21, get 38, interpolate 65 seconds. That’s just shy of 11 minutes, still close enough for what we’re doing.

Now, the tradeoff is that you have a ten-minute delay before your MK5 Extruder can begin squeezing out parts. That may not be a big deal if you’re looking at an hour of extrusion for each part and, after the head is up to operating temperature, it just keeps on running.

Adding more power increases the final equilibrium temperature and increases the initial temperature rise, so the head gets up to operating temperature faster. More power adds more stress to the resistors and shortens their life, which is nasty, brutish, and short even at the power levels I’m using.

You can run the numbers the other way. Measure the temperature rise at 0.09 time constant, multiply by 10, and you’ve got the final temperature rise. If you’re running 60 W into those heaters, a firmware lockup or thermistor failure that leaves the heaters jammed on will stabilize at maybe 350 °C over ambient. Nope, the resistors aren’t going to survive that experience…

The data from the high-temperature experiment suggested that I’d been unjustly maligning the thermocouple in the MK5 Extruder head.

This graph compares the raw MK5 thermocouple temperature against the adjusted reading from the Fluke 52 T2 thermocouple.

 

 

The regression line says the slope matches to within 0.01 °C and the offset is less than half a degree. While the line is a bit bumpy, that has more to do with the hasty data-taking than anything else: the points corresponding to the last reading at each power level are very close to the line.

Given that the adjustments to the T2 readings make them match the average of five thermocouples, not including the MK5 unit, I’d say the MK5 is spot on.

This doesn’t affect any of the conclusions I’ve come to over the last few days; I always put the MK5 thermocouple bead in a non-critical location.

Based on the earlier measurements, the Thermal Core seems like a reasonably isothermal setup. That means the reading displayed in the ReplicatorG control panel accurately reflects the Core temperature.

Based on the thermal coefficients found there, it looks like the MK5 Extruder head can reach operating temperature with only 15-ish watts of power, rather than the usual 60 W. While a factor-of-four more power certainly reduces the delay from power-on to building parts, the stress it puts on the resistors causes early failures.

What I’m trying to find here is the minimum power required to heat the head to 225 °C, in order to guide some future tweaks. The resistors will be operating outside their specification at anything more than 1 W each, but reducing the maximum power dissipation can’t possibly be a Bad Thing.

With that in mind, I wrapped the ceramic cloth insulation around the Thermal Core. Because the Core-to-ambient thermal coefficient was 12 °C/W without insulation and 16 °C/W with cotton cloth insulation, I didn’t do a thorough job of taping the gaps. Any insulation is better than none, but I knew I was going to dismantle the poor thing several times over the next few days.

The MK5 thermocouple is now in its intended position, clamped under the washer at the rear of the Thermal Core. I added a pocket of Kapton tape to electrically insulate it from the Core to prevent a resistor failure from shorting +12 V to the MAX6675 thermocouple interface chip, but that didn’t actually work: the bead punched through the tape. I think a small epoxy blob is in order.

I used just the Fluke 52 dual-thermocouple meter, in addition to the MK5 thermocouple:

Name	Meter	Location
TOM	MK5 t-couple	Standard location, Kapton wrap
T1	Fluke 52	Resistor
T2	Fluke 52	Core edge adjacent to resistor
CA	Craftsman A	not used
CB	Craftsman B	not used
MPJA	MPJA meter	not used

For the previous test, I read the temperature as the head warmed up at a specific power level. In this test, I picked the power level, read the temperatures until they sort of stabilized, then increased the power. As a result, the numbers aren’t quite comparable to what you’ve seen before: the head is not at a stable temperature.

The adjusted temperature readings, taken every 10 minutes:

Power	TOM	T1	T2	Time	Current
0	22.6	22.5	23.0	0	0.00
6	64.8	65.1	61.5	15	0.77
6	80.6	80.7	77.0	20	0.77
6	86.9	87.1	83.5	30	0.77
6	90.0	89.8	86.1	40	0.77
14	139.6	139.5	132.7	52	1.18
14	155.4	155.5	148.8	62	1.18
14	160.6	161.2	154.7	72	1.18
20	194.3	195.2	186.9	83	1.41
20	205.9	205.4	197.2	93	1.41
20	208.0	208.2	200.1	103	1.41
25	221.7	222.2	212.6	110	1.58

The Core was still heating with 25 W applied, but I couldn’t resist sticking an ABS filament into the Extruder, at which point my data-taking went downhill. Suffice it to say that 25 W heats the core well beyond 225 °C; I found a power level of 22 W (a current of 1.5 A) maintained the Core temperature at 225 °C.

One thing popped right out: the adjusted values for the MK5 thermocouple seem completely out of line, which isn’t surprising given what I saw in the isothermal block during calibration. I’ll have more to say about that in a bit, but the calculations you’ll see here use the raw MK5 thermocouple reading.

The last two lines show that the MK5 head, even with my crappy insulation job, can reach operating temperature with a total power under 25 W. That’s far less than the 58 W; it looks like running at about half power will be feasible.

The temperature differences:

Power	R – Edge	Edge – TOM	R – Ambient	Edge – Amb	TOM – Amb
0	-0.4	0.0	0.0	0.0	0.0
6	3.5	-1.5	42.6	38.6	40.0
6	3.7	-1.0	58.1	54.0	55.0
6	3.7	-0.5	64.6	60.5	61.0
6	3.6	-0.9	67.2	63.2	64.0
14	6.8	-1.3	117.0	109.8	111.0
14	6.7	-0.2	133.0	125.8	126.0
14	6.5	0.7	138.6	131.7	131.0
20	8.3	0.9	172.7	164.0	163.0
20	8.2	0.2	182.9	174.2	174.0
20	8.0	1.1	185.7	177.2	176.0
25	9.6	0.6	199.7	189.7	189.0

And, from those, the thermal coefficients for the boldified lines (which are as stable as you’re going to get for this dataset):

	R – Edge	Edge – TOM	R – Ambient	Edge – Amb	TOM – Amb
6 W	0.6	-0.1	11.2	10.5	10.7
14 W	0.5	0.0	9.9	9.4	9.4
20 W	0.4	0.1	9.3	8.9	8.8
25 W	0.4	0.0	8.0	7.6	7.6

You can’t compare the Resistor-to-Edge coefficient to the previous numbers, as the Core is getting heated from both resistors. Indeed, I think that column is totally bogus; using half the power gives a number comparable to the previous measurements, but I’m not certain that’s valid.

The rate of heat loss increases with higher temperatures: the Core-to-Ambient thermal coefficient is half of its previous value. The crappy insulation wrapper contributes to that, but the decline tracks the temperature for both types of insulation.

In round numbers:

• Heating the core to 225 °C requires maybe 25 W
• The resistors run 10 °C higher, with thermal grease
• This operation really stinks up your Living Room

This test determines the effect of thermal compound between the resistor and the Thermal Core on the MK5 Extruder head. The setup is essentially the same as before, with cotton fabric insulation wrapped around the Core.

I applied a thin layer of Thermalloy Thermalcote II from a small bottle that I’ve had since the days when you could actually use trichloroethylene as a solvent. It’s rated to 200 °C, so it won’t last long at full throttle, but it’s not nearly as permanent as epoxy.

That’s the thin blue line around the base of the resistor. You can actually have too much of the stuff, so I applied this by rubbing a dab from a scrap of paper onto the resistor’s base and squooshing it in place.

The instrumentation is the same as the last time around:

Name	Meter	Location
TOM	MK5 t-couple	Top of core
T1	Fluke 52	Resistor
T2	Fluke 52	Core edge adjacent to resistor
CA	Craftsman A	Bottom of core
CB	Craftsman B	not used
MPJA	MPJA meter	not used

After once again wrapping the core up in cotton cloth, I skipped directly to the higher power levels and sampled the data at 20-minute intervals.

The adjusted temperature readings:

Power	TOM	T1	T2	CA	Time	Current
0	22.6	21.5	22.8	22.8	926	0.00
4	69.0	73.1	70.0	66.1	946	0.89
4	84.8	87.7	84.6	80.5	1006	0.89
4	90.0	92.6	89.5	85.5	1026	0.89
4	91.1	93.8	90.8	86.6	1046	0.89
6	114.3	119.0	114.3	108.2	1108	1.10
6	120.6	125.0	120.3	114.4	1128	1.10
6	122.7	126.7	122.2	116.0	1148	1.10

Notice that the T2 reading on the block starts out a bit higher than the T1 reading on the resistor; I didn’t wait quite long enough for the heat of my hands to settle out inside that insulating blanket.

The corresponding temperature differences:

Power	R – Edge	Top – Bot	Edge – Top	Edge – Bot	R – Amb	Edge – Amb
0	-1.3	-0.1	0.1	0.0	0.0	0.0
4	3.1	2.9	1.0	3.9	51.6	47.2
4	3.1	4.3	-0.2	4.1	66.2	61.8
4	3.1	4.6	-0.6	4.0	71.1	66.7
4	3.0	4.5	-0.3	4.2	72.3	68.0
6	4.6	6.0	0.1	6.1	97.5	91.6
6	4.6	6.2	-0.3	6.0	103.4	97.6
6	4.5	6.7	-0.5	6.1	105.2	99.4

And now for the long-awaited and much anticipated thermal coefficients of the insulated and greased Thermal Core:

	R – Edge	Top – Bot	Edge – Top	Edge – Bot	R – Amb	Edge – Amb
4 W	0.8	1.1	-0.1	1.0	18.1	17.0
6W	0.8	1.1	-0.1	1.0	17.5	16.6

The grease reduces the thermal coefficient by about 20%, although I admit the numbers going into that calculation are getting pretty close to the limits of my instrumentation. Assuming the value remains the same at 30 W, the resistors will rise about 24 °C above the Thermal Core temperature to 250 °C, their maximum rated temperature. At that temperature, remember, their maximum rated dissipation is 10% of their 25 °C value: a whopping 1 W.

The R – Ambient and Edge – Ambient coefficients show that the insulation has about the same effect as before, which is comforting.

Now, to mull all this over for a bit…