## Monthly Aphorism: On Precision Instruments

• Any sufficiently precise instrument is a thermometer

That’s in addition to whatever it’s supposed to be measuring, of course, but it’s amazing how temperature effects creep into those last few digits without you noticing anything different.

The differences between precision, accuracy, and resolution remain relevant, if commonly misunderstood. In particular, precision is not the same as resolution. A good introduction is there.

I stand in awe of the analog IC design folks who can build temperature compensation into a chip by tweaking junction areas and currents. A tip o’ the cycling helmet to ’em!

## ATX Power Supply vs Heatsinks

Having just installed resistors on those three heatsinks, here’s the results from that ATX power supply.

 Loads +5V +12 V -ON -> Gnd PS Tester 0 4.94 11.76 13 17 1 4.81 11.82 70 72 2 4.79 11.92 129 130 3 4.77 12.02 189 192

Notice that the +12 V output increases under load, which turns out to be true because all the outputs share the same transformer: supporting the load on the +5 V output requires more flux, which tends to increase all the other outputs.

The last two columns are the input power from the AC line, with the -ON pin shorted to ground and with a black-box power supply tester that evidently draws a watt or three.

The resistors on each heatsink dissipate a total of 46.6 W:

• 1 Ω -> 4.82 / 1 = 23.0 W
• 6 Ω -> 11.92 / 6 = 23.6 W

The box sees 140 W with all three heatsinks powered up.

Three of those fans draw 1.1 A from +12 V, adding another 13.4 W.

Grand total: 153 W. Close enough!

Power supply efficiency at full load is 81% = 153 / 189. Not as good as you’d like, but it’ll suffice for my simple needs.

The drop across a single Molex power connector pin is 11 mV @ 5 A and 4.3 mV @ 2 A: call it 2 mΩ. They’re rated for 11 A with a 30 °C temperature rise, which means I really should use PowerPoles.

## Bugged ATX Supply

I intend to use an ATX power supply as a cheap source of bulk +12 V and +5 V power for the resistors on those heatsinks. I have a 250 W box on the shelf (harvested from a dead donor PC) that seemed ideal; they run more efficiently with higher loads and I only need 150-ish W.

Being that type of guy, I opened it up to see what’s inside…

Huh. Looks like some small creature of the night immolated itself down there in the lower left corner, tucked against the transformer. There’s nothing more than black goo and charred filaments left over, with green-blue corrosion creeping up the resistor lead.

Or maybe it’s actually toxic snot from the manufacturing line. Hard to say at this point.

The power supply tester says the juice comes out fine & dandy, so I might use the thing after trying to get the gunk out.

## Heatsink Thermal Coefficients: Forced Air

Continuing the experiment with forced air, I added some of those fans. Same thermocouple arrangement as before, heatsink sitting on the bench with the fins horizontal and the fan blowing across the bench. This obviously leaves a bit to be desired as far as air flow control goes, but it’s close enough to get a general idea of what’s going on.

With the fan in the air flow straightener, exit about 1 diameter (4 inches, 100 mm) away from the heatsink, flow perpendicular to the heatsink body, 24 W to the 6 Ω resistor:

• R = 27.7 °C
• Bot = 23.4 °C
• HS = 66 °F = 18.9 °C
• Thermal resistance resistor to heatsink: ΘRB = 0.18 °C/W
• Thermal resistance heatsink to ambient: ΘHA = 0.16 °C/W

That’s more like it!

With the bare fan sitting on the bench, exit about 1 diameter (4 inches, 100 mm) away from the heatsink, flow perpendicular to the heatsink body, 24 W to the 6 Ω resistor:

• R = 26.7 °C
• Bot = 22.4 °C
• HS = 64 °F = 17.8 °C
• Thermal resistance resistor to heatsink: ΘRB = 0.18 °C/W
• Thermal resistance heatsink to ambient: ΘHA = 0.12 °C/W

The bare fan actually does a better job than the flow straightener. Just from the general feel of the breeze, I think the fan’s air flow entrains a bunch of ambient air and slams it across the entire heatsink, rather than hitting just the central area.

Encouraged by that, I doubled the power to 50 W: 2.6 A in the 6 Ω resistor = 41 W and 3 A (the limit of my bench supplies) in the 1 Ω resistor = 9 W. Because the heatsink is now getting energy from two sources, the heatsink temperatures won’t be directly comparable to the previous ones.

With the bare fan in the same position as before, 50 W dissipation:

• R = 36.8 °C
• Bot = 29.4 °C
• HS = 72 °F = 22.2 °C
• Thermal resistance resistor to heatsink: ΘRB = 0.18 °C/W
• Thermal resistance heatsink to ambient: ΘHA = 0.14 °C/W

The heatsink will be 7 °C above ambient at 50 W and the resistors 4.5 °C at 25 W each above that. The resistors will be 11.5 °C = 21 °F over ambient.

Again, the average of the Bot and HS temperatures might be more meaningful.

The heatsink has fins on both side, but so far I’ve been using only one fan. Putting another bare fan on the opposite side, also 1 diameter away, so the heatsink gets ambient air from sides, thusly:

That produces even better results:

• R = 33.9 °C
• Bot = 24.9 °C
• HS = 66 °F = 18.9 °C
• Thermal resistance resistor to heatsink: ΘRB = 0.22 °C/W
• Thermal resistance heatsink to ambient: ΘHA = 0.067 °C/W

The 6 Ω resistor is dissipating 41 W, rather than the 24 W I plan to use: figuring the resistor-to-heatsink thermal coefficient at 0.2 °C/W seems OK. The heatsink-to-ambient coefficient is breathtakingly good: cooling both sides seems like it ought to cut the thermal resistance in half and it does! Call it 0.1 °C/W.

So, bottom line: with two fans and 50 W, the heatsink will be 5 °C over ambient. Dissipating 25 W in each resistor will raise them 5 °C over the heatsink and 10 °C = 18 °F above ambient.

With the box ambient at 140 °F and two fans per heatsink, the resistors should tick along under 160 °F. That’s plenty toasty, but only slightly above my rule of thumb: If you can’t hold your thumb on it, it’s too damn hot. And, heck, we’re building a heater here, right?

On the other paw: six fans?

In reality, that layer of thermal goop between the case and heatsink determines much of the resistor temperature. One fan per heatsink should be entirely adequate. I should try this with one fan blowing parallel to the fins, with the notion of putting a fan directly upstream of each heatsink or between each pair of heatsinks.

The raw data:

## Heatsink Thermal Coefficients: Convection

Having plugged the previous holes, I screwed down a pair of power resistors atop some heat sink compound: 6 Ω and 1 Ω. The general idea is that a stock PC power supply will dump about 50 W into the heatsink: 2 A @ 12 V = 24 W and 5 A @ 5 V = 25 W.

Thermocouples atop the 6 Ω resistor (R), on the opposite side of the heatsink just below that resistor (Bot), and at the far end of the heatsink (HS). They’re all held in place with foam blocks, in the hope that the steady-state air temperature between the foam and the resistor / heatsink will be pretty close to the temperature of the source.

Ambient in the Basement Laboratory is hovering around 60 °F = 15.6 °C, which is pretty mumble chilly. The numbers use the actual ambient for each test.

The first test powered just the 6 Ω resistor, because I wanted to find the natural convection capability of the heatsinks, which will be pretty low. Reaching steady state required a bit over an hour in each case; I recorded temperatures every ten minutes, which really chops up the day, but prevents forest fires.

Heatsink flat on its back in the worst possible orientation, atop a pair of wood blocks 35 mm off the bench:

• R = 64.3 °C
• Bot = 60.9 °C
• HS = 133 °F = 56.1 °C
• Thermal resistance resistor to heatsink: ΘRB = 0.14 °C/W
• Thermal resistance heatsink to ambient: ΘHA = 1.7 °C/W

Heatsink on edge, fins horizontal, clamped in a vise a few inches off the bench:

• R = 57.0 °C
• Bot = 52.4 °C
• HS = 121 °F = 49.4 °C
• Thermal resistance resistor to heatsink: ΘRB = 0.19 °C/W
• Thermal resistance heatsink to ambient: ΘHA = 1.4 °C/W

Heatsink on end, fins vertical (best orientation), same vise:

• R = 51.6 °C
• Bot = 46.4 °C
• HS = 114 °F = 45.6 °C
• Thermal resistance resistor to heatsink: ΘRB = 0.22 °C/W
• Thermal resistance heatsink to ambient: ΘHA = 1.2 °C/W

All those numbers are suspect, of course, but the general trend is comforting. The heatsink temperature might be better figured as the average of the Bot and HS values, but they’re pretty close.

Figuring ΘRB = 0.2 °C/W says the resistor will be 5 °C above the heatsink, which means we’re not dealing with insanely high temperature differentials. This is a Good Thing and shows that thermal compound helps.

Figuring ΘHA = 1.5 °C/W means convection really isn’t going to work, because at 50 W the heatsink will be 75 °C above ambient. That’s much too hot, but I need forced air flow to circulate hot air inside the box, anyway, so this is something of a worst-case situation.

The heatsinks will probably be in the second configuration, with fans blowing along the horizontal fins. If the fans fail, things will get downright toasty. We need a mechanical thermal cutout switch, heatsink temperature monitoring, and fan status feedback.

The raw data:

## Makerbot Thing-O-Matic MK5 Extruder: Resistor Abuse!

Having a Thing-O-Matic on order, I’d been browsing the doc and came upon the MK5 extruder specs, which uses a pair of 25 W resistors (* wrong: see bottom) similar to the 50 W resistors I’ve been building into the Hot Box Disinsector oven.

However, the head puts the two 5 Ω resistors in parallel, directly across the +12 V supply: each 25 W (* see bottom) resistor dissipates 29 W. To make matters worse, the heater block is wrapped in ceramic cloth tape thermal insulation, bundled up in Kapton, and servo-controlled to something over 200 °C.

What’s wrong with that picture?

Here’s the note I put up on the Google MakerBot Operator’s group a few days ago, with slightly cleaned-up formatting:

———————–

## MK5 / Thing-O-Matic Heater Problems

My TOM is on order, halfway through its 7-week leadtime. In the meantime, I’ve been reading the mailing lists and poring over the documentation. One thing stands out: a disturbing number of “my MK5 extruder stopped heating” problems.

Right up front, I’m not slagging the folks at MakerBot. I attended Botacon Zero, toured their “factory”, and ordered a Thing-O-Matic the next day. This is my contribution to tracking down what looks like a problem, ideally before my TOM runs into it. I *want* to be shown that my analysis is dead wrong!

What follows is, admittedly, a technical read, but that’s what I *do*.

### Background

The heater uses two 5-ohm 25-watt panel-mount resistors in parallel across the 12 V supply to raise the thermal core to well over 200 C. Some folks run their extruders at 225 C, which seems to be near the top end of the heater’s range.

The resistors are standard items from several manufacturers. The datasheets can be downloaded from:

KAL (Stackpole) http://www.seielect.com/Catalog/SEI-kal.pdf
Ohmite http://www.ohmite.com/catalog/pdf/89_series.pdf

### Possible Problems

My back-of-the-envelope calculations suggest several problems with the heater, all of which combine to cause early failures.

1) Too much power

Putting 12 V across a 5 ohm resistor dissipates 28.8 W. Allowing for 0.5 V drop in the wiring, it’s still 26.5 W.

That exceeds the resistor’s 25 W rating, not by a whole lot, and might be OK at room temperature, but …

2) No temperature derating

The 25 W power rating applies only when mounted to the heatsink specified in the datasheet at 25 C ambient temperature. Above that temperature, the maximum allowed power decreases linearly to 2.5 W at 250 C: 0.1 W/C.

When the resistor is not mounted to a heatsink, its maximum free-air rating is 12.5 W. That limit declines by 0.044 W/C to the same 2.5 W limit at 250 C.

What this means: at 200 C *and* mounted on a heatsink, the resistors must not dissipate more than 4.7 W. The MK5 heater runs them at 28 W, six times their 200 C rating, and they’re not on a heatsink.

3) Excessive heat

The resistors will always be hotter than the thermal core: they are being used as heaters. The temperature difference depends on the “thermal resistance” of the gap between the resistor body and the core.

The MK5 resistors are dry mounted without thermal compound, so the gap consists largely of air.

I recently measured the thermal resistance of the 50 W version of these resistors on an aluminum heatsink using ThermalKote II compound in the gap. In round numbers, the thermal resistance is about 0.2 C/W: at 28 W the resistors will be 6 C hotter than the thermal core.

The default air-filled gap to the MK5 thermal core will make the resistors *much* hotter than that. With the core at 225 C, the resistors will probably heat beyond their 250 C absolute maximum operating temperature.

4) Insulation

The datasheet ratings for the resistors assume mounting on a heatsink in a given ambient temperature, so that the resistors can dump heat to the heatsink (that’s why it’s called a *sink*) and to the surrounding air. The MK5 thermal core and resistors live inside ceramic insulation and Kapton tape, specifically to prevent heat loss.

### Conclusion

The resistors operate with far too much power at too high a temperature, inside a hostile environment with too much thermal resistance to the core. They will fail at a high rate because they are being operated far beyond their specifications.

Given that, the failures I’ve read here over the last few weeks aren’t

This picture (linked from the first message) shows a severely burned resistor slug:

http://img.skitch.com/20101108-nhrj8rjx68ffxrdq6p2fgwjcqx.jpg

I do not know what fraction of the MK5 extruders those messages represent. There are about 1000 members of this group, but not everybody has a MK5 extruder head. Assuming 250 MK5 heads, that’s a 2% failure rate.

The number of problem reports seem to be increasing in recent weeks, but that can be a fluke.

### Observations

Depending on the room temperature, a MK5 thermal core can probably reach operating temperature with only one functional resistor, but it will take much longer than normal.

Indeed, I suspect some of the “my MK5 has difficulty extruding” problems may come from a thermal core that’s nominally at operating temperature, but with one dead resistor: the steel block is cooler on the side with the failed resistor. The thermistor reports the temperature at the block’s surface, not inside where the plastic actually melts.

It’s entirely possible that a resistor failure can lead to an extruder motor failure: too-cool plastic → difficult extrusion → high motor load → extruder motor failure. That’s a guess, but it seems reasonable.

### Diagnosis

The symptoms fall into two categories, with what I think are the obvious causes:

Slow heating = one resistor failed
No heat at all = both resistors failed

To discover what’s happened, disconnect the heater power cable from the extruder controller, then measure the resistance across the wires. You should find one of three situations:

1) 2.5 ohms = both resistors good = normal condition
2) 5 ohms = one failed resistor
3) Open circuit = two failed resistors

The resistance value may vary wildly if you move the wires at the extruder head, because a failed resistor element can make intermittent contact. If you measure the resistance at the extruder controller connector end of the cable, leaving the thermal core alone, you should get more stable results.

### What to do

Given that the resistors operate under such hostile conditions, I think there’s not much you can do to make them happier. Some untested ideas:

1) Use the remainder of the anti-seize thread lube as thermal compound between the resistors and the thermal core. It’ll stink something awful until the oil boils off, but ought to keep the resistors significantly cooler by improving heat transfer to the core. Standard PC CPU thermal compound (Arctic Silver, et al) deteriorates well below 225 C, so it probably won’t survive in this environment.

2) Rearrange the thermal wrap to expose the ends of the resistor leads, which will cool the resistor element end plugs and reduce the deformation causing the slug to work loose inside the aluminum shell.

3) Use thicker connecting wire, without insulation, outside the thermal wrap, to dump more heat from the resistor leads.

The last two changes will cause more heat loss from the thermal core which means the controller will turn the resistors on more often. Perhaps reducing the thermal stress on the weakest part of the resistors will delay the failures, but I don’t know.

When my TOM arrives, I’ll instrument the thermal core with a handful of thermocouples, measure what’s going on inside, try some of those ideas, and report back.

If you get there first, I’d like to know what you find!

———————–

* From above

Now, as it turns out, my TOM arrived on Christmas Eve! Given the usual holiday distractions, I’m only now getting down to construction & measurements, but one thing pops right out: contrary to what I’d assumed / read somewhere, those resistors are rated for 10 W at 25 C. Everything I wrote above applies to 25 W resistors: the situation is actually much worse: a 10 W resistor dissipating 30 W while tucked inside an insulating blanket?

This is not going to have a happy outcome…

[Update: The story continues there.]