The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Category: Electronics Workbench

Electrical & Electronic gadgets

6C21 Triode
Aitch bestowed this gem on me while cleaning out his collection:

6C21 Triode

It’s a 6C21 triode, originally used as a radar modulator, atop a letter-size sheet of graph paper. The plate terminal is on top, the grid sticks out to the side, and the filament is common with the cathode through the base pins.

It has impressive specs (datasheet and pictures):
- 30 kV plate voltage
- 15 A pulsed plate current, 100 ms max
- 7.5 V filament at 15 A = 112 W (!)
- Pulse duty cycle 0.2%
The gray film inside the bulb shows that it’s been used, but the filament still has continuity. Ordinarily, you could turn something like this into a night light by running the filament at a voltage somewhat under its rating, but my bench supply maxed out at @ 3 A without even warming it up; a dim orange night light that burns maybe 75 W is Not A Good Idea.

The base has some intriguing holes, originally used for forced-air cooling, that lead directly to the glass envelope:

6C21 Triode – base

One could mount discrete LEDs in those holes, maybe a slightly turned-down 10 mm cool-white LED in the middle flanked by red and blue, and run a low-power Arduino-based mood light; by some cosmic coincidence, the hole spacing matches up almost perfectly with those LED strips. Or one could go full analog with three red LEDs driven by the WWVB signal.

I’m thinking a plain black acrylic case, with the tube base sunk into the middle, would be about right. No readouts, no dials, no buttons, just a gently glowing tube.

Maybe a 3D printed socket holding everything in place?
2013-03-21
Kensington Expert Mouse Trackball: Scroll Ring Aperture Alignment

That comment suggested scroll ring failures on a Kensington Expert Mouse (it’s a trackball) might occur when the apertures become misaligned from the IR emitter-detector pair, although later results were equivocal. I tore apart a failed unit to see what the alignment looked like for a known-bad scroll ring.

The right side view shows the receiver roughly centered in an aperture:

Kensington Expert Mouse – Scroll Ring aperture – right

The left side view shows that the ring is almost flush against the circuit board, with the isolating cutout just in front, and it’s not obvious how to lower it any further:

Kensington Expert Mouse – Scroll Ring aperture – left

So I think there’s no way to realign this one, other than to raise the aperture ring a bit, but that doesn’t seem like it would make any difference: the detector already has a good view of the emitter.

If your trackball has a failed scroll ring, tweaking the aperture ring’s alignment certainly can’t hurt: try it and report back.

If you don’t expect a miracle, you probably won’t be disappointed, alas.

The pix come from the Canon pocket camera mounted on the macro lens / illuminator, handheld with manual focus. The dust speck on the detector is just slightly out of focus, but you get the general idea.

Update: 2015-07-29 – A success story from Tom:

Hi, I wanted to leave a comment for your page here: [this url]

I’ve got an expert mouse trackball that was having intermittent scroll ring problems, then finally quit working altogether. Dismantled it easily using the instructions on this site.

Cleaned it and it still wasn’t working. Tried changing the alignment of the IR emitter/detectors and it still wasn’t working. Then we kept on fiddling with the alignment and voilà.

Like others have said, the alignment seems to be SUPER sensitive. So if any others are reading this with the same problem, keep persevering.

Thanks to everyone who has posted to help find solutions!

Another update: Seven years in the future, a real fix appears!

2013-03-18
Stepper Motor Thermal Coefficient vs. Thermal Compound and Forced Air

Prompted by that comment, a bit more data emerges.

This unsteady ziggurat barely supports the aluminum CPU heatsink atop a PC CPU exhaust duct; the two came from different PCs and have no relation to each other. The vise in the background keeps the whole affair from falling over. The fan sucks air through the heatsink and exhausts it out the front.

NEMA 17 Stepper – Heatsink with Fan

Throughout all this, the stepper driver runs at a bit over 10 k step/sec, tuned to avoid the howling mechanical resonances in that stack. At 1/8 microstepping, that’s 6.25 rev/s = 375 RPM, which would drive the Thing-O-Matic at 210 mm/s and the M2 at 225 mm/s. Your speed will vary, of course, depending on the pulley diameter / number of teeth / belt pitch, etc.

Under the same conditions as before (i.e., no thermal compound, fan off), the stepper stabilized at 143 °F = 62 °C in the 57 °F = 14 °C Basement Laboratory ambient, with 1.91 A peak current (I don’t believe that second decimal place, either) and a 6.6 °C/W case-to-ambient coefficient. That’s close enough to the 63 °C and 6.7 °C/W coefficient from the earlier test, so the conditions seem roughly the same.

Smoothing a thin layer of heatsink compound on the butt of the motor, then squishing it firmly atop the heatsink, cut the temperature to 130 °F = 53 °C without the fan. That suggests the case-to-ambient coefficient is now 5.3 °C/W: the thermal compound helps by 1.3 °C/W.

Turning on the fan drops the case temperature to 84 °F = 29 °C, which works out to a coefficient of 2.1 °C/W. Obviously, moving air over that heatsink helps the cooling a lot: the heatsink felt cold to the touch and the motor case was barely warm.

Increasing the current to 2.37 A dissipates 11.2 W, which would be scary without the heatsink and air flow. The temperature stabilized at 91 °F = 33 °C, for a coefficient of 1.7 °C/W.

At 2.83 A = 16 W, the temperature rises to 100 °F = 38 °C, with a coefficient of 1.5 °C/W. While it’s not unstoppable with that much current, the motor has plenty of torque! The motor becomes pleasantly warm, the heatsink stays just above cool, and all seems right with the world. I suspect the windings get a bit toasty in there, but they can’t possibly be worse off than inside a case at boiling-water temperatures.

Using the original insulated-motor coefficient of 19 °C/W, 16 W would cook the motor at 320 °C. Perhaps the case would make a nice extruder heater after it stopped being a motor?

[Update: See the comments for the results of just blowing air over the motor case.]

2013-03-12
Stepper Motor Thermal Coefficient
You’ve probably seen this exchange on whatever DIY 3D printing forum you monitor:
1. My stepper motors get scorching hot, what should I do?
2. Turn down the current!
3. That worked great, but …
4. … now all my objects have a shift in the middle.
5. Your motor is losing steps: turn up the current!
6. Uh, right.
NEMA 17 Stepper on cloth

So, with that setup on the bench, I ran a simple experiment with current, temperature, and heat transfer. Most DIY 3D printers have stepper motors attached to a plywood chassis or plastic holder, so the first data point comes from a motor with no mechanical thermal path to the outside world (which is the Basement Laboratory at 14 °C ambient).

Running at about 1200 step/s with a winding current of 1 A peak from a 24 A supply, the motor stabilized at 52 °C = 125 °F after half an hour.

Both windings have a 2 Ω resistance and carry 1 A peak = 0.7 A rms, so the total power dissipation is:

2 × [(1 A / √2)² × 2 Ω] = 2 W

That’s the same power produced with the motor stopped at a full step position, where the peak current flows in a single winding and the other winding carries zero current:

(1 A)² × 2 Ω = 2 W

The temperature rise suggests a thermal coefficient of about 19 °C/W = (52 °C – 14 °C) / 2 W.

The next current setting on the driver is 1.46 A, which doubles the power dissipation to 4.3 W. Assuming a large number of linearities, that would cook the motor at 82 °C = 180 °F above ambient. Even though the motor could probably withstand that temperature, for what should be obvious reasons I didn’t go there.

Instead, I parked the motor atop a big CPU heatsink harvested from an obsolete PC, sans thermal compound, mechanical fitting, and anything more secure than gravity holding it in place:

NEMA 17 Stepper on Heatsink

The results:

Ambient 14 °C

Winding 2 ohm

A pk A rms Power W Case °C °C/W amb °C/W incr

1.00 0.71 2.0 28 7.0 7.0

1.46 1.03 4.3 42 6.6 6.2

1.91 1.35 7.3 63 6.7 6.9

The thermal coefficients represent the combination of all interfaces from motor case to ambient, but the case and heatsink stabilized to about the same temperature, so the main limit (as always) will be heat transfer to ambient air. Obviously, the heatsink sits in the wrong orientation with little-to-no air flow, not to mention that the butt end of a stepper motor isn’t precisely machined and has plenty of air between the two surfaces. Improving all that would be in the nature of fine tuning and should substantially lower the coefficient.

What’s of interest: just perching the motor on a big chunk of aluminum dropped the case temperature 24 °C without no further effort.

Blowing air over the case (probably) won’t be nearly as effective. Epoxy-ing a liquid-cooled cold plate to the end cap would improve the situation beyond all reasonable bounds, plus confer extreme geek cred.

Hmmm, the Warehouse Wing does have some copper tubing…
2013-03-11
Stepper Driver Waveforms: Current Control

A bit more data from this setup:

HB-415M Driver – test setup

As you saw earlier the low-speed waveform looked reasonably good, although the HB-415M driver produces only 71% of its rated current (so it’s actually 1 A peak, not the 1.5 A in the caption):

HB-415M 8-step 1.5A 20V

The driver runs in 1/8 microstep mode, which means 1 revolution = 8 × 200 step = 1600 steps. Each cycle of that stepped sine wave has 32 microsteps = 4 full steps/cycle × 8 microsteps. One cycle is about 27 ms, so 1 step = 840 µs → 1200 step/s → 0.74 rev/s → 44 rpm. The Thing-O-Matic runs at 47 step/mm → 34 mm/rev, so this speed corresponds to travel at 25 mm/s, roughly the usual printing pace.

Admittedly, that hairball on the bench isn’t a realistic arrangement, because the motor runs with no load. On the other paw, assuming you’ve done a good job eliminating mechanical binding, then it’s probably pretty close to what you’d see during constant-speed travel.

Cranking the pulse generator to 6400 step/s = 133 mm/s produces this waveform:

HB-415M 1A 8step 24V

The power supply was 24 V, but there was no visible difference at 20 V. The driver evidently can’t control the winding current on the downward side of the waveform. Adding some frictional torque by grabbing the yellow interrupter wheel improved the situation, but not by much.

A box of 2M542 drivers just arrived from a nominally reputable supplier, although they were actually labeled M542ES. Under the same conditions, they produce this waveform:

M542ES 1A 8step 24V

So there’s something to be said for larger drivers; the HB-415M drivers were operating at their upper limit and the M542ES at their lower limit, both producing close to 1 A peak.

2013-03-10
Gratuitous Engine Jeweling

While pondering whether I should use the carcass of an old Dell PC to house the stepper drivers and control logic for the LinuxCNC M2 project, I bandsawed a scrap of aluminum sheet to about the right size. It had some truly nasty gouges and bonded-on crud, so I chucked up a wire brush cup in the drill press and had at it:

Machine jeweled baseplate

It’s obvious I haven’t done jeweling in a long time, isn’t it? Even a crude engine jeweling job spiffs things right up, though, even if a cough showcase job like this deserves straighter lines and more precise spacing. The aluminum sheet is far too large for the Sherline, which put CNC right out of consideration, and I’m not up for sufficient crank spinning on the big manual mill.

I match-marked mounting holes directly from the harvested motherboard and drilled them, whereupon I discovered that the aluminum is a dead-soft gummy alloy that doesn’t machine cleanly: it won’t become the final baseplate.

Memo to Self: Use the shop vacuum with the nozzle spinward of the brush, fool.

2013-03-02
HB-415M Stepper Driver Heatsinking: Lack Thereof

Just to see what’s inside, I took those HB-415M drivers apart. They’re not all identical inside:

HB-415M Driver – interior top

The other side shouldn’t come as much of a surprise:

HB-415M Driver heatsinking

Now, admittedly, I’ve applied a heatsink to the top of an epoxy package, but that DIP package has thermal tabs that should connect to the heatsink through a low-thermal-resistance path. A dab (!) of heatsink grease and what might be a thermally conductive plastic sheet atop the package seem, well, insufficient.

The driver chip sports an Allegro A3992 marking that might be genuine. The datasheet goes into some detail as to how you should lay out the PCB; none of its recommendations made it into the finished product. In particular, the hulking current sense resistors surely have more inductance than you’d like.

The resistor color code seems odd: black red red silver brown.

HB-415M current sense resisors

Using black as the first band is unexpected, but it’s probably the only way to indicate a low-value resistance without printing the numbers: 0.22 Ω ±1%.

Ah, well, the peak current isn’t as high as they claim, so it all probably works out in the end.

2013-02-27