Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
The 36 V 350 W power brick for the improved M2 HBP arrived and seems to work fine, apart from a distinct smell of hot electronics under load. Dialed back to 30.1 V at the terminals (to match the HBP spec) and with the HBP connected through the same length of 12 AWG wire as before, the supply draws 150 W from the AC line.
It draws 160 W at 31.7 V and stabilizes at about 100 °C. The heater resistance is 7.6 Ω before it has a chance to cool off, so the heater runs at 4.17 A and 132 W. The supply efficiency is 83% = 132/160, about what you’d expect. The fan runs intermittently with that load.
In order to dissipate 150 W in the panel at the same resistance, the voltage must be 33.5 V at 4.5 A. I’d want to install it in the M2 and make some measurements before jumping to any conclusions.
The SSR’s forward drop runs around 1.0 to 1.1 V at 4 A, which suggests a drain-source resistance near 0.25 Ω, rather more than you’d expect for a bare MOSFET, but probably about right for an up-armored device. Or it could just be a crap MOSFET inside there…
So I think the brick will wind up at about 35 V to make up for the SSR drop. The SSR will dissipate about 5 W and won’t need much heatsinking; just bolting it to an aluminum chassis may suffice.
I picked up five 12 V 40 W cartridge heaters from the usual eBay source for some extruder experiments and did a quick check to make sure they actually worked:
Cartridge heater test
The bench supply is good for 3 A, which isn’t quite enough to light them up all the way, but at 8 V they drew anywhere from 2.67 to 2.20 A, declining by about 0.1 A as they heated over the course of maybe 5 s, which is about as long as you want to run them outside of whatever they’re supposed to be heating.
Those dissipations are a bit lower than I expected; at 8 V you’d expect to see about 27 W = 2/3 * 40 W, not the 18 to 21 W I actually measured. Current & power don’t scale linearly, so I must gimmick up a larger block and make some better measurements when I get the LinuxCNC hardware set up.
The insulating tubes on the wires emerging from the cartridge, inside the main sheath, show the usual attention to detail I’ve come to know and love from eBay suppliers:
Cheap cartridge heater insulation
Ah, well, it keeps my toy budget under control…
There’s a story behind the dark vertical smudge just to the right of the cartridge. More on that in a bit.
My stock of single-row header pins seems to be running short, so it’s time for another slitting session:
Header pin slicing
Manual CNC, typing bare G-Code directly into LinuxCNC Axis: no reason to turn the cranks by hand.
This makes absolutely no economic sense, but it’s a sticky-hot day and the Basement Laboratory has the dehumidifier. Some day I’ll run into a killer surplus sale of single-row headers and that’ll solve the problem forever…
The Basement Warehouse Wing has an essentially unlimited supply of pristine CD cases (remember CDs?) that, with a bit of deft bandsaw work, will each emit a pair of 4×4 inch sheets of perfectly transparent acrylic plastic. The sheets are about 1.3 mm = 50 mils thick, which is just about exactly what you want for a Nixie-style display that doesn’t require high voltages, because you can edge-light a sheet with 0603 amber SMD LEDs. Obviously, this is not a Shining New Idea, but this post collects my doodles so they don’t get lost along the way.
The Squidwrench StickerLab session prodded me into lashing a prototype together to see how this would work; they have a Silhouette Cameo vinyl cutter driven with Robotcut that works well. I’d hoped to do some laser cutting at the subsequent session, but our schedules didn’t mesh.
The compelling advantage of laser cutting is that you could crack the CD cases apart, throw out the CD holder gimcrackery, lay the sheets flat on the cutter table with the latches & other junk upward, and burn the digits out of the middle without any further preparation. I think I could get much the same effect, at least for a crude prototype, by milling & engraving with the Sherline.
The sheets are about 4 threads of 3D printed plastic extruded at the M2’s default 0.4 mm width. You could print a black baseplate with slots to hold the sheets, put two threads between each sheet, and have the sheet 6 threads apart on center = 2.4 mm spacing:
Tab vs 3D thread size doodle
Ten such sheets would produce a standard 0-to-9 display about an inch deep, plus protective sheets front and back, so the whole affair would be maybe 1.25 inch deep. You’d probably want to offset the tabs on adjacent sheets to reduce light leakage between LEDs. The baseplate fits atop a PCB with LEDs at the right locations, so you get an opaque holder for the sheets that’s easy to produce and easy to assemble:
Sheet tab layout doodle
If you were clever, you could have different tab locations on each sheet so they’d fit only in the proper orientation; that might be important for cough mass production.
The M2 has a platform big enough to build an entire clock base in one pass, plus a matching piece to capture the tops of the digits. I think edge-lit acrylic needs a complete opaque surround for each digit sheet to block light leaking from the edges; it might be easier to build the mount in the other direction, lying flat on the platform, stack the mounts together with the digit sheets, then bolt the whole assembly from the front; that would ensure perfect alignment of everything.
In that case, the 3D printed layers are 0.25 mm (or smaller), but the resolution for the tabs would be 0.4 mm. If you were exceedingly brave & daring, you could lay the digit sheets in place during the build and come out with a monolithic unit; that might require a bit of clearance atop each sheet, as a grazing touch from a hot nozzle would be painfully obvious.
There’s also no reason you couldn’t use a wider “digit” sheet and engrave, say, the days of the week or the units of measurement or something like that on each panel.
If the display will be 30 mm deep, then the digits must be large enough that the depth doesn’t turn each digit into a tunnel. Large Nixe tubes had digits about 40 mm tall, so I went with a 30 x 45 panel, plus 1 mm tabs on the top and bottom:
Crude edge-lit acrylic panel vs vinyl stencil
The “engraved” digit on the left came from a vinyl mask similar to the one on the right, using fine sandpaper to rough up the acrylic surface. I deliberately started with a battered old CD case in order to prevent myself from getting too compulsive with neatness; as you’ll see, edge-lit acrylic reveals any surface imperfections, so cleanliness is important.
The black border could be a light-shield gasket around the outer edge of the display panel to reduce glare from the edges. This might be more important for laser-cut pieces with highly reflective edges or for milled pieces with diffuse edges; there’s no way to tell without actually building one to see. I simply bandsawed the sheet around the edges of the mask, then filed off the larger chunks: the edges are very, very rough, indeed.
There doesn’t seem to be an easy way to stash the Inkscape SVG file on WordPress.
I solder-blobbed some wire-wrap wire, a 1206 SMD resistor, and a 0603 LED together:
Crude 0603 SMD LED lashup
The 0603 SMD LED fits neatly along the edge of the sheet:
0603 SMD on CD case edge
A 3rd hand holds it upright on the bench over the LED lashup:
Edge-lit acrylic – front layout
It looks marginally better with the lights out, but you can see all the scratches:
Edge-lit acrylic – front detail
The hot spot at the bottom of the digit isn’t nearly that awful in person.
A top view shows the glowing edges, plus the nuclear glow from the LED:
Edge-lit acrylic – top view
A touch of soft focus, plus moving the LED under a tab location, helps a bit:
Edge-lit acrylic – front soft focus
You’d want two LEDs per digit and maybe one at the top, but that’s in the nature of fine tuning.
All in all, I like how it looks. Getting from this crud to a workable display will require far more effort than I can devote to it right now…
The first two charges for those Baofeng BL-5 batteries show that the actual capacity isn’t quite up to the 1800 mA·h spec:
Baofeng BL-5 Packs – First two charges
The (meager) instructions say that the batteries will reach “full capacity” after three charges. Unless there’s a miracle waiting in the wings for that third charge, I very much doubt that they’ll get any better than the 1400 to 1500 mA·h you see in that graph. Note that the two batteries have quite different capacities and that the capacity for Pack B decreased on the second charge (purple vs. green trace).
Compare that with the Wouxun batteries (plotted with Gnuplot, rather than a screen grab):
Wouxun 7.4 V Packs
Those are all at 250 mA, which is certainly less than the peak current and probably more than the average current. It’s close enough for now, anyway, and shows that the Wouxun batteries actually live up to their spec.
Huh. Who’d’a thunk it?
It looks like the blinky lights should go into power-save mode under 7 V, because there just isn’t that much capacity left when the cells start rolling over the edge of the cliff.
Although the M2’s heated build platform works well enough, somebody who knows what he’s doing (you know who you are: thanks!) sent me an improved version. It’s a PCB heater, laid out to compensate for the usual edge cooling, firmly attached to a tempered glass plate with genuine 3M thermally conductive tape:
Improved M2 HBP – test setup
They designed the heater around the 30 VDC power supply used in their other equipment. Although I had high moderate hopes that a boost power supply would convert the 24 V supply I already had for the stepper driver bricks into the 30 V for the heater, it was not to be. So there’s a 36 V 9.7 A 350 W supply arcing around the planet that (I think) should work better: adjust the voltage down as far as it’ll go, soak up another few volts in the solid-state relay, and Things Should Be Close Enough to 30 V. One can buy a genuine 30 V supply, but it costs surprisingly more than either 24 V or 36 V supplies on the surplus / eBay market and won’t really provide the proper voltage without upward tweaking anyway.
I replaced their standard 0.156 inch square terminals with Anderson Powerpoles, soldered a length of shielded cable to the 100 kΩ thermistor pads, and gimmicked up a connection to the 24 V supply; it delivered 23.7 V at the PCB terminals. The thermistor is 100 kΩ at 25 °C and 11.4 kΩ at 77 °C. The PCB heater is 5.9 Ω at 25 °C and 7.3 Ω at 77 °C; it dissipates 77 W at 77 °C (no, that’s not a typo).
The ultimate temperature looks to be about 90 °C with a 24 V supply, which isn’t quite enough for ABS (which I’m not using in the M2 right now, but probably will eventually). The time constant, assuming the 1-e-1 point is 66 °C, works out to about 9 minutes; it’ll be up to final temperature in half an hour. Those numbers aren’t quite as accurate as one might wish, because the heater power drops as the temperature rises and the copper resistance increases.
A 30 V supply would dissipate 120 W at 77 °C and rumor has it that the ultimate temperature is around 125 °C, which would be fine for ABS. Goosing the power a bit would produce more heat, but I’v been running the Thing-O-Matic at 110 °C and that’s good enough. More power, of course, gets it to the temperature setpoint faster, which is probably a Very Good Thing.
Obviously, you need PWM to control the temperature; given a 9 minute time constant, a bang-bang controller will work perfectly well.
The original data, including the thermistor resistance after I got my act together, plus a cute little temperature-vs-time graph:
Improved M2 HBP – 24 V supply
The colored flyspecks are part of the paper; I salvaged a stack of fancy menu cards from a trash can and padded them up as geek scratch paper.
The Smell of Molten Projects in the Morning 