Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
The big rectangle at the top will heatsink the main p-MOSFET power switch, which shouldn’t dissipate much power at all. The two lower rectangles heatsink the n-MOSFETs, although I think they may each require a stand-up metal tab to handle the dissipation during high-duty-cycle blinkiness.
The silver line around the edge is soldered copper foil tape connecting the top and bottom ground planes; a dozen or so additional Z wires will connect the planes at high current nodes. It’ll get a bunch of flying signal wires, too, because I’m not a fanatic gotta-embed-all-the-wires kind of guy on stuff like this.
The PCB is 30 mil FR4, which (once again, I make this mistake over and over again) seems a bit bendy for surface-mount parts; I must print a simple nest to stabilize the poor thing. Some of the drilled holes look white, because I hadn’t rinsed out the remains of the silver plating powder; the surfaces are a lot more silvery in person.
Before I etched the back side, I realized I’d made a classic layout blunder: the high-current return path from the center MOSFET flows around the Hall effect sensor near the center of the board. So I filled in the grid pour with a fat point black Sharpie to get more copper in that area:
Hall Effect Brassboard – added etch masking
I think it probably wouldn’t matter either way, but nothing exceeds like excess. FWIW, I use the grid pattern as a way to verify the end of the board etching: when all the holes in the ground pour come clear, all the traces are done, too.
The etched backside came out OK, although with a few etched squares sprinkled in the Sharpie masking:
Hall Effect Brassboard – bare back
The three leads for the Hall effect sensor are just to the right of the center, with the middle lead connected to the ground pour. Given the millivolt-level signals, this isn’t a good place for ground bounce…
It’s etched with ferric chloride, rubbed with a sponge, and took under ten minutes on each side. That’s less than the usual time, which suggests the PCB is plated with half-ounce copper (that’s 0.5 ounce / ft2), rather than the one-ounce copper on the other boards I’ve done recently; just one of the hazards of buying surplus PCB stock. Doesn’t really matter, as the peak currents will be under half an amp and now I know not to use this batch of raw board stock for high current circuits.
All in all, it looks good enough. Now, for some component soldering.
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.
The Smell of Molten Projects in the Morning 