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.
Who’d’a thunk it?
Thinking about springs to apply downforce on plotting pen holders suggested magnets, so I extricated some neodymium bars from my collection of power toothbrush heads:
A snippet of magnetic field visualization film shows a dipole pattern:
Snapping two of them together in line:
… produces a quadrupole:
Now, if only I had some magnetic monopoles, this whole thing would be easier!
This one took us by surprise:
“As Advertised”, indeed, although the flyer got it right.
We mentioned it at the Meat Department and discovered they don’t print the signs, so Mary reported it to The Front Office.
It’s not as if I’ve never run afoul of a heterograph or perpetrated the occasional typo …
A control head from an ancient Brother BAS-311 sewing machine emerged from a recent Squidwrench clearing-out session:
The sturdy metal enclosure ought to be good for something, I thought, so I rescued it from the trash.
One of the ten button-head screws galled in place and resisted a few days of penetrating oil, so I drilled it out:
The PCB has no ICs! It simply routes all the LED and button pins through the pillar into the sewing machine controller:
The ribbon cable alternates the usual flat strip with sections of split conductors:
The split segments let it roll up into the pillar, with enough flexibility to allow rotating the head. I’ve seen segmented twisted-pair ribbon cable, but never just flat conductors.
Maybe the control head can become Art in its next life?
Spotted behind a small strip mall during one of our walks:
Perhaps the cable wasn’t rated for outdoor use?
The earth ground conductor isn’t insulated and the nonconductive filler strands look scary, but neither should kill you outright.
As far as I can tell, the insulation around the individual conductors remains intact, but it’s surely brittle and ready to fall off at the slightest touch.
The breaker box and cable are out of reach and, I suppose, out of mind.
Being the kind of guy who lives under a rock, I thought this thing lying at the end of the driveway might be a USB widget:
But the contacts are all wrong:
It has an opening on the other end:
An easy teardown produces a yard sale of parts:
The fiber snippet inside the coil carries the same sickly sweet scent as exhaled by passing vapers.
Some casual searching suggests it’s a Juul Vape Pod. The Juul site insists on lower browser armor than I’m willing to grant it; you’re on your own.
The heating coil press-fits into slots cut in the contacts:
It’s about 1 Ω cold, so I foolishly assume there’s a current limiter somewhere in the circuitry.
The little steel tube goes into the Tray o’ Cutoffs, where it might come in handy some day, the debris hits the trash, and I washed my hands up to the elbows.
Ya learn something new every day around here and, obviously, I must get out more …
Feeding 50% PWM at 1 kHz into the simpleminded 24 V BLDC driver produces the results you’d expect:
The upper trace shows the MOSFET drain voltage, the lower trace is the current at 200 mA/div.
The fan is connected from +24 VDC to the drain, so it’s getting power when the MOSFET is turned on and the drain is at 0 V. When the MOSFET turns off, the drain goes high and the drain current flow stops dead in its tracks.
Of course, the fan current doesn’t drop to zero, because inductance. The drain voltage rises until the MOSFET body diode enters avalanche breakdown, whereupon the energy in the magnetic field burns down across the voltage difference as usual.
Weird current waveforms happen all the time:
Or like this:
I think we’re looking at a sensorless BLDC controller trying to figure out the fan RPM from the back EMF after rebooting during each PWM cycle.
The fan turns at 2600 RPM at 50% PWM, close enough to the 2580 RPM I measured at 12 VDC.
In any event, the drain voltage in the upper trace tops out around 120 V, because the IRF530 MOSFET has a 100 V absolute maximum VDS spec: you’re watching avalanche breakdown happen. A transistor rated for 14 A of avalanche current isn’t in much danger quenching only 200 mA, though, so it’s all good, apart from slapping the fan with -100 V across what used to be its +24 V supply.
A closer look at the turn-off end of the pulse:
Eyeballometrically, the drain current decreases at 100 mA / 500 ns = 200 kA/s with the drain voltage clamped at 120 V, during the division just right of center. The other side of the fan sits at +24 VDC, so the effective inductance looks like 480 μH = 96 V / 200 kA/s. I’m unwilling to tear the blower apart just to measure the motor winding inductances.
In any event, because we’re seeing the output of a 24 V three-phase fan controller being reverse-biased at 100 V, I doubt those numbers mean anything, other than that you shouldn’t PWM-chop the current going into a BLDC fan controller, of course.
One of the March snowstorms dumped about a foot of wet, sticky snow on our yard throughout the day and evening:
The high-pressure sodium street light behind the tree glows orange, with LED yard lights on the right providing blue highlights.
The faint purple disk dead center in the image comes from the Pixel XL’s IR laser (so they say) rangefinder reflected in 1950-era window glass. Another image, with the Pixel pressed flat against the glass, shows two reflections:
Mary took a similar picture in the morning, standing in the patio just outside the front door:
The downed branch will require some chainsaw work, but, if past experience is any guide, the sticks will vanish from the end of the driveway within a day. The previous storm dropped a tree on the power lines half a mile northward, leaving us in the dark for about 18 hours.
Funny thing about major snowstorms, though: there’s not much looting in their aftermath.
The Smell of Molten Projects in the Morning 