Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Category: Science
If you measure something often enough, it becomes science
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.
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.
For reasons that will become apparent in a while, I got a pair (*) of boost power supplies from the usual eBay source, allegedly capable of boosting a 10-to-32 VDC input to a 12-to-35 VDC output, up to 10 A and 150 W:
Boost power supply
After establishing that it would not produce 30 V into a 5.9 Ω load (5.1 A, but 152 W), I got systematic.
A 100 Ω resistor drew 1/4 A while I set the output to 28 V. Doubling up the resistors showed that it worked OK at half an amp:
Boost supply – 50 ohm load
Four 6 Ω resistors in series draw 1.2 A, then (channeling the true spirit of DIY 3D printing) two in series drew 2.3 A:
Boost supply – 12 ohm load
That’s 32 W each and, yes, they did get toasty, but, no, I didn’t leave them turned on all that long.
But a 6 Ω resistance still didn’t work, so the supply can’t provide 4.7 A at 130 W. In case you were wondering, that’s two 6 Ω resistors in series and a pair of those strings in parallel, so each resistor still sees 32 W.
In terms of driving the actual load, these supplies aren’t going to light it up.
Ah, well, whaddaya want for five buck from halfway around the planet?
(*) Davy’s Aphorism: Never buy only one of any surplus item, because you’ll never find another. Get at least two, maybe three if it’s something you might actually use.
A month ago I tossed a new bag of silica gel into the basement safe and put the used one on the workbench to see how much more water it would adsorb. The numbers worked out like this:
Bag + staples: about 8 g
Dry weight: 500 g of silica gel beads
At 24%RH: 575 g = +67 g water
At basement ambient, about 50%RH: 652 g = +144 g water
At upstairs ambient, about 65%RH: 673 g = +165 g water
At 50%RH, the capacity is about 27% = 135 g of water, which is close to the measured 144 g. The logger recording groundwater temperature says the average humidity hovers just under 55%RH, in which case 28% capacity = 140 g of water: as close as you could possibly hope for.
At 65%RH, the capacity is about 32% = 160 g of water, which is very close to the measured 165 g.
The safe humidity remains flatlined at the logger’s 15%RH minimum level, with one blip when I installed the door gasket strips:
Basement_Safe – 2013-08-28
After I accumulate a few more used bags, we’ll see how well they regenerate.
A bit of rummaging in the Big Box o’ Weatherstripping produced the stub end of a spool bearing 1/4 x 1/8 foam tape that exactly fills the gap between the Basement Safe’s door and liner:
Basement Safe – Foam door seal – latch side
The hinge side of the door has tape between the door liner and the safe wall, because that closes in compression rather than shear:
Basement Safe – Foam door seal – hinge side
There should be a big bump in the humidity record marking that installation, but I don’t expect any immediate difference. If the silica gel lasts more than two months, I’ll consider it a win.
The Smell of Molten Projects in the Morning 