Archive for category Science
Monthly Science: Baking Powder Desiccant
Being that type of guy, I tucked a bag of silica gel desiccant and a humidity indicator card into a #10 can of baking powder, then recorded the bag weight whenever I refilled the kitchen container:
For reasons not relevant here, we pretty much stopped using baking powder a couple of years ago, so there’s a protracted silence between the last two data points:
That last point emptied the can and, after a few days in the 60% RH basement, the bag weighed 243 g. The slope of the line suggests it’s been near 240 g for a while, although the humidity card showed the air was reasonably dry in there.
At our current rate, we’ll open the next can in a year or so …
Sapcicle Season
Posted by Ed in Photography & Images, Science on 2019-02-23
Once again, maple sap rises from the ground and falls from damaged branches:
Sapcicle – 1 
Sapcicle – 2 
Sapcicle – 3 
Sapcicle – 4 
Sapcicle – 5
And, sometimes, a tiny sweet treat during our walks …
“5 W” G4 COB LED Specsmanship
Posted by Ed in Electronics Workbench, Science on 2019-02-19
A bag of G4 COB LEDs arrived from halfway around the planet:
Those are “5 W” and “4 W” cool white modules, respectively, with another set of 4 W warm white looking pretty much the same. There’s no provision for heatsinking, which makes the wattage seem suspect; halogen G4 bulbs run around 20 W, for whatever that’s worth.
The silicone overlay becomes nearly transparent when seen through an ordinary desktop document scanner:
Highlighting the PCB copper pours shows 18 LEDs arranged in three series groups of six LEDs in parallel:
The “smart IC” touted in the writeup turns out to be a bridge rectifier for AC or DC power:
The SMD resistors on all 15 modules measure 27.6 Ω, more or less, and seem randomly oriented face-up or face-down. I assume that one is face-down; maybe it’s just unlabeled on both sides.
Back of the envelope: there’s no way it will dissipate 5 W. The bridge drops 1.4 V = 2×0.7, the LEDs drop maybe 9 V, leaving the resistor with 1.6 V to pass all of 60 mA, so call it 700 mW.
Some measurements:
With 12 VDC applied to the pins, the bridge drops 1.6 V, the LEDs 8.2 V, and the resistor 2.2 V, with 80 mA through the whole affair dissipating just under 1 W.
Huh.
Cranking the supply until the current hits 200 mA puts 15.7 V across the pins for a total dissipation of 3.1 W, burning 1.7 W in the LEDs and 1.1 W in the resistor.
Cranking the supply to 21.3 V drives 410 mA, dissipates just under 9 W total, produces a curl of rosin smoke from the PCB, and maybe delaminates the silicone around some of the LEDs.
OK, now I have a crash test dummy.
Given complete control over the application, I’ll strip everything off the PCB and bond it to a heatsink of some sort. With 6 LEDs in parallel, 120 mA (6 × 20 mA) total current might be reasonable and 200 mA (6 × 30 mA) probably won’t kill the things outright. Plus, I have spares.
An external 18 Ω resistor should suffice. Perhaps a pair of 6 Ω SMD resistors on the PCB, with fine-tuning through an external resistor. Call it 250 mW apiece: don’t use little bitty SMD resistors.
Epoxy Joint: Test to Destruction
Posted by Ed in Electronics Workbench, Machine Shop, Science on 2019-02-03
Some years ago, I put the LED power supply for one of the Kenmore 158 machines atop a plastic project box with an adjustable boost supply inside:
The LEDs connected through a coaxial power jack on the far side of the box, held in place with a generous blob of epoxy:
A closer look:
I’m adding a light bar, similar to the one now going onto the Juki TL-2010Q, which needs a direct connection to the 12 VDC supply. Rather than add another coaxial jack, I ripped out the existing jack and installed a DE-9 connector (serial ports being a fading memory by now), giving me an opportunity to test the epoxy joint:
Which required grabbing the connector with a pair of pliers and twisting / bending / abusing until it popped free. I don’t know how much grip the scored lines added to the joint, but the connector definitely didn’t give up without a fight; it wasn’t going to fall off on its own.
To be fair, the epoxy had a better grip on the coaxial jack than on the plastic plate, perhaps because the bottom of the jack had all manner of nooks and pins intended for PCB mounting. Ya use what ya got, sez I.
The new connector looks exactly like it should and, because it’s held in place by a pair of screws, should last forever, too:
More about all that, later …
Monthly Science: Lithium Cells vs. Low Discharge Current
Posted by Ed in Electronics Workbench, Photography & Images, Science on 2019-02-01
The amount of energy you can extract from a battery depends strongly on the discharge current, which is why the advertised capacity always exceeds the real-world capacity. Testing the NP-BX1 batteries for my Sony HDR-AS30V at about an amp produces a reasonable estimate of their run time in the camera:
Even though defunct cells lack enough capacity to keep the camera alive during a typical bike ride, they should power a microcontroller or astable multivibrator for quite a while.
My CBA II has a 100 mA minimum test current, which is far higher than the 15-ish mA drawn by the Arduino Pro Mini / Nano and SK6812 LEDs in a vacuum tube light, so these tests should provide a lower bound on the expected run time:
The two dotted lines show a “good” battery (Wasabi 2017 K) tested at 100 mA has a 1 A·h capacity similar to the “defunct” batteries. Testing at 1 A drops the capacity by a factor of two and eliminates the relatively constant voltage part of its discharge curve.
Handwaving: a 15 mA load on a battery with 1 A·hr capacity should run for 66 hours, ignoring nuances like the Arduino’s minimum voltage requirement and LED minimum forward voltages.
A few days of informal (“Oh, it stopped a while ago”) testing showed 50 hour run times, with little difference in the results for batteries with 800 mA·h and 1300 mA·h capacity:
The red power LED remains on long after the SK6812 LEDs dim out and the Arduino stops running. The blue and green LEDs fade before the red LED.
The run time test data:
The 100 mA graph plotted against watt·hours has a similar shape:
You’d use those results for a constant power load similar to a camera or, basically, any electronics with a boost supply.
DMM Probes
Posted by Ed in Electronics Workbench, Oddities, Science on 2019-01-25
After the Great DMM Probe Debacle, I picked up similar-but-different set of cheap probes and clip leads.
The needle-tip probes carry a 20 A current rating:
If you look out along the wire, though, you’ll find a 10 A rating:
Now, even though 20 AWG wire in silicone may carry a 17 A spec, the corresponding 200 °C temperature seems excessive for a test probe. Limiting the current to 10 A would reduce the power dissipation by two thirds, which should limit the temperature rise. Whether the wire actually contains 20 AWG of actual copper strands remains an open question.
The kit also had banana plug / test hooks with no particular rating, although the wire allegedly has 16 AWG conductors:
The banana plug / alligator clip combo claims 30 A, also with 16 AWG conductors. Who knows? It could be true.
For comparison, the Siglent SDM3045 DMM came with these probes:
The probes carry a 10 A rating and, although the wires aren’t branded, I’ll assume they have good-enough QC to ensure the copper matches the claims. The production values seem a bit higher, too, even if they bear a striking resemblance to the cheap probes.
And, for reference, the probes with the cold solder joint also claim 20 A:
Wouldn’t trust any of ’em for more than a few amps, tops …
Tensilizing Copper Wire
Posted by Ed in Electronics Workbench, Machine Shop, Science on 2019-01-18
The “bus bars” on the battery holders are 14 AWG copper wire:
Slightly stretching the wire straightens and work-hardens it, which I’d been doing by clamping one end in the bench vise, grabbing the other in a Vise-Grip, and whacking the Vise-Grip with a hammer. The results tended to be, mmm, hit-or-miss, with the wires often acquiring a slight bend due to an errant whack.
I finally fished out the slide hammer Mary made when we took a BOCES adult-ed machine shop class many many years ago:
The snout captured the head of a sheet metal screw you’d previously driven into a dented automobile fender. For my simple purposes, jamming the wire into the snout and tightening it firmly provides a Good Enough™ grip:
Clamp the other end of the wire into the bench vise, pull gently on the hammer to take the slack out of the wire, and slap the weight until one end of the wire breaks.
With a bit of attention to detail, the wires come out perfectly straight and ready to become Art:
The wires start out at 1.60 mm diameter (14 AWG should be 1.628, but you know how this stuff goes) and break around 1.55 mm. In principle, when the diameter drops 3%, the area will decrease by 6% and the length should increase by 6%, but in reality the 150 mm length stretches by only 1 mm = 1%, not 3 mm. My measurement-fu seems weak.
Highly recommended, particularly when your Favorite Wife made the tool.
The Harbor Freight version comes with a bunch of snouts suitable for car repair and is utterly unromantic.
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