Archive for category Science
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.
Monthly Image: Red Oaks Mill Dam Flow
Posted by Ed in Photography & Images, Science on 2019-01-15
Some of our regular walks take us over the Rt 376 bridge downstream of the Red Oaks Mill dam and I try to take a picture whenever we cross.
For reference, two years ago in December 2016:
Red Oaks Mill Dam 2016-12-11
January 2018:
Red Oaks Mill Dam – 2018-01-13
April 2018:
Red Oaks Mill Dam – 2018-04-08
October 2018:
Red Oaks Mill Dam – 2018-10-29
November 2018:
Red Oaks Mill Dam – 2018-11-18
December 2018:
Red Oaks Mill Dam – 2018-12-31
The dam breast seem from the north (left in above pictures) in December 2018:
Red Oaks Mill Dam – north view – 2018-12-30
Searching for the obvious keywords will produce far more pictures than the subject may deserve.
Getting hydropower from the rubble would require considerable capital investment …
Red Fox Visitation
A Red Fox came trotting around the garden on the day before Christmas, then nosed up to the back of the house:
Red Fox visitation – 2018-12-24
Presumably, it was in search of a snack. We wish it good hunting.
A few hours later, the fox walked quickly across the back yard with half a dozen turkey toms close behind, perhaps urging it away from their hens. Everybody remained calm and collected, knowing their roles in this particular play.
FWIW, Marist College fields Red Foxes athletic teams. The women’s teams are Lady Red Foxes, as “vixen” carries entirely inappropriate connotations.
Astable Multivibrator: 2N7000 MOSFET IDSS and Vthr Tests
Posted by Ed in Electronics Workbench, Science on 2019-01-07
Building an astable multivibrator from MOSFETs for longer time constants and more reliable operation suggests I should know a bit more about their operation with minuscule currents and low voltages. I have a small stock of low-threshold ZVNL110A MOSFETs, but using something less obsolete seems in order.
Dirt-cheap 2N7000 MOSFETs have a maximum IDSS around 1 µA at room temperature, which would be way too high in this situation; there wouldn’t be much difference between their ON and OFF states.
The test setup is simplicity itself:
2N7000 IDSS – calibration
The initial reading from a 4 V bench supply was 0.00 µA on the Siglent SDM3045, my best low-current meter, so I put a 10 MΩ resistor across the drain and source terminals:
Astable – 2N7000 – IDSS cal
Close enough, particularly given the silver fourth band on that old carbon composition resistor and its no-doubt unclean surface.
The rest of the 2N7000 MOSFETs have IDSS ≤ 10 nA, which you can’t distinguish from zero on that scale.
The 2N7000 datasheet specs give a threshold voltage from 0.8 to 2.5 V for 1 mA drain current, bracketing a 2.1 V typical value, which would be too high for a nearly dead lithium cell.
I calibrated the VGS(thr) current at 11 µA with a 348 kΩ resistor:
2N7000 MOSFET Id cal
Which produced 11.49 µA at 4 V, just as it should, so I plugged in a MOSFET and twiddled the trimpot for a nice round 10 µA:
2N7000 MOSFET Vthr test
Most transistors conducted 10 µA with the gate at 1.42 V, with a few outliers spanning 50 mV on either side. Close enough and low enough!.
Now, to conjure an astable.
Blog Summary: 2018
Posted by Ed in Administrivia, Oddities, Science on 2019-01-01
Page views for 2018:
At this late date, the RepRap site has a much better G-Code reference, at least for the weird and wonderful assortment of 3D printer commands.
Given that I’m at best a secondary reference for Toyota Sienna ABS trouble codes, things must be getting grim out there in the minivan crowd.
And, as always, houses (and especially plumbing) are trouble!
As for everything else, well, it’s just me and my shop notes …
WordPress reports 101 k ad impressions per month for 24.6 k “page views”, suggesting most folks see four ads per page. If you’re not using an ad blocker, start now!
Those seem to be the most aggressive (and thus highly desirable to advertisers) video ads, because WordPress pays me a whopping 8¢ per kilo-impression; a few percent of the Youtube rate. The numbers are dropping, though, suggesting ads will never push me into the ranks of the thousandaires.
Now you know.
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