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Archive for category Science

Epoxy Joint: Test to Destruction

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:

Needle LEDs power supply - exterior
Needle LEDs power supply – exterior

The LEDs connected through a coaxial power jack on the far side of the box, held in place with a generous blob of epoxy:

Needle LEDs power supply - interior
Needle LEDs power supply – interior

A closer look:

Kenmore 158 COB LED - power supply jack
Kenmore 158 COB LED – power supply jack

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:

Kenmore 158 COB LED - power supply jack - epoxy bond
Kenmore 158 COB LED – power supply jack – epoxy bond

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:

Kenmore 158 COB LED - power supply
Kenmore 158 COB LED – power supply

More about all that, later …

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Monthly Science: Lithium Cells vs. Low Discharge Current

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:

Sony NP-BX1 - Wasabi GHIJK - 2017-09-01 - annotated
Sony NP-BX1 – Wasabi GHIJK – 2017-09-01 – annotated

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:

Defunct NP-BX1 for Blinkies and Glowies - AmpHr - 2019-01
Defunct NP-BX1 for Blinkies and Glowies – AmpHr – 2019-01

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:

Arduino Pro Mini - NP-BX1 cell - SK6812 - blue phase
Arduino Pro Mini – NP-BX1 cell – SK6812 – blue phase

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:

Arduino Pro Mini - NP-BX1 run times - single SK6812 - 2019-01
Arduino Pro Mini – NP-BX1 run times – single SK6812 – 2019-01

The 100 mA graph plotted against watt·hours has a similar shape:

Defunct NP-BX1 for Blinkies and Glowies - 2019-01
Defunct NP-BX1 for Blinkies and Glowies – 2019-01

You’d use those results for a constant power load similar to a camera or, basically, any electronics with a boost supply.

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DMM Probes

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:

No-Name DMM probes - needle tip - 20 A
No-Name DMM probes – needle tip – 20 A

If you look out along the wire, though, you’ll find a 10 A rating:

No-Name DMM probes - needle tip - 10 A wires
No-Name DMM probes – needle tip – 10 A wires

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:

DMM Clip Leads - 16 AWG
DMM Clip Leads – 16 AWG

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:

Siglent DMM probes - 10 A
Siglent DMM probes – 10 A

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:

No-Name DMM probes - 20 A
No-Name DMM probes – 20 A

Wouldn’t trust any of ’em for more than a few amps, tops …

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Tensilizing Copper Wire

The “bus bars” on the battery holders are 14 AWG copper wire:

Astable - NP-BX1 base - detail
Astable – NP-BX1 base – detail

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:

Slide Hammer
Slide Hammer

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:

Slide Hammer Snout
Slide Hammer Snout

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:

Straightened 14 AWG Copper Wires
Straightened 14 AWG Copper Wires

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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Monthly Image: Red Oaks Mill Dam Flow

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

Red Oaks Mill Dam 2016-12-11

January 2018:

Red Oaks Mill Dam - 2018-01-13

Red Oaks Mill Dam – 2018-01-13

April 2018:

Red Oaks Mill Dam - 2018-04-08

Red Oaks Mill Dam – 2018-04-08

October 2018:

Red Oaks Mill Dam - 2018-10-29

Red Oaks Mill Dam – 2018-10-29

November 2018:

Red Oaks Mill Dam - 2018-11-18

Red Oaks Mill Dam – 2018-11-18

December 2018:

Red Oaks Mill Dam - 2018-12-31

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

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 …

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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

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.

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Astable Multivibrator: 2N7000 MOSFET IDSS and Vthr Tests

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

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

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

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

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.

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