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.

Category: Electronics Workbench

Electrical & Electronic gadgets

DC-DC Boost Converter: Another QC FAIL
Each LED emitter in the Kenmore 158 endcap light contains six chips in series:

LED mount – lighting test

Even though the current has the usual exponential relationship to the terminal voltage, the slope at 200 mA (100 mA each, assuming they share & share alike) remains low enough that I (think I) can get away with just dialing in a voltage and leaving it at that; changes due to small temperature variations won’t cause meaningful differences in the current.

That’s easier than building an adjustable current regulator, anyway.

The heap disgorged two cheap DC-to-DC boost converters from halfway around the planet, with about the right specs:
- 10 to 32 V DC in
- 12 to 35 V out
- 10 A
- 150 W
They couldn’t produce their rated output, but a pair of LEDs shouldn’t pose much of a challenge.

So I wired one up to the bench supply, set it for 12 V, turned it on, and wham it maxed out the supply at 3 A with no load on the converter’s output.

Huh.

Adding a suitable load resistor brought the input current down, but the voltage adjustment trimpot didn’t have much effect and the bench supply would still wham hit 3 A with no provocation, so the load resistor didn’t actually make any difference. Eventually, I figured out that simply pressing my finger on the trimpot caused the output to vary wildly.

Given that fairly broad hint, this became obvious:

Boost Converter – trimpot pins

Evidently, I had used the other converter for the previous tests. Huh.

With that trimpot pin soldered in place, the converter worked fine. Eyeballometrically speaking, the LEDs seem bright enough at 100 mA total (50 mA each) for my purposes, which happens at 18-ish V. Dissipating only 2 W won’t require nearly as much heatsink as they’re presently mounted on, although I should wait for warmer weather before concluding that they’re doing OK while crammed inside the end cap.

Before declaring victory, I took a closer look at the board and found this mmm oversight:

Boost Converter – masked 78L09 tab

Notice the big pad under the 78L09 regulator, with six thermal vias to an expansive copper pour on the other side of the board, completely covered with red solder mask.

Removing the regulator show the regulator’s footprint didn’t include the tab:

Boost Converter – 78L09 footprint

Maybe they decided, after a careful analysis, that the regulator couldn’t possibly dissipate enough power to warrant the additional solder required for the entire thermal pad. Heck, pocket a fraction of a yuan on ten million boards and you’re livin’ large.

Scraping the mask off, fluxing everything in sight, and soldering the regulator down probably won’t make any difference:

Boost Converter – scraped and soldered

Yes, The Bigger The Blob, The Better The Job strikes again. It does make me feel better and that’s all that counts.
2015-01-07
Kenmore 158 LED Heatsink: Epoxy Sculpture

The LED mounting plate inside the sewing machine’s end cap sits 30° from the vertical axis of the needle. Even though the surface-mount LED emitters have a broad pattern, it seemed reasonable to aim them toward the needle to put the brightest spot where it’s needed.

The LEDs must have enough heatsinking to pull 2+ W out of the solder pads, so I figured I’d just epoxy them firmly to the mounting plate, rather than try to gimmick up a circuit board that would interpose a fiberglass slab in the thermal path.

Combine those two requirements and you (well, I) get a wire fixture that provides both power and alignment:

LED mount – wire fixture

The LED body is 5 mm square, sin(30°) = 0.5, and the rear wire raises contact end by 2.5 mm. This still isn’t an exact science; if the center of the beam lands in the right time zone, that’s close enough.

Testing the LED assembly at low current before entombing it shows the emitters have six chips in series (clicky for more dots):

LED mount – lighting test

The grotendous solder job follows my “The Bigger the Blob, the Better the Job” principle, modulated by the difficulty of getting a smooth finish on bare wires. Indeed, the first wires I painstakingly bent, set up, and soldered turned out to have an un-solderable surface, much like the header pins from a while ago. That hank of wire now resides in the copper cable recycling bucket; you’re looking at Version 1.1.

Two strips of Kapton tape under the ends of the wires hold them off the (scoured and wiped clean!) aluminum plate, with more tape forming a dam around the nearest edges:

LED mount – epoxy pour

Despite being steel-filled, JB Weld remains nonconductive, the epoxy-filled gap under the wires insulates them from the plate, the wires aren’t shorted together, and there’s a great thermal bond to the heatsink. Good stuff, that JB Weld!

A view from the back side shows the epoxy sagging over the wires before I added another blob:

LED mount – epoxy pour – rear

The LED assembly just sits there, without being anchored, until the epoxy cures. The epoxy remains thick enough (in the rather chilly Basement Laboratory) so that it doesn’t exactly pour, can be eased into place without too much muss & fuss, and stays pretty much where it’s put.

After the epoxy stiffened a bit, I gingerly positioned stranded wires not-quite-touching the LED wires and applied a dot of solder to each. Powering the LEDs from a bench supply at 500 mW each took the chill off the heatsink and encouraged proper curing:

LED mount – heated epoxy cure

Fast forward to the next day, return the heatsink to the Sherline, and drill a hole for the power cable. It’s centered between the wires in Y and between the fins in X, which is why I couldn’t drill before mounting the LEDs:

LED mount – drilling cable hole

It’s not like I’m building this from any specs…

Trim the wires, solder the cable in place, cover the wire ends & joints with JB KwikWeld epoxy, and it’s done:

LED mount – final epoxy

With the LEDs running their 230 mA rated current, the entire heatsink gets pleasantly warm and the mounting plate isn’t much warmer than that. I loves me a good JB Weld job…

However, I suspect they’ll shine too brightly at full throttle, which means an adjustable power supply looms on the horizon…

2015-01-06
Pilot InstaBoost: Product Cheapnification in Full Effect

After rebuilding the battery clamps on the Pilot Instaboost jump starter, something on the back of the package caught my eye:

Pilot InstaBoost – clamp picture

The un-modified joint on the as-delivered clamp has a plastic stud and nothing through the spring:

Battery Clamp – original joint

Compare the first picture with our modifications:

Battery Clamp – improved joint

Looks like Pilot applied some cost reduction between taking the picture and shipping what we have now.

I bet they cheapnified something else, too. Something that cost them a lot more and the absence of which can’t be verified by most consumers…

2014-12-31
Sony 64 GB MicroSDXC Card: Speed Reduction Thereof

So one of my Genuine Sony 64 GB MicroSDXC cards stopped working in my Genuine Sony HDR-AS30V action camera, failing to record video after starting normally.

For example:

The RCVER status display doesn’t appear anywhere in the manual, but also occurs when the camera must rebuild its metadata indexes. Or something like that. Anyhow, it’s obviously unhappy about what just happened in the course of recording.

After several weeks of having Sony ignore my emailed requests (no “email agent” never contacted me after the initial “we’re on it” autoreplies) and after several days of being blown off by their phone menu (800-222-7669 and 800-282-2848 lead to the same tree, after which 5 – 1 – 6 disconnects after one ringy dingy), I got another number by picking a reasonable (to me) option and bulldozing the pleasant voice off-script: 877-440-3453. It turns out that if you’re at the Digital Camera node in the Sony tech support tree, the helpful agent cannot find the model number of the SR-64UY MicroSDXC card in their database, even though I’m looking at the Sony Support web page describing it.

Anyhow, 877-440-3453 (or the “direct” 956-795-4660) produces a pleasant voice that directs me to their Media Services center in Texas and, after clicking on the Ordering Information menu item (isn’t that obvious?), produces a PDF that one fills in and sends with the failed media for their perusal.

Being that type of guy, I sent in a somewhat more extensive description than would fit in the tiny space on the form:

The problem with this SR-64UY MicroSDXC card (serial N73WAXOP) is that it cannot record video at the highest resolution produced by my SONY HDR-AS30V action camera: 1920x1080p @ 60 fps.

The formatted data capacity seems unchanged at 59 GB, so the problem is not a loss of capacity.

The camera starts recording and will continue for a few seconds or a few minutes, at which point it stops recording, flashes WAIT, then RCVER (“recover”), then returns to its idle mode. The recorded video is correct up to the failure.

I have reformatted the card in the camera, which does not correct the problem.

An identical SR-64UY MicroSDXC card (serial N73WA9JM), bought shortly afterward and not used, continues to operate correctly, so the problem isn’t the fault of the camera.

The failing card (XOP) has recorded less than 100 sessions since August, while the working card (9JM) has been sitting, unused, on my desk. Recording sessions generally run 45 to 90 minutes and the AS30V produces a 4 GB every 22 minutes, so each session involves 2 to 6 large video files, plus the same number of thumbnails. I transfer the files to a PC and delete them from the card after each session. The card has therefore recorded only 1000 GB of video before failing.

The XOP card can record video at 1920×1080 @ 30 fps and all lower resolutions. The camera requires a Class 4 speed, which means that the SR-64UY card no longer meets its Class 10 / U 1 speed rating.

Please replace this card with one that meets its speed rating.

Thank you…

The replacement card just arrived, so a speed reduction is a warranty failure.

I’ll test this one by plugging it into the high-amperage Micro-USB charger for the Kindle, aiming it at a clock, and letting it run until it’s either filled the card with excruciatingly boring high-data-rate video or crashed & burned in the attempt.

2014-12-30
Kenmore 158: Pulse Drive First Light
This worked right out of the box:

Pulse Drive – Tek 1 A-div

That’s roughly two half-cycles of the full-wave rectified AC with about 100 ms between pulses.

The upper trace comes from the differential amp, the lower trace from the Tek current probe at 1 A/div. The overall amp transconductance looks to be 1.3 A/V = 1.3 A/div, minus that small DC offset, so the ADC range is actually 6.5 A. That might be a bit too much, all things considered, but not worth changing right now.

Notice that the upper trace drops like a rock at the end of the pulse, while the Tek probe shows a gradual decrease. The missing current goes ’round and ’round through the flyback diode across the motor:

Pulse Drive – Flyback Diode – Tek 1 A-div

The Tek probe in the lower trace goes on the green wire connecting the diode to the bridge rectifier, oriented to match the diode polarity (+ current flows from motor to blue wire on collector to brown wire on rectifier to motor):

Motor flyback diode – installed

That nasty little spike in the middle of the diff amp output occurs when the collector voltage drops to zero and the ET227 shuts off, but the motor current continues to flow due to the winding inductance. In the first scope shot, the Tek probe doesn’t show any spikes in the motor current, because there aren’t any.

Compare that with the voltage and current of the motor running from an isolation transformer:

Rectified AC – 200 mA div – 875 RPM

As the pulse repetition frequency increases, the motor speed goes up and the current goes down:

Pulse Drive – Fast – Tek 1 A-div

The dropouts between successive pairs of half-cycles show where the firmware shuts off the current and goes once around the main loop.

The Arduino code making that happen:
```
PedalPosition = ReadAI(PIN_PEDAL);
if (PedalPosition > 190) {
	BaseDAC.setVoltage(Cvt_mA_to_DAC(3000),false);					// give it a solid pulse
	MotorDrive.ADCvalue = SampleCurrent(PIN_CURRENT_SENSE);			// measure current = half cycle delay
	MotorDrive.ActualCurrent = Cvt_ADC_to_mA(MotorDrive.ADCvalue);
	printf("%5u, %5u, %5u, %5u, %5u, %5u, %5u\r\n",
		MotorSensor.RPM,ShaftSensor.RPM,MotorDrive.State,
		MotorDrive.DACvalue,MotorDrive.ADCvalue,MotorDrive.ActualCurrent,PedalPosition);
	delay(3);														// finish rest of half cycle
	BaseDAC.setVoltage(0,false);									//  ... then turn it off

	delay(map(PedalPosition,190,870,100,0));						// pedal controls off time
}
```
The map() function flips the sense of the analog voltage coming from the pedal, so that more pedal pressure = higher voltage = lower delay. The pedal voltage produces ADC values from about 185 through 860, with a pleasant sigmoid shape that gives good speed control.

The maximum motor speed isn’t quite high enough for bobbin winding, but I like what I see so far!
2014-12-29
Improved Pilot InstaBoost Jumpstarter Clamps

The Sienna now spends all its time sitting outdoors in an apartment parking lot and gets even less driving (hence, battery-charging) time than we used to give it. Fortunately, Santa being my kind of guy, our Larval Engineer received a Pilot InstaBoost jumpstarter, which is basically a 10 A·h / 40 W·h lithium battery with husky plug & socket connectors, a pair of 10 AWG wires, and big alligator clamps. The package claims a 400 A peak discharge rate, but the tiny inscription on the back of the case reports 200 A; either of those seems mmmm somewhat optimistic to me.

The customer reviews suggest that the plastic battery clamp handles feature a crappy hinge joint which disintegrates under moderate stress on a cold winter night, firing the spring into the nearest snowbank and rendering the clamp completely useless. The joint consists of a plastic post on each side of the inner handle that protrudes into a hole in the outer handle:

Battery Clamp – original joint

I assigned her some Mandatory Quality Shop Time to improve the joint. She found some brass tubing that fit the existing hole and cut two pieces to length:

Battery Clamp – cutting brass tube

A 1 inch stainless screw was just barely long enough (that’s Loctite Red in the nut), but the end result certainly looks durable enough:

Battery Clamp – improved joint

It’s along the same lines as the improvement I applied to my old Park Tool MTB-7 Rescue Tool.

Apart from that, the clamps look pretty good. There’s even a husky braid between the two jaw pads, ensuring at least one reasonably low resistance joint to the battery post:

Battery Clamp – jaw strap

With a bit of luck, we’ll never know how well it works as a jumpstarter. She can use the USB port to keep her phone charged, which may provide enough motivation to keep the thing topped up and ready for use…

[Update: two days after this post went live, someone found it by searching for:

how to repair clamps pilot instaboost 400

You have been warned!]

2014-12-28
Kenmore 158: Recalibrated Optoisolator Drive

Because the motor will draw more current during pulsed operation, the ET227 needs more base drive. The existing circuit topped out around 2.5 A, so I reduced the current sampling resistor by a bit:

Optoisolator Driver

If you care about the exact current, you’d use a 1% resistor, but if you care about the current, you’ll be doing closed-loop feedback to compensate for the transistor gain variations. Compared to those, the resistor doesn’t matter.

Running the MCP4725 DAC through its range produces a nice graph:

Current Calibrate – DAC – 270k Hall 2.7k opto

The X axis comes from the Tek Hall-effect current probe, so the numbers don’t depend on the ferrite toroid & differential amp calibration. They do, of course, require a bit of eyeballometric calibration to extract the flat top from the waveform, as shown by this old waveform:

Motor current – ADC sample timing

Ya gotta start somewhere.

The linear fit to those dots gives the DAC value required to produce the observed current, at least for these particular transistors at whatever temperature they’re at in a rather chilly Basement Laboratory.

Of course, the observed current tops out at 1.2 A: the motor’s peak current during normal linear operation. The line looks so pretty that I’ll assume it continues upward to the maximum 12-bit DAC value of 4095 and the corresponding ET227 current. Working backwards, that will be 3.1 A and should suffice for all but the highest peaks at high line voltage.

2014-12-26