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.

Author: Ed

  • Nonmagnetic Tweezers: Don’t Believe The Hype

    A small package of 6000 SMD resistors just arrived from a Hong Kong eBay seller. It showed up promptly despite traveling halfway around the world, had neat packaging, and I’ll give ’em good feedback.

    Also included was a free needle-tip tweezers, just exactly what you need for plucking those little ceramic rectangles from their packages. I  already have a bunch of needle-tip tweezers in my rack, but you can never have too many tools and this one won’t go to waste.

    Gooi TS-11 tweezers
    Gooi TS-11 tweezers

    The package has what appears to be comprehensive instructions in both Chinese and Japanese (to my untrained eyes, anyway). Not much in English, other than that Anti-magnetic, anti-acid and non-corrosive Stainless Steel line; perhaps this isn’t the export model. Indeed, it lacks the obligatory country-of-origin labeling, but, given where the package came from, one may reasonably assume the usual Chinese origin.

    The tweezers are (almost illegibly) stamped STAINLESS NON-MAGNETIC and bear a tidy sticker: gooi TS-11 ANTIMAGNETIC.

    Gooi TS-11 Antimagnetic sticker
    Gooi TS-11 Antimagnetic sticker

    The build quality and surface finish are, um, a bit rough, but Gooi seems really proud of their non/anti-magnetic properties.

    Needless to say, a magnet sticks firmly…

    I have no convenient way to test their anti-acid (whatever that is) and non-corrosive properties, but I’m betting these suckers are plain old Chinese mild steel, made from recycled US scrap. Perhaps the previous iteration was stainless and we’re stepping down the cost-saving ladder? If they would just change the packaging to match reality, that would be fine with me.

    [Insert standard observations about Chinese quality control here.]

    Y’know, come to think of it, I’m sort of wondering about those 6000 SMD resistors. With any luck they’ll actually work when I get around to using them. If not, I suppose it serves me right for buying direct from Hong Kong via eBay, eh?

    And, yes, I know some stainless steel is magnetic.

  • MAX4372 Sense Input Protection: The Story Continues

    Measured vs setpoint currents
    Measured vs setpoint currents

    As noted here, there’s a difference between the current setpoint (controlled by the PWM analog outputs) and the measured values. As it turns out, there’s a better way to look at those datapoints.

    This is a graph of measured current against the setpoints. Looks pretty good to me, apart from a teensy offset error. There really isn’t much in the way of a gain error over the entire range.

    Having had a bit of time to think this over, the measured current-sink current should generally be numerically equal to the setpoint value, simply because there’s an external op-amp forcing that to be true. The twiddlepot adjusting the op-amp gain doesn’t enter into this, because the loop forces that voltage to match the PWM output. So, duh, the purple line should be spot on, at least up to the point where the sink transistor saturates.

    What’s more interesting is that, over this range, the MAX4372 output is also spot on, which is not obvious from the previous chart. It flattens out when the common-mode voltage at the sense resistor drops below a volt, more or less, which is what the datasheet leads you to believe.

    The datapoints comes from the same panel on a different day, so the points don’t quite line up if you’re comparing them. The brown solar panel voltage curve flattens out when the current sink transistor saturates, but the panel can continue to supply increasing current into a dead short, so the current continues to rise for a bit.

    After I get the Circuit Cellar column laid to rest, I gotta figure all this out from first principles, then run the current up to 300 mA from the dreaded bench supply.

    But the short answer seems to be that the Schottky protection circuitry doesn’t have much effect up through 75 mV. Which seems reasonable, come to think of it.

  • MAX4372 Sense Input Protection: Results

    Solar Photovoltaic Panel Maximum Power Point Cloud
    Solar Photovoltaic Panel Maximum Power Points

    Here’s the result of using the Schottky diode input protection circuit I proposed there. I used 10 and 5 ohm resistors, twice the values shown in that schematic.

    The circuit runs a load test and determines the maximum power point (MPP) of a solar photovoltaic panel every minute. The points represent the results of about two hours of winter-afternoon sunlight.

    The test program applies an increasing load in steps of 10 mA (it’s a small panel, OK?) and records the corresponding panel voltage. One combination of current and voltage extracts the most power from the panel; that’s the MPP.

    Obviously, the MPP varies with the amount of sunlight falling on the panel, so the result of the test is a cloud of points. That’s what you see in the graph: the highest points represent the most intense sunshine, the lower points come from shadows and changing sun angle.

    The load is applied through a current sink that draws 100 mA per volt, as generated from a microcontroller PWM output. That’s pretty well calibrated by twiddling a gain pot, so I think it’s quite close.

    The graph shows that the 10 mA steps recorded by the now-well-protected MAX4372 high-side current amp are low by about 10%, regardless of the absolute current level. That’s more error than predicted by the SPICE model (even with the larger resistors) and may represent contributions from something other than the protection network.

    However, it does look as though a simple calibration routine could compensate for the error. That’s a simple matter of software, right?

    Most important of all, the MAX4372 has survived the usual mistreatment. The load test is essentially DC, where the inductor counts as a piece of wire. Under those conditions, the combination of a stiff voltage source and an imposed load exceeding 600 mA produces a lethal differential voltage across the current-sense resistor. The diode clamps that voltage to 300 mV or so, which is enough to protect the MAX4372.

    Rumor from my source at Maxim says the protection circuitry inside the MAX4372 can withstand maybe 50 mA, so the high-value external resistor approach (without the diode) may be the better way to go. Getting rid of the nonlinear diode should be a win…

    Update: A different plot shows a different result. I think the offset comes from something other than the protection circuitry.

  • They’re Getting Bolder!

    Turkey on the Patio
    Turkey on the Patio

    Got up this morning, looked out the kitchen window, and there stands a turkey on the patio!

    They’ve been edging closer and closer for the last week or so; we think the snow cover is making the seeds under the feeder look more attractive. As nearly as we can tell, though, they have yet to venture across the patio to the feeder: no tracks in the snow.

    What would be really impressive: a row of turkeys lined up on the patio railing, just like they do on our neighbor’s split-rail wooden fence.

  • Logitech Trackball: Tilting Thereof

    Trackball platform
    Trackball platform

    The right-hand trackball by my keyboard is a Logitech Cordless Optical Trackman, which I fixed a while ago with a laying-on-of-hands repair. If you do a lot of typing and want to save your wrists, a trackball might be just what you need.

    This trackball’s shape is strongly right-handed and I found that my wrist was happier when I tilted the trackball about 30 degrees to the right, making the ball almost vertical and the thumb buttons to the upper left. Evidently my wrist wants to work at a more clockwise angle, not at whatever Logitech found suitable.

    I made the platform from thin oak-veneer plywood left over from a bookshelf project, with oak wedges holding it up. Polyurethane glue, my favorite wood adhesive, holds everything together. I presented the bottom to the belt sander to get a nice flat surface and bevel the down-side edge of the platform, then applied non-skid rubber stair tread tape to the wedges.

    Conveniently, Logitech held the trackball’s case together with four plastic-tapping screws. I removed a screws at each end, drilled two matching holes in the platform, and used similar-size machine screws. The threads don’t quite match, but it’s close enough.

    Rotated trackball in use
    Rotated trackball in use

    Here’s what it looks like in use…

    The platform makes battery replacement a bit more tedious. Much to my surprise, the two AA cells run for half a year at a time, so that’s not a big issue.

    However, the trackball occasionally (every few weeks) loses sync with its base receiver, requiring a poke of buttons on both units. I think that’s partly due to the Logitech wireless mouse on my esteemed wife’s desk ten feet away.

    On the whole, I like it a lot. If Logitech made one for southpaws, too, I’d get a bookend set, but they don’t.

    Oh, yeah, if only evdev allowed button reconfiguration, without using a bunch of batshit kludges, I’d be ecstatic. As of the last time I fiddled with it, the standard mouse xorg driver couldn’t handle the number of buttons and evdev didn’t allow button mapping. Mostly, it works, but I’d like to reassign a few of the buttons.

  • Unsolderable Header Pins

    Unsolderable pin headers
    Unsolderable pin headers

    Speaking of things that don’t work, these header pins from my stash have developed some sort of rot. They’re genuine Brand Name pins, albeit a few decades old, and have been stored in the original bag in various basements along the way.

    What’s supposed to happen: you touch a pin with a soldering iron and some solder, the solder melts and wets the pin. If the pin is in a circuit board at the time, the solder bonds it to the pad surrounding the hole. Nothing exciting here, except that when I tried to use these pin headers, that didn’t happen.

    The symptom is that the headers are unsolderable: the solder doesn’t wet the pins.

    Non-solderable header pin detail
    Unsolderable header pin detail

    The detail view shows what does go on. When I touch a the pin, the original solder plating scoots out of the way, exposing the underlying metal (or whatever it is). Neither tin-lead nor tin-silver solder wets the surface, so the pin can’t be soldered.

    The flux forms a layer over the new surface and doesn’t do its usual job of cleaning the metal. Scraping the pin clean doesn’t seem to help, either. In fact, nothing helps: that whole bag of headers is a dead loss.

    I’m sure these things worked when they were fresh, but that was a long time ago. I’m not sure what sort of change could occur underneath the original solder plating.

    So I picked up some new headers with what passes for gold plating these days and they work fine.

    The pix come from my pocket camera on the binocular microscope, using my homebrew adapter.

  • NOAA N-Prime is Up at Last!

    Not that anybody pays attention to these things, but NOAA’s N-Prime earth observatory had a successful launch today.

    There’s a bit of backstory to this bird: Lockheed-Martin manged to drop the satellite during the final phase of its assembly, causing all manner of damage. Basically, they forgot to bolt the booster adapter down before trying to tilt the satellite over.

    It seems L-M ate the rebuild costs, which was a nice gesture on their part.

    I wrote about the event in my Dr Dobb’s Journal column some years back, in the context of how we do error checking in our projects. Bottom line: no matter how good you think your development process might be, alas, you’ll always miss something. The trick is to miss only small problems, not project-killers.

    Now, if only the bird works correctly…