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.
Electrical & Electronic gadgets
The running lights on our Tour Easy fairing sit just about perfectly level, despite how they appear in relation to the fairing edge:
And, because they’re firmly attached to the fairing mount, there’s no way to tilt them to extract the 18650 cell.
This took entirely too long to figure out:
The LC40 end caps have a recess exactly where it’ll do the most good: capturing the tab inside the cap means it can’t interfere with the rear contact spring:
Swapping cells no longer requires muttering!
Being that sort of bear, I (sometimes) note the date on cells when I change them, as with this notation on the AA alkaline cells in the Logitech trackball:
These Amazon Basics AA cells lasted almost exactly two years, compared with 15 and 20 months from the previous two pairs of Duracell AAs. A few months one way or the other probably don’t mean much, but the Amazon cells aren’t complete duds.
The new Amazon Basics cells have a gray paint job, so they’ve either changed suppliers or branding.
Most currents around here come in tens-of-milliamps, maybe a few hundred, tops, but it’s worth noting some curves from the Tektronix AM503 current amplifier manual for A6302 Hall effect probes:
The maximum current drops from 20 A for frequencies above 20 kHz:
There’s a 100 A·μs pulse charge limit:
Because the probe is actually a pulse transformer, its internal termination imposes a (small) load on the input circuit:
The specs are 100 mΩ at 1 MHz and 500 mΩ at 50 MHz, which means the load is essentially zilch for the circuits and signals I deal with.
The Tektronix Probes for Current Measurement Systems has some useful descriptions.
Memo to Self: Should any of those limits matter, rethink what you’re doing.
An interesting story about the AM503 design from someone who lived through it.
The question occasionally comes up as to why one would want a Tektronix A6302 Hall effect current probe and AM503 amplifier. The answer is simple: non-contact, essentially non-invasive current monitoring.
I used the venerable dead-battery Astable Multivibrator to check out the rebalanced A6302 probe:
The scope screen in the background shows the two base voltages at the top, plus the overall battery current along the bottom:
The current at 1 mA/div shows plenty of noise, but the 200 ms LED pulse is barely 1 mA tall. The two AA alkaline cells have faded to 2.5 V, so the “wearable” white-LED-with-dyed-overcoat runs far under its nominal 3.6-ish V spec.
There’s basically no other way to get that result, because inserting a current-sense resistor into the circuit will alter the results, plus be intractably difficult to measure, particularly if you need the current in a non-ground-referenced branch of the circuit.
The AM503 has terrible thermal drift, by contemporary standards, but after the first half-hour or so it’s manageable for short durations. I’m thinking of epoxying a small knob to the screwdriver-adjustable twiddlepot to simplify the baseline adjustment.
Alas, even non-working probes and amps have become eBay collectables. You could, of course, buy new.
A package deal of two Tektronix A6302 current probes arrived from eBay, with one probe having a small crack across its case (shown in the description and bought accordingly).
The other probe worked fine and was quite clean inside:
The cracked one couldn’t be balanced, with the twiddlepot on the AM503 amp unable to bring the signal down to 0 V from a positive offset on any of the ranges.
The current transformer might have suffered some stress on the upper-left corner of the main part (in the probe body), but it doesn’t have any obvious damage:
The small ball to the left of the transfomer lid provides the slide detent; it’s an ordinary 3/32 = 0.094 inch bearing. Which, as it happens, is a Good Thing, because there’s another one exactly like it somewhere in the litter under the Electronics Workbench.
Protip: follow the disassembly procedure in the instruction manual and do it over a towel or, at least, a shallow dish. You have been warned.
Extracting the transformer from the body revealed a numeric value I didn’t recognize at the time:
The top slide contacts looked awful, but they’re actually covered in semi-dried contact grease and cleaned up easily:
Swapping the “bad” transformer into the P6302 probe I got a while ago showed it wouldn’t balance, either, but the offset was far off into negative voltages. Putting the “good” transformer into the “bad” probe produced a similar too-positive offset. Conclusion: the transformer was probably good and Something Else was wrong.
Spending more time with the manuals produced this hint in the AM503 Amplifier circuit description:
Fortunately, the AM503 probe connector has pin labels:
Note the absence of pins G and I, probably to eliminate any confusion with “ground” and “one”, respectively.
Continuity checking reveals the left end of the 34.8 kΩ resistor connects to pin H:
Huh. Even a blind pig occasionally finds a truffle: where have we seen that value before? Apparently Tek measured each transformer / Hall sensor and wrote the appropriate offset resistor value exactly where it’d do the most good.
Although I don’t pretend to know why the transformer offset has changed, if Tek can select a resistor to correct the offset, so can I:
The 82 kΩ value roughly centers the offset twiddlepot span around 0 V; it’s the result of a binary search through the resistor drawers, rather than a complex calculation.
With the resistor in place and the probe reassembled in reverse order, everything works the way it should:
The lower trace is a square wave from the scope’s arb waveform generator into a (likely counterfeit) Fotek DC-DC solid-state relay, with the bench supply dialed to 5.7 V to put 5 V across a hulking 100 Ω power resistor, thus 50 mA through the probe. The purple trace comes from the repaired probe, with the other one turned off for pedagogic purposes:
That wasn’t easy, but seems to solve the problem.
Dang, I loves me some good Tek current probe action …
I hoped this bit of roadside debris would yield a shiny new amber LED and driver:
But, alas, it uses an ordinary WY5W incandescent bulb:
That whole assembly seems to be the replaceable unit, as the lens is firmly snapped-and-glued to the housing. The white shell used to hold the wires, but those vanished when the collision ripped the mirror off the car.
After I pried off the shattered lens and extracted the bulb, I found a broken filament.
Ah, well, now we won’t be riding through plastic shards along the shoulder.
Both my Tek 2215A and HP 54602 oscilloscopes came with snap-on front covers to protect all those delicate knobs and connectors. Not so the Siglent SDS2304X, which is basically a flat shoebox with a handle: the case has no features for a cover to snap onto, Siglent doesn’t offer a padded carrying case, and it’s too thick big for any of the laptop bags around here.
I’ve been lugging it to Squidwrench meetings and can easily visualize a gash across the LCD panel or a knob rammed against a door frame.
So I trimmed a pair of foam angles, punched holes to fit around the knobs along the right edge, cut up a cardboard tray from the heap, and duct-taped the whole mess together:
The cover is equally ugly from the outside:
A Velcro bellyband around the whole affair / through the handle holds it together.
I considered 3D printing a set of corners and screwing them to a flat plastic plate, but came to my senses just in time.
The Smell of Molten Projects in the Morning 