The Smell of Molten Projects in the Morning
Ed Nisley's Blog: shop notes, electronics, firmware, machinery, 3D printing, and curiosities
Blinking & glowing LEDs
Given that I put them into a gadget intended to use partially dead alkaline cells, the “05” date on the cells, and that it’s been blinking since last November, I cannot complain when this happens:
I might just fill the battery holder with vinegar and let it fizz for a while:
Even my simple discrete LM3909 circuit can blink a blue LED from a battery producing under 1 V, but those cells were flat dead. Gotta look over there more often, I suppose.
Two years ago, a quartet of new BatMax NP-BX1 batteries performed about as well as could be expected and, by last fall, had deteriorated about as much as expected:
In round numbers, the total capacity declined from 3.25 W·hr to 2.5 W·hr, which means a single battery can’t quite power the camera for the duration of our normal hour-long rides. I do not know what voltage trips the camera’s decision, but the batteries definitely shut down sooner.
So, based on their previous track record, I bought another quartet of Batmax batteries. Being that type of guy, I tested both the old (2020) and new (2022) sets:
The blue traces are the C/D batteries from the as-new tests back in early 2020, the green traces are C/D after two years of use, and the red traces are the “new” quartet after their first charge in the Official BatMax Charger.
It looks very much like BatMax is selling used batteries repackaged as new items, because they are indistinguishable from my used ones. They definitely are not the “Premium Grade A cells” touted in the description.
I returned them for a refund and sent the test results to BatMax; they sent “new replacements” even though I said I would not pay for any future shipments. The batteries had a slightly different wrapper, but the test results were still indistinguishable from used batteries. I offered to return the package and was told that would not be needed.
Just a few more batteries for the blinkies.
So I bought a trio of NP-BX1 batteries from Newmowa, an Amazon supplier with a few more vowels than usual, and repeated the exercise:
It seems three good batteries now cost about as much as four crap batteries, under the reasonable assumption chargers are essentially free.
Three batteries isn’t quite enough for my usual rotation and, for unknown reasons, one cannot buy only batteries, so in short order I will have two chargers and six batteries.
The consolidated test results:
The color code:
I stopped writing Amazon reviews after having a few detailed-writeups-with-graphs rejected for the usual unspecified reasons. As the Finn put it, “You wanna download, you know the access code already.”
Dropping the ordinary flashlight bulb into the drawer where it belonged revealed what I think is a halogen flashlight bulb, so I rebuilt the blinky test setup:
This time I used a BUZ71A MOSFET (13 A, 100 mΩ RDS) driven with a 10 V gate pulse to make sure it acted like a switch instead of a current sink.
The first attempt looked … odd:
The gate pulse is yellow, the drain voltage is magenta, the bulb current is cyan at 1 A/div, and the timebase ticks along at 2 ms/div.
Moving the magenta trace to the supply voltage on the other side of the bulb produces even more weirdness:
Apparently, slugging a 3 A bench supply with a 3 A pulse lasting only 4 ms causes distress of the output tract.
Kludging a hulking 22 mF (yes, 22000 µF) cap across the power supply provides enough local storage to make things work properly:
With the cap in place, the drain terminal looks less unruly:
The drain voltage starts at about 600 mV with the 3 A pulse, a bit more than you’d expect from the alleged 100 mΩ drain-source resistance, but those numbers are generally aspirational and the test setup leaves a lot to be desired.
A 10 ms pulse produces a distinct flash, rather than a dull orange blip (timebase now at 10 ms/div):
A 30 ms pulse reaches full brightness as the filament settles at normal operating temperature:
A 20 ms flash might suffice for decorative purposes, in which case each pulse requires 90 mW·s = 3 V × 1.5 A × 20 ms of energy. Running it all day requires 7.8 kW·s = 2.2 W·h, so it’s even less appealing than that old skool tungsten bulb.
Which is, of course, why LED flashlight bulbs are a thing.
A flashlight bulb emerged from the clutter, which prompted me to ask if it might make an interesting blinky. Spoiler: probably not.
The bulb had “2.4 V 0.7 A” stamped on its shell, so the test setup looked like this:
A list seems helpful:
The function generator has a 50 Ω output, so depend on it to limit the base current just like it was a resistor. The output voltage is symmetric around 0 V, so apply an offset of half the peak-to-peak signal to get a positive-going pulse:
A 150 ms pulse gives the bulb just barely enough energy to light as a little orange blip, with the collector voltage dropping as the filament heats up and its resistance increases:
Given 350 ms to heat up, the bulb produces a nice white-hot flash:
The poor transistor acts as a 600 mA constant current sink, which isn’t surprising given its 300 mA absolute maximum current rating.
Homework: figure the base drive and current gain
Protip: don’t do that to a cherished transistor
The bulb resistance starts out at 0.5 Ω and rises to 2.5 Ω when the filament glows white-hot at the end of the pulse.
Something like 250 ms produces a noticeable blink, requiring 360 mW·s = 2.4 V × 600 mA × 250 ms from the power supply. Blinking once every ten seconds all day means 8640 pulses for a total energy of 864 mW·hr; call it 1 W·hr.
A pair of (fresh) AA alkaline cells provide 7.5 W·hr for maybe a week of blinkiness.
A not-dead-yet 18650 lithium cell might offer 15 W·hr, but running the bulb from 3.7-ish V, rather than 3-ish V, increases the energy per pulse by 20% and decreases the run time correspondingly.
Surely not worth the effort …
With a generous dollop of JB Plastic Bonder left over from a set of Bafang brake sensor magnets, I tried coating the ersatz plate cap of a triode tube:
That’s the result after leaving it hanging upside-down while it cured to push all the drips to the top.
For comparison, the uncoated cap back in the day:
Seeing as how the urethane is an adhesive, not a coating, I’d say it looks about as bad as expected.
As with all 3D printed things, one must embrace imperfections and striations, rather than endlessly strive for perfection.
Now, if I had a resin printer …
These two discrete LM3909 circuits recently stopped blinking:
The green LED (on the left) took six months to wear its pair of not-dead-yet AA alkalines from 2.7 V down to nearly zero.
The blue LED in the radome took two months to go from 1.0 V (!) to nearly zero. It didn’t start very bright and went decidedly dim along the way, but the LM3909 circuitry still managed to jam a few microamps through the LED.
In both cases, one of the cells was reverse-charged by a few hundred millivolts, although neither leaked.
Both got another set of not-quite-dead AA cells and they’re back in action.
One of those LED spotlights may have barely outlasted its worthless warranty, but not by much, and has been languishing on the back of the bench with “Flickers hot” scrawled on its side.
The metal base didn’t respond to twisting, so I slit the threads with a cutoff wheel:
Applying the screwdriver removed the base to reveal a silicone rubber casting:
The small wire emerging near the edge of the plastic case seems to be the neutral contact to the shell, with a poor enough joint to suggest it might have been why the lamp flickered when it got hot.
Some brute force snapped the silicone off at the bottom of the plastic case and broke the two wires bringing AC to the PCB:
Digging around inside produced a debris field of silicone crumbs, broken resistors, torn caps, and various other components, with zero progress toward removing the shell:
A little lathe work converted a chunk of PVC pipe into a crude mandrel supporting the mangled case:
A few millimeters of sissy cuts released a silicone O-ring sealing the shell against the reflector:
Continuing the cuts eventually revealed the three screws holding the shell to the reflector and the two wires powering the LED:
Chopping off the screws with a diagonal cutter freed the shell and revealed the top of the PCB:
It really does have a surprising number of components!
Those three screws connected the LED panel / heatsink to the shell through the back of the double-walled reflector. More brute force peeled the outer shell away and released the panel:
Each of the 5050 packages contains a pair of white LEDs with 5.2 V forward drop for the pair, at the very low test current. They’re all in series, so you’re looking at well over 60 V total forward drop:
Note that the wiring, which nobody will ever see, follows the electrical color code of white = common and gray = hot.
Perhaps I should turn the lens into an interesting art object …
The Smell of Molten Projects in the Morning 