Looking at what comes out of various USB chargers, with the Tek current probe monitoring the juice:
First, a known-good bench supply set to 5.0 V:
The yellow trace is the Glass Tile
Heartbeat output, which goes high during the active part of the loop. The purple trace shows the serial data going to the SK6812 RGBW LEDs. The green trace is the USB current at 50 mA/div, with the Glass Tile LED array + Arduino drawing somewhere between 50 and 100 mA; most of that goes to the LEDs.
The current steps downward by about 10 mA just after the data stream ends, because that’s where the LEDs latch their new PWM values. The code is changing a single LED from one color to another, so the current will increase or decrease by the difference of the two currents.
A charger from my Google Pixel 3a phone (actually made by Flextronics and, uniquely, UL listed), with Google’s ever-so-trendy and completely unreadable medium gray lettering on a light gray plastic body:
The current waveform looks only slightly choppy:
An AmazonBasics six-port USB charger
from tested by Intertek:
A blackweb (their lack of capitalization) charger, also
made tested by Intertek:
Finally, one from a lot of dirt-cheap chargers from eBay:
Which has the most interesting current waveform of all:
A closer look:
From the 75 mA baseline, the charger is ramming 175 mA pulses at 24 kHz into the filter cap on the Arduino Nano PCB! The green trace has a few seconds of (digital) persistence, so you’re seeing a lot of frequency jitter; the pulses most likely come from a voltage comparator controlling the charger’s PWM cycle.
It’s about what one should expect for $1.28 apiece, right?
They’re down to $1.19 today: who knows what the waveform might be?
Update: Having gotten a clue from a comment posted instantly after I fat-fingered the schedule for this post, I now know Intertek is a testing agency, not a manufacturer.
5 thoughts on “Glass Tiles: USB Charger Current Waveforms”
Isn’t Intertek the testing lab, not the manufacturer?
Haven’t a clue. Everybody seems proud of ’em, though.
Of course, there is no guarantee that the Intertek mark is genuine. A lot of Chinese junk manufacturers will slap a mark on anything, whether it has been tested or not.
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