Advertisements

Cheap WS2812 LEDs: Test Fixture Current

With the WS2812 test fixture neatly mounted, I plugged it into a six-port USB charger allegedly capable of supplying 2.4 A per port and captured a trace with nearly all 28 LEDs displaying full white:

WS2812 4x7 array - 200 mA VCC - all on

WS2812 4×7 array – 200 mA VCC – all on

At 200 mA/div, the top trace shows a bit under 1.2 A, a bit under the 1.68 A = 28 × 60 mA you’d expect; in round numbers, each RGB pixel draws 43 mA. Actually, the WS2812 specs don’t specify the maximum / typical LED current and, on belief and evidence, I doubt these units meet any particular specs you’d care to cite.

Also, the supply voltage (measured across the LED array “bus bar” wires) hits 3.37 V, well under the 5 V you’d expect from a USB charger and less than the 3.5 V called for by the WS2812 specs. Although the WS2812 nominally limits the LED current, there’s no telling how it varies with supply voltage.

A cheap USB 1 A wall-wart charger produced far more hash:

WS2812 4x7 array - 200 mA VCC - all on - cheap 1A wart - 20 uF

WS2812 4×7 array – 200 mA VCC – all on – cheap 1A wart – 20 uF

That’s with an additional 20 µF of tantalum capacitance across the power bus bars. The peak current looks like 1.4 A, with marginally more supply voltage at 3.56 V.

Bumping the trace speed shows the wall wart produces nasty current spikes, at what must be the poor thing’s switching speed, as it desperately tries to produce enough juice for the LEDs:

WS2812 4x7 array - 200 mA VCC 50 us - all on - cheap 1A wart - 20 uF

WS2812 4×7 array – 200 mA VCC 50 us – all on – cheap 1A wart – 20 uF

The step over on the right looks like a single RGB LED going dark, as it’s about 50 mA tall.

The output voltage doesn’t show the same spikes, so the LED array acts like a constant-voltage load. Given that the WS2812 probably connects all the LEDs pretty much straight across the supply, that’s not far from the truth: we’re looking at the forward drop of those blue LEDs.

Now, to let it cook away in the cool and the dark of the Basement Laboratory…

Advertisements

  1. #1 by RCPete on 2017-02-25 - 11:48

    cool and the dark

    If you had any expectation that you’d really want to use one of these sorta-pixels, you might want to consider the old burn-in model. The mil-spec folks would test parts at room temp, then run them powered up for a week at the maximum specified ambient temperature. So, for an average mil-spec part, it would be 125C. The survivors would usually last after that. (1970s technology, though these were in hermetic packages, so the wire bonds didn’t have plastic pushing them around.)

    I’d modify this for room temp uses and this package, but a burn-in in a warm dry room (somewhere on the attic stairs in warmish weather) would help. The results of the sharpie test tell me that a modification of the 85C/85%RH burn-in wouldn’t leave anything useful. (The packaging engineers would use 85-85 to check the encapsulation in optocoupler plastic package development. That kind of stuff made me happy to be working at the wafer and chip level…)

    • #2 by Ed on 2017-02-26 - 14:23

      The back of the PCBs in that array run around 40 °C, which isn’t shouldn’t be anywhere near their limit. On the other paw, that stack-of-platters light killed strip LEDs at 50 °C, so maybe their thermal limit runs lower than anyone, least of all me, would imagine.