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I’m a theatre lighting technician (and electronics hobbyist), working on a project involving SK6812 RGBW addressable LED strips & strings. They use an 800kHz single-wire control signal, regenerated by each chip. That is to say, the first chip in the string receives the entire packet of N*4 bytes, takes the first 4 bytes as its R, G, B and W 8 bit PWM values, then regenerates the data packet of (N-1)*4 bytes for the next chip, etc.

The strips work as expected, but I’ve been having difficulties with the pixel strings. Unlike the strips, each diode has a 75 ohm (750 code) current limiting resistor in series with the 5V supply! Using an oscilloscope, I compared the data stream coming into the first pixel from the controller, and what was generated by the pixel. The input stream was perfect, but the output sagged significantly to ~3.3V after ~500 microseconds into each packet. Note that according to the datasheet the SK6812 requires a >3.4V logic high level. I suspect the timing coincides with the 1.2kHz PWM frequency.

Why would this resistor be added? The strips have no resistor and work just fine. The resistor seems an addition that makes things worse! I haven’t gotten around to measuring the inrush current but regardless 75 ohms seems extreme. Any suggestions on workarounds?

The power supplies are generic (read: low quality) 5V 60A supplies, but there is <1% output ripple, and only sags by 0.5V (to 5V from 5.5V) at 15A current draw (based on tests with a programmable load). The LED strip draws a max of maybe 4 amps with everything at full. The voltage drop was barely noticeable with oscilloscope.

Also worth noting, I have made this pixel string work fine at home on a better power supply, but I haven’t gotten around to checking the difference in output data stream from the first pixel with that supply.

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  • \$\begingroup\$ added an empty line after each of your paragraphs – as you can see, that makes the paragraphs work as such :) \$\endgroup\$ – Marcus Müller Jun 24 at 6:34
  • \$\begingroup\$ the SK6812 has a built-in constant current source ... you should not be using an external resistor \$\endgroup\$ – jsotola Jun 24 at 7:00
  • \$\begingroup\$ I am not adding anything. These are pixel strings delivered from factory. I just did a proper continuity-check of the whole board, and confirmed that the resistor is in series with the 5V VCC (and before decoupling capacitor). AFAICT this must be a factory mistake, as others report 75 ohm resistors in series with DIN (for filtering). \$\endgroup\$ – cortices Jun 24 at 7:57
  • \$\begingroup\$ @cortices is it possible these pixel strings are rated for a higher voltage? \$\endgroup\$ – Marcus Müller Jun 24 at 9:29
  • \$\begingroup\$ No, they specifically say 5V on them. And voltage drop is proportional to current, so if run on higher voltage, they would fry whenever not turned on (drawing less current) \$\endgroup\$ – cortices Jun 24 at 9:43

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