SK6812 RGB led sanity check

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BlindAssassin111
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Joined: Tue Jun 03, 2014 8:31 pm

SK6812 RGB led sanity check

Post by BlindAssassin111 » Thu Nov 30, 2017 1:36 pm

Okay so I am designing a pcb for a project and was going to be using SK6812 leds, from my research I have had issues seeing how people actually wire the power for these, as most of the schematics just say "+5V" and "GND" and don't show the actual wiring from there. I will be using 120 to 132 LEDs, not fully decided on how many but it will be in that range, and while drawing the schematic I realized I wasn't 100% sure I was doing it correctly.

I assume they are supposed to wired in parallel, correct? Seeing as I will be using a 5V/10A brick to power it all, I would need parallel to supply the 5 volts and, need to just run a higher amperage to make sure they light up fully.

How I have these layed out is in a 20x6 (up to a 22x6) matrix would it be fine if I wired the columns and rows like in the below picture where the top wire is +5V and the bottom is GND? Or should I run it so they are in parallel individually, for some reason I can't figure it out when looking at it, as there are so many leds I can't wrap my head around how the power is actually being delivered to them.
SK6812_Power.PNG

BlindAssassin111
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Re: SK6812 RGB led sanity check

Post by BlindAssassin111 » Fri Dec 01, 2017 4:05 pm

Anybody have an answer for this? I am literally at a stand still until someone helps me...

n1ist
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Re: SK6812 RGB led sanity check

Post by n1ist » Sat Dec 02, 2017 6:29 pm

The data sheet shows a 100nF cap between Vcc and GND for each one (it goes right next to the power pin). As for how to wire it up, the layout is more important here than the schematic. Remember that the traces all have resistance, and the voltage drop is proportional to the resistance Use thick traces. Wiring them in groups like you show is fine as long as the number of LEDs per group is small. Also, I would feed the power supply to the board in multiple places with separate thick wires (say 16AWG), and don't forget the fuses.
/mike

paulvha
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Re: SK6812 RGB led sanity check

Post by paulvha » Sun Dec 03, 2017 4:39 am

Have not worked with SK6812 before. but looking at the data sheet it can be confusing. (as they mix the pin numbers in the same document)

I would stick to the mechanical size and pin map. That is different than the pin numbers you apply in the schema. Pin1 (vss) has a small marker in the corner. Hold that bottom right. pin2 (DIN) is above, pin4 (DOUT) is left from pin1, and above pin3 (power). The power has to be provided in parallel (VDD +5V, VSS 0V/gnd). The capacitors are to remove/drain any noise on the power line (e.g. from the max 800Khz data) that COULD cause interference. For safety this can be done after each and every LED, but that is also depending on your board design.

The power is an important point as mentioned already. I read somewhere that for SK6812RGB (30 leds full brightness) the max estimated power draw is ~9W (1.8A@5V). if you plan 132 leds, one could say it could be up to 132/30 ~ 4 x 1.8A = 8A. Your power supply is 10A, to it should be able to hold, but I fully support the points of n1ist earlier.

BlindAssassin111
Posts: 43
Joined: Tue Jun 03, 2014 8:31 pm

Re: SK6812 RGB led sanity check

Post by BlindAssassin111 » Sun Dec 17, 2017 2:34 pm

So forgot to comeback and check this thread as I got busy.

First, I know that there needs to be caps on each LED, that isn't what I needed help on...just weren't placed on the schematic in those columns at the time and it provided a cleaner picture for the question.

Since before this I had to reroute the entire pcb because I realized I would benefit from most of the components being flipped 180 degrees and that meant a full rework of the layout, took a couple days to fix but now I am almost done.

My question about the picture still stands, can I run the power as shown and it be considered parallel or is it more complicated than that? Reason for asking is I need to run much wider 5V traces in order to carry the current needed to power this many LEDs(decided on 126). I will not be running wire in multiple points as that is a bad practice and a band-aid rather than an actual solution.

How I have it laid out atm is via option 1 in the below picture, and was wondering if it was possible to actually wire it up like option 2 and it be considered parallel so that I can use the 5v brick. I will be limited to 5 amps as that is all that a barrel jack can actually take which won't be that bad of a thing as I will be able to drive the LEDs at high enough current that they would be considered too bright anyways. sorry for the crappy drawing, it is easier than trying to explain the board layout and components.
led power help.PNG

n1ist
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Re: SK6812 RGB led sanity check

Post by n1ist » Mon Dec 18, 2017 9:43 am

Number 2 is a much better solution. With number 1, current for the lower right LED must travel all the way to the left side and back, with a much higher drop. Route ground the same way. It doesn't take much drop for pixels to start turning pink.

Feeding power into multiple points with a parallel bus is pretty standard (and was common even with ICs back in the TTL or ECL days...) . There's a limit to how much current you can push through a flex PCB; Feeding it using larger gauge wire will get around this.
/mike

paulvha
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Re: SK6812 RGB led sanity check

Post by paulvha » Mon Dec 18, 2017 11:27 am

The more parallel the better indeed... but what goes up.. must come down.. make similar considerations for the GND connection. Otherwise you get a problem there..

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