Author Topic: PICAP questions regarding WS2815 strip, GPIO use, and current limitations  (Read 465 times)

Offline n8huntsman

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I've got a FPP that I will be using on my RZR side by side to run some LED whips and seasonally with some temporary pixels.  I got a picap to run the pixel strip whips.  The strip is WS2815.  I chose this because the extra data wire and they are 12v.
https://www.aliexpress.com/item/WS2815-DC12V-addressable-full-color-RGB-5050-LED-strip-60leds-m-5m-waterproof-in-silicon-coating/32943534376.html?spm=a2g0s.9042311.0.0.4ae44c4dy10aeD


I have the WS2815 up and running with the E6804 V5 beta firmware but will be transitioning it over to the picap now that I know my setup works.  Is there any issues with WS2815 and the picap?


I'll be bringing in two signal wires to trigger GPIO events.  Whats the best way to access these with the picap on?  Solder to the exposed side of the picap?


The stips above claim to be 72 watts per roll.  Do you think I could possibly power two rolls, one off of each output?  I know that will be 6amps each.  But will I actually see that much current if I go full white or is that just some theoretical number?  Could the power input headers handle the combined current of both outputs at full current?


Thanks




 

Offline CaptainMurdoch

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Re: PICAP questions regarding WS2815 strip, GPIO use, and current limitations
« Reply #1 on: February 20, 2019, 02:38:16 PM »
I don't know if anyone has tried ws2815 with the PiCap, but it should work.

For attaching to the GPIO pins while the PiCap is installed, you could use a shim module so that you don't have to solder to the PiCap:  https://www.newark.com/piface/2445464/piface-shim-module/dp/31AC4613

For the current, you can run the strips at a reduced brightness with FPP v2.x and that will cut your max current below the rated 5A max.
-
Chris

Offline Bos Lights

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Re: PICAP questions regarding WS2815 strip, GPIO use, and current limitations
« Reply #2 on: February 20, 2019, 03:10:50 PM »
For the 60 leds / m you will most liky have to power inject.

Offline n8huntsman

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Re: PICAP questions regarding WS2815 strip, GPIO use, and current limitations
« Reply #3 on: February 20, 2019, 03:23:43 PM »
The strips say you don't need to inject but I did notice the end of the strip was not behaving properly.  Limiting the brightness solved the issue.  In hindsight it would have been a good idea to inject power but its probably too late now.  Ive got them all assembled and these is no way to run a wire at this point.  If I make another set I'll keep that in mind.  For now, limiting the brightness in FPP sounds like a good solution.

So no problem running 10amps on the input header?

Offline CaptainMurdoch

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Re: PICAP questions regarding WS2815 strip, GPIO use, and current limitations
« Reply #4 on: February 20, 2019, 05:27:36 PM »
So no problem running 10amps on the input header?

I don't know of any issues.  The outputs are fused at 5A each and I believe the board is designed to handle it.

If you have any concerns, you can always power inject right at the PiCap and just run the data and ground lines from the PiCap to the first pixel with the power and ground from the first pixel connected to a fused power source.

Offline n8huntsman

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Re: PICAP questions regarding WS2815 strip, GPIO use, and current limitations
« Reply #5 on: February 23, 2019, 10:30:23 PM »
Is there a small water proof-ish enclosure that will fit the pi and picap? My TA-200s are too small :(

Offline n8huntsman

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Ive got a basic electrical fundamental questions. Using power injection method Ive always connected my pixels ground to the power supply and ran an additional wire to the controller to ensure a common ground as is required. Essentially you end up with two parallel paths to ground. What prevents 50% of the current from being pulled across the controller ground traces? Its never been an issue, just curious.
In this case, my picap power and pixel power are coming from the same 12v system. Is it still necessary to connect a common ground, as close as possible, between the controller (picap) pixel output ground and pixels ground to minimize a difference in potential? Or will the differential be negligible?

Offline pixelpuppy

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What prevents 50% of the current from being pulled across the controller ground traces?


Ohms Law my friend.   

When parallel paths are available to carry the flow of current, the current across each path will be inversely proportional to the relative resistance of each path.    It they are exactly equal resistance then they will carry exactly equal current (50/50).  In reality, they won't be exactly equal resistance and therefore won't be exactly split 50/50
-Mark

Offline n8huntsman

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Thats kinda what I was afraid of! The current could still be substantial. Im thinking it might be best to just have ground to controller and separate ground to pixels. Since the grounds are from the same source hopefully that will be enough?

Offline pixelpuppy

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Thats kinda what I was afraid of! The current could still be substantial. Im thinking it might be best to just have ground to controller and separate ground to pixels. Since the grounds are from the same source hopefully that will be enough?


What is "substantial"?   With a typical Power Injection scenario, one of those ground paths is back through the 18-22awg ground wire in a longer length pixel string (and likely through several pigtail connectors with added resistance) vs. a 12-18awg shorter ground wire with a direct path back to the power supply.   Even though resistance is low on either path, the proportional resistance between them is significant enough to cause more of the current flowing on the dedicated PI ground wire and less current backflowing through the pixel string and controller ground traces.

 

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