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Voltage drop versus pixel spacing

Discussion in 'RGB Lights - Intelligent Pixels and 3-Channel RGB' started by nutz4lights, Jul 5, 2013.

  1. nutz4lights

    nutz4lights Full Time Elf

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    Hey all,

    A few months ago, when I was hooking up my first pixels, I spent a decent amount of time trying to understand the voltage drop of 5V WS2811 pixel strings. The reason is that, for the first decoration element I wanted to use them in, I was pretty sure I was going to need 20-25cm (8-10") pixel spacing. I had ordered some 5V 25cm (10") WS2811 pixels from Ray and was seeing horrible performance. My first instinct is to try and understand what the @#$% is going on and that resulted in the following thread:

    http://auschristmaslighting.com/forums/index.php/topic,4155.0.html

    So, I ended up deciding that the higher pixel spacing just wasn't gonna cut it (didn't like the idea of injecting power after 35-40 pixels) and I ordered a bunch of strings for my twelve trees with the standard 10cm (4") pixel spacing. When I placed that second order, I also ordered several different spacings so that I could make some more voltage and current measurements. I ordered these lights:

    http://www.aliexpress.com/store/product/50pcs-DC5V-input-WS2811-LED-pixel-light-with-all-black-wire-IP68-rated/701799_1032448178.html

    with the following spacings between pixels: 50mm (2"), 75mm (3"), 100mm (4"), 150mm (6")

    First off, I should mention that, in a very repeatable fashion, the pixel spacings are all off by 25mm. No joke. While it doesn't matter for the current application (wrapping trees), it would matter if one was attempting to grid something out. I have another application coming down the line where I pretty much want 75mm pixel spacing, so I guess I will have to order 50mm to get it...

    Just like in the first link above, I did all testing with a standard 5V power supply and my P12R controller. I measured voltage at the beginning and end of 50 pixels with the above spacings and I also measured current. Here are the results:

    [attachimg=1]

    I included the pixel spacing I asked for as well as the actual pixel spacing. The top and bottom tables are identical, but the one is in "I'm from the U.S." units and the other is in "I'm not from the U.S." units. You will see voltage at pixel #1 and pixel #50 for white, red, green, and blue, as well as current for white, red, green, and blue. Important take-aways are:
    • Although voltage at pixel #1 actually increases for higher pixel spacing and longer pixel strings, voltage at pixel #50 decreases with higher pixel spacing and hence longer pixel strings.
    • Current draw decreases across the board as pixel spacing and pixel string length increase, inevitably due to the fact that there is higher voltage drop through the wire and less available to the resistors associated with the pixels.
    The data seems to show that the voltages at pixel 50 are near or in the range of the activation voltages of the blue and green portion of the LED but only the really high pixel spacing is below 3.0V. I don't know if it was my brain telling my eye what to see, but I did notice more pinkish reddish color when in white mode for those strings... I suppose I could get my wife to help and do a blinded random test...

    Anyhoo, just some more data to throw into the pool of useful data around here. 5V? Higher pixel spacing? Definitely going to need to inject power after 35-40 pixels instead of 50... but maybe most folks already know that?!

    Thanks for looking.
     
  2. ecbailey

    ecbailey Full Time Elf

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    Extremely helpful, very bizarre pattern with the spacing. Maybe how your measuring the spacing and how the factory is measuring it is a bit different?


    I presume if one was to order strip then they would get the spacing right. However, modules may indeed have the same problem?


    Thanks for posting the info :)
     
  3. fasteddy

    fasteddy I have C.L.A.P Global Moderator Generous Elf

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    Great info for those not sure about the relationship between cable length and voltage drop and how larger spacing will effect the level of voltage drop. Thanks for putting this together.
     
  4. OP
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    nutz4lights

    nutz4lights Full Time Elf

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    No worries. I am measuring the spacing pixel center to pixel center. Now that I think about it, those pixels are around 20-25mm square... I bet they are counting 75mm pixel spacing as the length of wire between the pixel bodies... which could or could not make sense, depending on how you look at it... me personally, I'd rather measure pixel spacing light to light, not the length of wire... If the cut the wire 75mm for a 75mm pixel space, they would wind up with 75mm + 10-12mm + 10-12mm = ~ 100mm which is what I'm seeing...
     
  5. OP
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    nutz4lights

    nutz4lights Full Time Elf

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    You're very welcome Eddy, just trying to keep up with all the other contributors out there!
     
  6. fasteddy

    fasteddy I have C.L.A.P Global Moderator Generous Elf

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    traditionally spacing has been measured from centre to centre of the lights and not the cable length, but in China it may not be consitantly done that way
     
  7. AAH

    AAH I love blinky lights :) Community Project Designer

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    Nice work Louie
    A request or 2 and a couple questions.
    -Any chance that you could post a picture with some more pixels in it? My poor old eyes must be getting old.
    -Did you measure the power supply volts under load at all? I'm wondering if the reason for the lower volts at pixel 1 were due to the voltage drop in the power supply, across the fuse/s in the P12 or down the lead wire. I myself would have liked 4.9 volts or thereabouts at pixel 1.
    -Did you or can you measure the quiescent (standby) current of the string with all pixels turned off?

    I am a bit horrified at those figures just quietly. White should be the sum of the red, green and blue all turned on. This should be somewhere in the region of 2.1A based on your figures but you are measuring between 1.28 and 1.1 depending on the length. According to the spec sheet for the ws2811 the current per channel in constant current mode is 18.5mA. For each of the colours this gives a current of 50x0.0185=0.925A and a total of 2.775A. Unless my maths is badly wrong or Ray has the pixels current limited to lower than what the spec sheet says I would imagine that the pixels are actually at an average brightness of half what they could be when they are all on white and there is no power injection :(
     
  8. OP
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    nutz4lights

    nutz4lights Full Time Elf

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    Alan,

    Let's see if this attachment method works a little better.

    [smfattach]1[/smfattach]
     
    Last edited by a moderator: May 27, 2017
  9. OP
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    nutz4lights

    nutz4lights Full Time Elf

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    Alan, ok, on to your other questions.

    Thought I would clarify how everything was hooked up really quick. Ray Wu 5V 200W power supply connected to AC mains with 2m 16AWG/3 cable. That power supply was then connected to the P12R with a 30cm piece of 20AWG stranded wire. I may repeat that tomorrow with a little heavier wire, but I'm only running the P12R and the one string of 50 pixels... Anyway, the 50 pixels are connected directly to the P12R (which I realize is not a real world setup, but I wanted to measure the pixel string without any additional contributions to voltage drop from an extension cord). I am measuring the pixel 1 voltage at the output connector on the P12R. Obviously the end of the string measurement is done at the end of the string, not a whole lot of ambiguity there.

    I also was expecting higher voltage at pixel 1 and I must have spent 1/2 hour today trying to come up with some explanation for why the pixel 1 voltage is so low... I don't know what you mean by measuring the power supply under load, but I think I am doing that with this test? It is technically under load... I mean, I am only using 10% of the output rating of the power supply... I would hope that 10% of the rated power draw would not kill the voltage on it...

    The quiescent current on the string was in the low mA range... I caught it out of the corner of my eye when I was setting up, but can't remember what it was. I will try to go measure that tomorrow.

    In the post I linked in the first post above, there were many posts in which I was questioning the current draw versus what we expect the current draw to be and we had some very good discussion I felt. I think the one problem is, we have to make quite a few assumptions in dealing with these pixel strings. The main reason for my interest in making detailed measurements is that I felt we needed some more concrete current draw measurements to go with all the other detailed information we have on this site.

    Thanks for linking that 2811 spec sheet. I had come across that at some point, even had some of the electrical engineers at work take a look at it with me (I am a lone chemist working in microelectronics with 50-75 EE's, which obviously has its advantages!) Now that I think about this a bit more... That spec sheets claims 18.5mA constant current for each red, green, and blue... but references a single 100 ohm resistor. If that resistor is the current limiting resistor, that also is responsible for dropping the remainder of the voltage the LED doesn't drop, that could explain why I was routinely measuring higher current for red compared to green and blue. The red forwarding voltage is typically 2.3V compared to 3.2V for green and blue, right? Would Ray have to use separate current limiting resistors? I would think you would want to run different current limiting resistors for each color.

    As you can calculate from my spreadsheet, most of the color data (non-white) is showing up as 13.5mA to 15.0mA for current draw, which is pretty close... those numbers go way down for white, though... 8-10mA, so I agree with your "half the brightness" comment.
     
  10. kane

    kane Dedicated Elf

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    I had some custom length ones done a few weeks back, and I'd always recommend that you give them a diagram so that there is no confusion. This is what I sent:
    [attachimg=1]
     
  11. AAH

    AAH I love blinky lights :) Community Project Designer

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    I'm not going to do a quote as it will take up too much space.
    By under load I meant checking the voltage at the power supply itself when the pixels are on. Theoretically it shouldn't vary by more than a percent or 2 over the full load range as there is voltage sensing as part of the circuitry.
    There will be a voltage drop across each of the 2 fuses that the power is going through. According to my reading of mini fuse spec and ATO fuse spec there should be <100mV drop across the 2 fuses with only 1 output and only 1.2Amps being drawn. This will get markedly worse as more strings get added.
    The 110V power lead will have no effect on anything. The 0.02 ohms in that 20 gauge wire should be responsible for about 20mV drop. Somewhere something is stealing more volts (or part thereof) than you'd expect.
     
  12. OP
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    nutz4lights

    nutz4lights Full Time Elf

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    Kane... that is a great idea and I will definitely do the same thing in future orders where the spacing is critical.
     
  13. OP
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    nutz4lights

    nutz4lights Full Time Elf

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    Ok... Alan... I have an update...

    First, I measured the quiescent current through a 50 pixel string with 75mm spacing and it was 40mA.

    Second, I went and measured the voltage at the power supply outputs with the same string running and I was seeing around 100mV difference between that reading and the reading at the output of the P12R. That is in agreement with your suggestion below (albeit the high end) for voltage drop in the fuse on the P12R with one output running at 1.2A so we're good there... So that puts the voltage at the output of the power supply at 4.29V (the reading at the output of the P12R to the string - pixel 1 - was 4.19V).

    With that in mind... I started thinking... "what if the power supply is the issue?"... I mean, the voltage out of the PS is 5.0 ± 0.1V with no load... This power supply is a 5V 200W supply from Ray (normal PS, not one of the IP67 rated units). I had just received some 5V 350W power supplies from ray (again, normal, not weatherproof) so I snagged one of those and wired it up. Although I don't like to change two variables at once, I also snagged a piece of 18AWG stranded lamp cord to hook the PS up to the P12R... I re-ran the test on the string and saw the following:

    Voltage @ PS (white light): 4.81V
    Voltage @ pixel 1 (white light): 4.73V
    Voltage @ pixel 50 (white light): 3.58V

    Very interesting... on to current measurements:

    Current Draw (no light): 0.04A
    Current Draw (white light): 1.59A
    Current Draw (red light): 0.80A
    Current Draw (green light): 0.73A
    Current Draw (blue light): 0.70A

    So that shows that the individual color current draws were unchanged (± 0.03A) but the white light current draw increased dramatically (0.31A). That to me says that something is wrong with the 200W power supply, right? Well, I grabbed a second 200W power supply I had from that first shipment and repeated the test. The numbers came out in exact agreement with the other 200W power supply (i.e. lower than the new 350W power supply).

    This opens up a whole new can of worms... should I be irritated? Try to get in touch with Ray? There's only one string plugged into the P12R drawing 6W of power out of a 200W power supply and it is struggling...

    I'm curious to see your guy's response to this new data...

    Thanks!
     
  14. OP
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    nutz4lights

    nutz4lights Full Time Elf

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    So I was out working on some pixel stuff tonight and this thread popped back into my head... and I remembered that I never posted a final post on this subject matter. If you read through this thread, you will see that I was experiencing high voltage drop on a 200W Ray Wu power supply with 50 pixels and that high voltage drop went away when I switched to a newly-received 350W Ray Wu power supply...

    ... well... the difference ended up being that, when I received the 350W power supply, I went and switched the mains voltage over to 220VAC, which is what we use here in the U.S. ... and I thought to myself "did I do that on the 200W power supply?"... which I hadn't. When I finally switched the 200W power supply over from 220VAC to 120VAC... the higher voltage drop went away.

    Now, I have no idea why I didn't do it on the 200W power supply when I started using it... I mean, the first thing that popped into my head when I got the 350W power supply out of the box was "go switch the mains voltage setting", so... whatever... :p

    Lesson learned... make sure to set the mains voltage properly... All of the numbers in this thread should be considered irrelevant at this point... I wouldn't even be disappointed if this thread disappeared from the internet entirely... :eek:

    Thanks,
     
  15. DeeJai

    DeeJai Is that Magic Smoke?!?

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    see the good news for us in Aus, if we set the switch to 120, we get a loup pop followed by the magic smoke coming out of the psu. We then know we should have checked :p

    glad you found the culprit tho.
     

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