Questions after reading Eddy's/Lukes Articles in PC Mag

Discussion in 'RGB Lights - Intelligent Pixels and 3-Channel RGB' started by BundyRoy, Apr 14, 2014.

  1. BundyRoy

    BundyRoy Senior Elf

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    I've just read both Eddies and Devo's articles in the PC mag. I will read the rest but thought I'd ask some questions that came to mind before they leave. Article is http://www.planetchristmas.com/Magazine/April2014/#/14/ for those that haven't seen it yet.

    Both articles were very good. I have very little understanding of the topic as yet and so I learnt heaps. Struggled with some of the bits about universes/channels and the power/current but I'm not sure how you can explain that in words without learning through trying. I'm just going to list the questions. Was going to ask in chat but thought others might learn if it was answered in the forum.

    1) Dumb strip lights. They are either all on or all off and all one colour when on. You can't do a chasing sequence down the strip. Not sure that terminology is right but effectively I mean part of the strip is on and part is off at the same time.

    2) The total voltage drop across a series of LED needs to be close to the total voltage input. For example. Say a LED has a voltage drop of 3.2V and I have 240V supply, I would need 75 LED in series (disregard No of channel issues), so 75x3.2=240V. If I only had say 50 LED in series I would need a resistor with a 240-50x3.2=80V drop across it. Did I understand this correctly.

    3) Is there any pros/cons of Art-Net vs 1.31. What is the difference. Seem similar to me but not quite sure.

    4) In the article it says you can see how a USB dongle becomes uneconomical in large shows. I don't know enough to be able to understand. I think from discussion I've had hear that with a dongle you are limited to one universe. If so I can see how 170 pixels/usb port would be a limiting factor.

    5) I gather that the power supply units can be used to power multiple items (eg 2 banks of a controller, power injection) as long as the power consumption of all the things combined is less than 85% of the rated power of the power supply.

    6) Whats the trick for isolating +ve power inputs when power injecting. I see you could just disconnect the +ve coming from the upstream lights but then you would have to resolder it back on next time you used it in a non power injection situation. Could become painful to change or do you find you basically need to inject all the time anyway so no hassles.

    7) It said that if a pixel blows in a pixel string all the downstream lights stop working. I had a pixel stop working in my pixel strip and all the lights downstream still work. Are they wired differently.
     
  2. mborg10

    mborg10 Michael Borg

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    I'll just comment on the ones I know about.
    1, your correct. A dumb strip is treated as one rgb light. The entire strip does the same thing for the whole length. A pixel strip has a data signal sent thru to the first pixel. The first pixel takes what data it needs then sends the data on to the next pixel so each pixel can do different things eg chase pattern.

    2, voltage drop isnt my strong point but it is not 240v. It would be stepped down to something more like dc12v or 5v. Red green blue LEDs work at different voltages. If you have too big of voltage drop along a set of lights you won't get correct colour. Eg white will have a pinkish look because blue which requires the highest voltage will be weaker and red which needs a low voltage which show up more. This is because white is all 3 LEDs at 100%. For me voltage drop is better measured than calculated. If in doubt just add more power injection.

    4, one universe off a dongle is limiting. 170 pixels would be a small show. My first year with pixels last year had 3000 pixels running from 2 controllers with 20 universes.

    5, depends on how good of quality power supplies you have but good rule of thumb. Worst case you just don't run everything on white for more than a few seconds. White uses most power.

    6, you need to power inject frequently and if it's from the same power supply you shouldn't have an issue. If you want to use separate power supplies for power injection as to what your controller is using then it can get tricky and beyond my experience to answer correctly.
     
  3. Jago

    Jago New Elf

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    I think you guys are getting getting mixed up with voltage drop caused by resistance in a conductor and Forward Voltage of a LED.

    As the last post explained most LEDs are run in parallel and not in series.

    In pixel strings there is a common v+ and v- that is connected to each IC (chip) controlling the LED.

    In 5volt version the +v can drive the IC directly and you only need a small resistor to drop to the LEDs voltage.
    In 12volt strips there is a resister that comes from the v+ to drop the voltage for the IC then to avoid a large resistor (and lot of wasted energy) 3 LEDs are place in series. By doing this the required voltage drop is not great. These 3 leads are treated as 1 pixel.



    This explains voltage drop you need to be more concerned about.
    v+ ----------------------------------------------------------------------
    | | | | |
    LED LED LED LED LED If this string was 5v by the end here (with full string)
    | | | | | you might be down around or below 3v and this can
    v- ---------------------------------------------------------------------- be a issue and why power injection is required (send 5v from this end as well)
     
  4. OP
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    BundyRoy

    BundyRoy Senior Elf

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    Thanks Michael/Jago,

    I think I'm getting there. The voltage drop along a line of leds is purely a result of the resistance of the length of wire upstream of it. So the two things you need to be aware of is the voltage drop down the line and the total power consumed by the line as a whole.
     
  5. fasteddy

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

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    I hope my article wasn't too confusing


    Forward Voltage is the voltage rating at which the LED runs at its optimal designed parameters. So for a 5050 LED used on strip, then this has a forward voltage of approx. 3.2v for green and blue and approx. 2.3 volts for red. Now with LED circuit design the basic rules are
    LEDs in Series = Add voltages up - Current stays the same
    LED in Parrallel = Voltage stays the same - Current adds up

    So for example a 12vdc strip is made up of 50 series circuits of 3 LEDs, each series circuit is connected in parallel to eachother.

    The best place to see this and understand this is using the LED calculator found here http://led.linear1.org/led.wiz
    Here is an example of a 12vdc strip of 1/2 metre in length that has 5 independent controlled sections of 3 LEDs each.

    [attachimg=1]


    Now voltage drop is an entirely different thing which has been explained by the other posts. But voltage drop is just not associated with resistance alone, you also have to factor in the amount of current as this also plays just as an important role as resistance when determining voltage drop, the more current the more voltage drop, the more resistance the more voltage drop because in Ohms law Voltage, Current and resistance are directly linked

    Example

    Volts=Current x Resistance so by changing either the resistance or the current you change the voltage drop

    5 amps x 0.5 Ohms = 2.5 volts dropped

    Only changing current
    2 amps x 0.5 ohms = 1 volt dropped

    Only changing resistance
    5 amp x 0.2 ohms = 1 volt dropped

    So both effect the end resultant voltage drop
     

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  6. OP
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    BundyRoy

    BundyRoy Senior Elf

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    Thanks Fasteddy. I've got to say I'm having a little trouble with this concept. My brain keeps telling me it can't be that hard as there is only three variables in the equation. I think I will just have to wait until all my gear arrives and then work through a real life example.
     

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