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Help with voltage suggestions for new project!

Discussion in 'Community Projects' started by Lampy, Mar 25, 2013.

  1. Lampy

    Lampy New Elf

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    Hi, i am making 8 led light tubes. they consist of two led strips in each. Each strip has 79 leds which are 0.3w each.The strips take 5v. Thats about 88 amps by my calculations. From my drawing could anyone tell me the best solutions for power.
    As you can see i have decided to split them into two groups 44 amps per side. Sometimes i would only use perhaps four units for example unit 1,4,5 and 8. this would mean ofcourse the cable distances would be longer between each unit perhaps 8metres between units instead of 5metres. My two questions would be, what power supplies should i use and secondly to distribute the 5volts without voltage drop what gauge cable do i need?


    Cheers
     

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  2. fasteddy

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

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    You will always get voltage drop, the amount of voltage drop is determined by 3 main factors

    1: Distance of the run (Total Resistance)
    2: Guage of the cable (resistance/metre)
    3: The amount of current being drawn

    So from those values we can get our voltage.

    1: Distance of the run

    The drawing shows approx a 15 metre run but we have to add both the +V and the Ground together to calculate the total distance. This would be 15 metres x 2 = 30 metres

    2: Guage size of the cable (ohms/metre)

    All cables have a resistance value in either Ohms/metre or Ohms/foot and the larger the diameter (guage) the cable is, the lower the resistance is.

    There is a table in the ACL 101 lighting manual that can be used as a general reference for different cable resistance values.

    we will us the common 4 core security cable 14/020 (21 guage) which has a resistance value of 0.043 ohms/metre

    We then multiply this amount by our tolat distance of 30 metres

    30 metres x 0.043ohms = 1.29 ohms

    3: The amount of current being drawn

    So using your calculations for total current draw then each strip will be drawing 5.5 amps


    So now we can calculate the max voltage drop for a 5vdc with a rating of 5.5amps with a total run of 15 metres

    Calculating Voltage Drop

    Volts = current x resistance

    5.5amps x 1.29 ohms = 7.095 volts dropped

    So as you can see there is no way you can power this with 4 core security cable as the voltage drop is higher than the actual voltage


    So now lets look at 16 guage and then 12 guage cable

    16 guage cable = 0.014 ohms/m x 30 metres = 0.42 ohms x 5.5 amps = 2.31 volts dropped

    12 gauge cable = 0.0053 ohms/m x 30 metres = 0.159 ohms x 5.5 amps = 0.8745 volts dropped


    So as you can see the biggest issue with using 5vdc is voltage drop, even with using a very chuncky cable (12 guage) you will still see nearly 0.9 volts dropped just for using the one strip.


    Solution

    The only real way is to do your layout is to use multiple power supplies located at each pair of strips as if we had the power supply just 1 metre away we then dramatically reduce the voltage drop.



    14/020 security cable = 0.043 x 2 metres = 0.086 ohms x 5.5 amps = 0.473 volts

    This cable even at this distance is no good and is underated but the cores can be doubled to effectivly 1/2 the voltage drop to an acceptable level.

    16 guage = 0.014 ohms/m x 2 metres = 0.028 ohms x 5.5 amps = 0.154 volts dropped

    12 guage = 0.0053 ohms/m x 2 metres = 0.0106 ohms x 5.5 amps = 0.0583 volts dropped


    So by showing the above you can then get an idea of why its so important to have the power supply very close to the lights when using 5vdc high current strips as voltage drop is the biggest enemy.

    Thats why i prefer to use 12vdc as it is more forgiving with voltage drop as the % of voltage dropped is less than when using 5vdc.
     
  3. AAH

    AAH I love blinky lights :) Community Project Designer

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    According to my maths it works out at 632 leds at 0.3W each which gives 189.6W. 189.6W at 5V is 37.92A.
    Based on my knowledge of 2811 chips they drive at a constant current of 18.5mA. Assuming that they are no transistors involved in the design then 632 leds times 18.5mA gives 11.692A. My knowledge gives me a power use of 0.09W per led so there may be something else going wrong or alternatively the 0.3W may be wrong.
    Regardless of whether it's 38A or 12A you can use a single 200-350W power supply. The logical solution to reduce voltage drop is to provide power injection to each of the 8 light tubes.

    After writing all the above I'm now assuming that you were misleading us ;) . What there is is 79 pixels or 3 leds per strip. This gets the power to 0.3W and current to 36A.
     
  4. fasteddy

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

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    If using the 64 LED/M 2811 strip then this has a calculated rating of 3.06 amps per metre so for 79 pixels the total current draw would be 3.78 amps x 2 strips = 7.56 amps per tube x 8 tubes = 60.48 total amps for the 8 tubes

    This strip is definetly power hungry and will need to have voltage drop closely managed

    My previous post on voltage drop calculations were based on 5.5 amps per strip but this will now change based on the calcultaed load of each tube which works out to be 7.56amps per tube.
     
  5. OP
    OP
    Lampy

    Lampy New Elf

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    Hi, thanks for the help chaps, feel like i am getting close to understanding what's going on here. Looks like i selected the
    wrong strip for the purpose. I guess due to the distances i should have used 12v strip. Having spoken to the supplier in china i can confirm that the strip is 0.3w per led.
    http://www.clenled.com/Products/64LEDs-ws2811-IC-built-in-digital-led-strip.html
    Have re-drawn my diagram to be more accurate. I guess using a 10amp psu at the base of each of the eight lights would work, though i has hoping to travel the data and power over one(three core) cable from the power/data source and daisy chained through each light 4 per side. The strip has three connections, data, ground and 5v. Was thinking to use 3pin dmx cable for the data and ground and to tag the 5v power cable to it to form one cable.
     

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