Why do I have to use power injection with RGB and Low Voltage Lighting

Discussion in 'Computers, Cabling & Other Miscellaneous Hardware' started by fasteddy, Feb 2, 2012.

  1. fasteddy

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

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    I have seen and been asked this question many times and thought i might show why power injection is required for low voltage applications and why a low voltage strip doesnt get the same length as a rope light

    It has to do with Ohms law and the resistance of a cable, the voltage and the current.

    An example is the difference between a 240v rope light and a 12vdc RGB strip, a 240 volt rope light can get much further runs then a 12vdc strip

    so with some calculations we can show the difference the voltage makes to current for the same rated wattage used.

    In this example we will use a 100 watt load


    So the current required would be:

    Current = power(watts) / Voltage

    Current = 100 watts / 240 volts

    = 0.42 amps @ 240 volts



    Now for the same wattage used but running at 12vdc

    current = 100 watts / 12 volts

    = 8.3 amps @ 12 volts



    The reason that strips are only 12vdc and 5vdc is because an LED has a rated forward voltage that it will run with, so the LEDs are connnected in series.

    Example. An LED may have a forward voltage rating of 3 volts, if you connect 3 of theses LEDs in series you will get 9 volts and 3 volts will be required to be dropped through a resistor. This allows the strips to have a much smaller cuttable section than what is found on rope lights. If a strip was rated at 240 volts or a 240v rope light used a similar rated LED then this would require around 70+ LEDs connected in series to become an efficient LED circuit.



    Now the next thing is resistance of a cable which effects the voltage and this as well can be shown using ohms law.

    Voltage = Current (amps) x resistance (ohms)

    So different cable sizes have a different resistance value, generally the larger the cable the lower the resistance.



    20 guage (approx 1mm2) wire has a resitance rating of 1.28 ohms per 100 feet

    16 guage (approx 2.5mm2) wire has a resitance rating of 0.40 ohms per 100 feet

    So the voltage drop differences for a 5 amp load over 100 feet would look like this



    20 gauge (1mm2): Voltage = 5 amps x 1.28 ohms = 6.4 volts dropped over 100 feet



    16 gauge(2.5mm2): Voltage = 5 amps x 0.40 ohms = 2 volts dropped over 100 feet



    So by using a lower voltage the % of voltage drop is much higher with lower voltages than with higher voltages over a given distance and this is why cable choice is also very important when using low voltage lighting and why power injection is used to get these extra distances.


    I hope that makes it a bit more easier to understand why it is so
     
  2. David_AVD

    David_AVD Good news, everyone!

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  3. zeph

    zeph New Elf

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    Good approach. There are a couple of tweaks tho.

    I don't find 1.28 ohms/100' in any tables for 20ga copper. Typically more like 1.05

    I like this chart: http://www.thelenchannel.com/1wire.php because it include various stranding combinations used to produce various guage wires, not just solid wire.

    I'm glad to see that the wiki article reminds newbies that you have to account for the voltage loss in both the power and ground wires. So putting something 100' distant usually means dealing with 200' of voltage drop.

    Dumb RGB strings are a bit more complicated, as the colors share one lead and each colors has its own wire for the other. Just calculate the drop for the shared lead (with full current in all three) and the individual lead (with current for only one color) and add them.

    Do folks down under still use that foot thing? I've always figured they were just humoring us Americans.
     
  4. OP
    OP
    fasteddy

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

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    Thanks for pointing out that error, i actually used the value of 21 gauge, damn eyes are getting bad. Anyway i got the figures from this chart and you are correct with the value for the 20 guage.

    http://www.cirris.com/testing/resistance/wire.html


    foot thing, i tried to keep it easy for our American friends ;) i just used feet as a general example as the figures from the chart used feet and the thread was just to give people a general idea surrounding low voltage and voltage loss as the maths is the same.
     
  5. GoofyGuy

    GoofyGuy Guest

    Between you and David Im not sure why the entire world isnt on board for the Blinkyness of this hobby! Your how to's and why's are almost always in terms that are easy to understand, and David whom I think of as the Mad Scientist of this hobby can pretty much create something that will till any needs to get power, signal, or both to a display.

    As for power injection in terms of power supplys and long runs, do you reccomend the power injection points for a strip be fed from the same power supply as the original. I know not every power supply will be the same output. Would it be harmful to the power supplys or the strip if theres a voltage conflict somewhere in the line?
     
  6. OP
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    fasteddy

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

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    Very good point to raise

    In most cases the power injection must come from the same power supply as most power supplies will have issues being connected together as they are not designed for parrallel connection. There are some expensive power supplies that are designed to do this
    Definetly the Chinese PSU that Ray sells would not do well if connected this way.
     
  7. GoofyGuy

    GoofyGuy Guest

    As always Eddy thank you. I assume using different power supplies for a parallel connection was a bad idea. Now one question that did not get answered that I have asked in the past or maybe it wasnt clear to me the answer.

    From one of my controllers that will be on the ground the lights connected will be 25 feet away. At that distance, and using a variable power supply would it be a good or bad idea to with a multimeter test the power at the end of the cable leading to the RGB string for voltage and correct the power with the power supply back to 12 volts, regardless of negative or positive?
     
  8. OP
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    fasteddy

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

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    I assume you’re talking about testing for voltage drop. If this is the case then it’s just a matter of connecting your test leads of your voltage meter to the positive and negative wires and take down the reading. Then do this same test at the power supply and then work out the difference between the two values. This test should be done when you have the cable connected with the actual load you’re going to use being on as current draw is also a factor in voltage drop.

    I hope that answers your question
     
  9. GoofyGuy

    GoofyGuy Guest

    Answers the question perfectly thank you.
     
  10. zeph

    zeph New Elf

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    If you are going to tweak the PS voltage to compensate for voltage drop on lights which may be partially on (eg: RGB strip with individual dimming per color, or pixel strand where some pixels may be on and others off), remember to look at the "almost all on" and "almost all off" edge cases (current drop) so as not to go too high nor too low.
     
  11. zeph

    zeph New Elf

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    That depends. The simplest answer is no. One definitely would not want to connect two power supplies in parallel with low resistance between them; if one's trying hard to maintain 5.1v and the other 4.9v, they will be fighting each other.

    It might be possible in some cases to inject power on both ends of a string if the resistance of the string is sufficient (and the supplies are not TOO divergent). Need to do your calcs first, tho! For example for static lights, the voltage might be 24.2 at one end, and 23.9 at the other - and 22.5 in the middle. The supplies would be sharing the load, not fighting each other. But if this doesn't make sense or the calcs are not your cup of tea - go back to answer 1 above!
     

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