12VDC Strings or 5VDC Strings, what is best to use

Discussion in 'RGB Lights - Intelligent Pixels and 3-Channel RGB' started by fasteddy, Aug 21, 2012.

  1. fasteddy

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

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    The question has come up many times in the past, what is better to use for RGB strings, 5VDC or 12VDC. Well this will depend on what the individual needs are, so below i have tried to explain the differences so people can make up their own decisions based on the information provided. Im sure there will be others that will add to this thread to help people make an informed decision.

    The differences between using 12vdc and 5vdc pixel strings both have advantages and disadvantages

    * 12VDC allows longer strings due to less % of voltage drop over the same distance as 5vdc
    * 12VDC will use and waste a lot more power than 5vdc which means providing more power supplies
    * Some 12vdc strings like the 180x series run in constant voltage mode as apposed to constant current mode. LEDs are a current devices so constant current is a better method of control. Im not sure if the 12VDC 2801 pixel string has been set to run in constant current or constant voltage mode.
    * 12VDC will dissapate a lot more heat than 5vdc due to the wasted energy.
    * 12VDC will let out less effective light output compared to 5VDC due to running at lower currents
    * 5VDC requires more power injection due to both the higher currents and higher % of voltage loss over the same distance as 12VDC
    * 5VDC requires larger guage wire compared to 12VDC for the same power requirements.
    * 5VDC is much more efficient than 12VDC and requires less power supplies for the same amount of light output.

    To see this we will compare a 12VDC and 5VDC string using the common 8mm LED

    8mm RGB LED Specifications

    Red - 20millamps - 2 volts
    Green - 20 milliamps - 3.2 volts
    Blue - 20 milliamps - 3.1 volts

    So to run the red LED we need to drop the voltage from 12VDC to 2VDC, as you can see there is a much higher % of volts that need to be lost to run at 12VDC compared to that of 5VDC. So there is a lot of wasted energy. One way that is used to to minimise the amount of lost energy through a 12VDC string is to run the LEDs at a lower current, so instead of running the LED at the rated 20ma for each colour, they will instead be set to run at 10ma (these values may differ), this then reduces the amount of wasted energy due to the lower current needs of the LEDs. But as LEDs are current devices, the lower current will reduce the light output. Now normally at 10ma the human eye cant really notice a great difference when looking at the LED compared to being run at 20ma, but what happens is the effective light output is reduced and this will show with a light meter, this will then reduce light wash and surrounding light generated by the LED.
    The reason why we see 12VDC strip and modules is because they have 3 LEDs connected in series, so the red LED circuit would then be 6vdc (3x2v), this makes much better use of 12VDC than using a single LED.
    5VDC is used for single LEDs and 12VDC is used for multiple (3) leds. 12VDC string were designed to help overcome the voltage drop and current needs of using 5VDC strings.

    So now knowing the differences and the advanatges and disadvantages should help people decide if they prefer to use 5vdc or 12vdc strings. Personally i use a lot of strip and modules because it is 12VDC and makes efficient use of the power used compared to running 12vdc strings. The little strings I do have are 5VDC because that is the correct and most efficient design for using single LEDs

    So really it goes down to

    12VDC = Longer string length, more power used, low efficiency
    5VDC = Shorter string length, less power used, high efficiency
     
    hammo131 likes this.
  2. trcons

    trcons New Elf

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    Thanks for taking the time to spell that all out in a single post. Saved me from starting a thread asking something like " hey which ones should I choose"

    Tom
     
  3. DeeJai

    DeeJai Is that Magic Smoke?!?

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    or my main reason for going 12V over 5V. I just bought the stuff when i got talked into it by Steve. + i didint understand back then.
     
  4. anon

    anon Apprentice Elf

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    I was struggling to work this one out (getting confused between strip and string) but then cottoned onto what was being said.



    Strip and modules use 3 LEDs and so can 'break down' the 12VDC with LEDs that produce light for the power/resistance. Pixels being a single LED must use a higher series resistor (that wasted energy) to keep the 2V across the LED.


    Pdissipated = IV. We will always have the same constant current (20mA assuming this is the case), but now the resistor must be specced to drop 10V instead of 3V. So we now have to dissipate 3.3x as much power out of the resistor to bring the voltage down. For each red led this will be 20mA * 10V = 200mW, compared to 20mA * 3V = 60mW. 100 LEDs and thats 20W of power generated as heat and wasted, compared to 6W.
     
  5. OP
    OP
    fasteddy

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

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    This is correct, but you will also find that 12VDC string LEDs will also run at a lower current and typically around 10ma, 1/2 of the rating of the 5050 LEDs, this is to help reduce the amount of wasted energy needed to run the LEDs, but as i mentioned before, looking directly at the LED then you wont see much visual difference, but the overall total light output is reduced thus reducing the amount of light wash

    The above statement is in relation to intelligent pixels where each pixel has its own LED driver. As far as dumb RGB led strings then im not sure what resistor value they use and what current they run the LEDs at.

    So the basic rule with LEDs

    Series LEDs = voltage is added together

    Parrallel LEDs = current is added together

    So by having 3 LEDs in series you would then add the voltages together, 2v + 2v + 2v = 6v so now you only have to reduce the voltage by 6 volts instead of 10 volts for the single LED pixel string.
    This is the reason you see with traditional mains voltage LED strings and rope lights with many LEDs in a single circuit, as the total number of LEDs for a 240 Volt string or ropelight requires many more LEDs in series than a 110 Volt string or rope light to get closer to the supply voltage. Each circuit will have a resistor to then match the circuit with the voltage.


    The below examples will show you the wasted energy dissapated through the resistor compared to the total energy used, this shows why 10 or 9 milliamp is typically used with 12vdc pixel strings as this then allows the string to to use a similar amount of energy as a 5volt string running at 20 millamps. But what you are getting is a LED that has reduced overall output which uses slightly more energy and is inefficient in its design as more energy is dissapated through the resistor than what is dissapated through the LED

    5VDC LED at 20mA
    Red - 20mA - 2 volts
    total power dissapated: 100milliwatts
    60mW through the resistor
    40mW through the LED

    Green - 20mA - 3.2 volts
    total power dissapated: 104milliwatts
    40mW through the resistor
    64mW through the LED

    Blue - 20mA - 3.1 volts
    total power dissapated: 102milliwatts
    40mW through the resistor
    62mW through the LED

    12VDC LED at 20mA
    Red - 20mA - 2 volts
    total power dissapated: 264milliwatts
    224mW through the resistor
    40mW through the LED

    Green - 20mA - 3.2 volts
    total power dissapated: 252milliwatts
    188mW through the resistor
    64mW through the LED

    Blue - 20mA - 3.1 volts
    total power dissapated: 250milliwatts
    188mW through the resistor
    62mW through the LED

    12VDC LED at 10mA

    Red - 10mA - 2 volts
    total power dissapated: 120milliwatts
    100mW through the resistor
    20mW through the LED

    Green - 10mA - 3.2 volts
    total power dissapated: 131milliwatts
    100mW through the resistor
    31mW through the LED

    Blue - 10mA - 3.1 volts
    total power dissapated: 132milliwatts
    100mW through the resistor
    32mW through the LED
     
  6. zeph

    zeph New Elf

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    Good info, FastEddy. You've covered the basics very well and at most I could hope to add some perspective.


    One note is that I've never seen 12V pixel strings or strips with 20ma/color, probably due to heat dissipation, so for pixels the first and last options (5V@20ma/color and 12v @ 10ma/color) may be the "real" ones to consider.


    If anybody can provide a link to pixel strings or strips with 20ma/color @ 12V, I will stand corrected. Otherwise, let's omit that part as confusing beginners with a non-existant alternative.


    (There is a larger variation in these matters with domes and modules, including using series LEDs at 12 or 24V, so that's a whole different game. And of course dumb strips can more easily stack LEDs in series so they too are a different game. This note is only about currently available pixel strings and strips where each RGB light is individually controlled).


    So the real world difference at this time is: 12V pixels use about 20% more power than 5V pixels, to deliver somewhat less brightness. This is to be expected as the 12V pixels only provide about half as much current to the LEDs. Some claim that the difference in visible brightness is minor (one source even measured it as much less than the 2:1 current difference would lead one to expect).


    AND - thus the 12V pixels are much easier to power if you have them spread out - because with half the current they can have half the voltage drop (unless smaller wires are used), AND they have a lot more margin before the voltage drop matters. This can be a major difference in some layouts. So it depends on how much the extra brightness of the 5V lights matters to one, versus the convenience of the 12V power options.


    The efficiency that matters at the full system level is "useful light per watt input", which is not really the same as "power dissipated at the LED per watt input" due to factors like varying LED efficiency and logarithmic response of the eye to brightness. In my case, full brightness of the 12V pixels has been more than enough, so the small extra brightness from 5V pixels would not have been a benefit. Other people may have different needs. This is not to advocate for 12 V pixels being best for everyone - and in fact I'm considering adding 5V pixels for other reasons (faster PWM cycle on WS2801'a)
     
  7. OP
    OP
    fasteddy

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

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    LithgowLights had some last year that he mentioned were 12Vdc and ran at 20mA so these ran very warm. Hopefully he may post some further info regarding these
     
  8. Sluggo Doug

    Sluggo Doug New Elf

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    Thanks for that Fast Eddy ,I was going to use 12 volt as i had several 25 amp power supply s ,But will now go 5v as it is more efficient and less heat .Learning as i cruze through the forum.
     

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