Need some expert advice on wiring mixed LED's

Discussion in 'The Projects Forum' started by SparkDog, Dec 8, 2014.

  1. SparkDog

    Thread Starter New Member

    Jan 7, 2014
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    Hello all

    I have a christmas Santa Claus lamp that originally used a 5W diachroic bulb and a stepping motor with coloured disc
    to illuminate various areas of santa via fibre optics.

    My plan is to completely do away with the motor and 5W bulb
    and use 3 of these: Slow-Changing-Cycle-RGB-LED-Mulitcolour
    and 3 of these: 5mm-White-LED-Ultra-Bright
    Powered by a 9V/2A AC/DC adapter.

    After looking at the specs on those links, and using a Vf of 3.2V for the whites, and 2.4V (as suggested) for the rainbows
    my calculations say that i require 180Ω resistors if i am going to connect them as shown in the attached image.

    Are my calculations correct ?
    Is it ok to connect them up as shown ?
    Are 0.25W resistors good enough for the job ?

    My proposed circuit leds.jpg

    Power supply specs power-adapter.jpg

    Thanks
    SparkDog
     
  2. KJ6EAD

    Senior Member

    Apr 30, 2011
    1,425
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    I don't think that's going to do what you expect it to.

    First, even if it worked electrically, the white will tend to wash out the color from the RGB and unless the different RGBs are driving into separate fiber sections, their colors will combine in an unpredictable way. Second, the RGB will produce a white of sorts on it's own as it cycles. Third, those color changing LEDs are not just LEDs for which you can calculate a limiting resistor value; they're three LEDs and a small integrated circuit driver that needs to be supplied a fixed voltage, usually 4V and as they change colors will draw between 20mA and 65mA.

    I submit that the seller of those RGB devices knows little about them and is pasting specifications from other products. You would do well to get the RGB devices and do some experimentation before proceeding further with detailed plans for their use. Here's an actual data sheet for a typical product of the genre. It even includes a timing diagram.

    http://dlnmh9ip6v2uc.cloudfront.net/datasheets/Components/LED/changingLED.pdf

    Then there's the matter of effectively coupling the light from a 5mm LED into a fiber that originally was driven from a larger diameter source. LEDs do couple well into plastic optical fiber but again some experimentation may be needed.
     
    Last edited: Dec 8, 2014
  3. ScottWang

    Moderator

    Aug 23, 2012
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    If you have volt meter or multimeter then you can measure the current of leds, make sure the current <= 16mA, that is for the led life and concerned the brightness.
     
  4. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    I assume that fibre optic bundle comes together at light source making a disc of about one cm dia. Easy to couple one LED to fibre but how could couple 6 LED's & as said it would be a mess of color. How about eleminating fibre & space LEDs around santa, or split fibre into 6 bundles?
     
  5. SparkDog

    Thread Starter New Member

    Jan 7, 2014
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    Hello KJ6EAD, ScottWang and Bernard

    Very sorry for any confusion caused, i should have explained things more clearly.

    OK, i'll try and do that now.
    The LED's will actually be arranged so that only the 3 rainbow's will direct light through the fibre optic bundle,
    and the 3 whites will be positioned to illuminate the main space inside of santa.

    I have a reflector out of an old LED torch that i am going to use for the rainbow LED's
    as it is a pretty good match diameter wise (25mm) to the bundle of fibre optics.
    I will simply attach that reflector to the end of the fibre optic bundle,
    either with a few dabs of hotmelt glue or some electrical insulating tape.
    reflector.jpg

    Testing with just one string (resistor > white-led > rainbow-led) on a breadboard with a 9V battery,
    and both LED's light up bright enough for my intended purpose.
    There wasn't any noticeable heat from any of the components, although i only had them running for a few minutes.

    My main concern is safety, i don't wish for anything to get so hot as to cause a fire,
    especially as it will be powered off the mains supply and not a battery.
    Hence why i asked if 0.25W resistors are up to the task, or would i be better off using 0.6W resistors ?

    Unfortunately i do not have a multimeter or any other kind of meter.

    Thanks
    SparkDog
     
  6. ScottWang

    Moderator

    Aug 23, 2012
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    A LED drawing less than 20mA and you using 0.25W is OK, if you can't make sure then you could using your fingers to touch the led to sense the temperature, if it just a little warm then it's ok.
     
  7. SparkDog

    Thread Starter New Member

    Jan 7, 2014
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    Hello ScottWang

    That's good to hear, thanks.
    That's exactly what i did, neither the LED's or the resistor felt warm.
    I just had them running again for about 30 minutes, and they still feel pretty cool to the touch.

    So, do you think it is safe to wire it up as per my original circuit diagram ?

    Thanks
    SparkDog
     
  8. ScottWang

    Moderator

    Aug 23, 2012
    4,855
    767
    If the temperature there is no rising then it's ok, I always used 0.25W for the led current <= 20mA.
     
  9. SparkDog

    Thread Starter New Member

    Jan 7, 2014
    6
    0
    Hello ScottWang

    OK thanks again.
    I'll wait a day or so in case anybody else has different ideas,
    then i will go ahead and wire it all together and see how it goes.

    Thanks
    SparkDog
     
  10. KJ6EAD

    Senior Member

    Apr 30, 2011
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    363
    You really should connect the color changing LEDs to a voltage source, not in series with the white LEDs.

    If you'd use a surplus phone charger (5V) and a 100Ω resistor in series for each white LED and use a 1N4001 rectifier diode in series with each color changing LED, you'd have a safe, efficient circuit that satisfies the white LED's requirement for current limiting and the color changing LED's requirement for a constant voltage between 3.5V and 4.5V. Additionally, each light emitting element will be on a parallel line independent of each other so a single failure will not affect the other five elements. The whole system would only present a peak current load of ≈255mA and an average power of ≈1W!
     
    Last edited: Dec 8, 2014
  11. SparkDog

    Thread Starter New Member

    Jan 7, 2014
    6
    0
    Hello KJ6EAD

    Not quite sure i fully understand what you are suggesting,
    could you please provide a circuit diagram to help explain things.


    I have managed to unearth an old Nokia charger, it's output is rated at 5V@350mA
    would that be suitable or is it a little under powered for what you suggested.

    5V-supply.jpg

    SparkDog
     
  12. KJ6EAD

    Senior Member

    Apr 30, 2011
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    Like this. Just repeat the white LED/resistor and the color changing/diode legs two more times each.

    [​IMG]
     
  13. Bernard

    AAC Fanatic!

    Aug 7, 2008
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    Checking peak current drain of 3 color LED yields something like this:
    Supply 5V, monitoring drop across R
    100 ohm, 2 V = 20 mA
    33 ohm, 1.2 V = 36 mA
    460 ohm, 2.5 V = 5.4 mA
    I would use a 100 ohm R for 3 color LED's
    If white LED is operated in series with color, white flickers in sync with colored LED.
     
  14. KJ6EAD

    Senior Member

    Apr 30, 2011
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    The automatic color changing "LED" is actually an IC combined with an RGB LED. It is a voltage driven device and cannot be accurately characterized based on the specifications of it's included LEDs.
     
  15. SparkDog

    Thread Starter New Member

    Jan 7, 2014
    6
    0
    Hi

    Apologies for the delay in responding.

    I have decided to go with the circuit in the attached image, using santa's original 12V AC power supply.
    final-circuit.png

    It has been running for over an hour and appears to be fine.
    The temp of the 7805 is hovering around 40 degrees C (room temp is 24)

    In the end i didn't need the white LED's as there is sufficient light coming from the 3 colour changing LED's.

    So i would just like to thank everyone who responded, it was much appreciated.

    SparkDog
     
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