Tricky LED Wiring - Need a Nudge

Discussion in 'The Projects Forum' started by Myedroid, Aug 12, 2013.

  1. Myedroid

    Thread Starter New Member

    Aug 12, 2013
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    Hey guys,

    Quick question for all you geniuses out there...

    I am trying to wire this RGB LED to get a nice shade of Purple.

    The site shows the following info for each die:

    I plan on using an output that can provide up to ~7.4v and ~2A

    If i wire these die in series, I will get ~3.7v per die which is fine for the blue, but is a little high for the red. Since I plan on using a Potentiometer on the red die, voltage is not a problem since i can vary my resistance.

    However, doesn't the second die get limited by the current the first die is drawing? Or, since my output will give the LED's what they want current wise, can they both pull 350 mA while wired in series and halve the voltage? Or will the first die pull 350 mA and then feed a pittance to the second die?

    Did I just answer my own question, that wiring in series will feed acceptable voltage to both die, and current will manage itself?

    Also, what type of potentiometer will i need, i am totally lost on that front...

    Any resources, or help that can point me in the right direction would be helpful. I feel like i'm so close to figuring this out but am simply missing something simple. Thanks so much guys!

    **EDIT**

    I found something here on the forums that I missed before... Looking at it this way, I need a linear pot, over 1 Ohm? Woefully inadequate, I apologize.
     
    Last edited: Aug 12, 2013
  2. wayneh

    Expert

    Sep 9, 2010
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    You need separate control over each color, so you need to wire the 3 in parallel. Calculate the current-limiting resistor for each color individually. I'd recommend starting at, say, 100mA each to see how things look. Only go to higher currents 1) if you really need to and 2) once you're sure that nothing is overheating.

    You'll need a big heat sink for that thing.

    Oh, and forget about putting a pot in series to control the power. Any pot that could handle that much current will be too expensive and, unless wired with a fixed resistor, risks sending too much current forward if someone turns the dial too far.
     
  3. Zenock

    Active Member

    Jun 1, 2009
    36
    1
    People can correct em if I get it wrong.

    Assuming you aren't wiring in the Green. you can wire the Blue and Red in series. Wired in series they will both get the same amount of current. There will be a voltage drop across red of 2.5 V to 3.0 volts There will be a voltage drop across blue of 3.2 to 3.8 volts. For a total voltage drop of 5.5 to 6.8 volts leaving you with between .6 and 1.9 volts pretty big range...

    .6/.350 = 1.71 Ohms 1.9/.350 = 5.43 ohms Might be worth building a current follower to get the .350 amps (350 mA) use a resistance to small and it will burn up your LEDs use a resistance too big and they won't turn on at all. The absolute smallest your resistance should be is about 2 ohms. Any smaller and you will burn up your LEDs and may burn them up with a 2 ohm if the voltage drop across the diodes is on the low end of the range.

    Personally I would use a 555 timer to drive the LEDs. Put the pot on the timer and use the timer to generate a pwm signal.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Yes, I'll ditto that recommendation to consider a PWM driver (3 of them) for this. That'll allow smooth and continuous control over each color. It'll also eliminate 3 hot resistors.
     
  5. LDC3

    Active Member

    Apr 27, 2013
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    160
    Okay, I believe you're wrong. The device has only 4 leads, so it is either a common anode or common cathode. You cannot wire the LEDs in series.
     
  6. tubeguy

    Well-Known Member

    Nov 3, 2012
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    197
    I agree. One end of each LED may be connected to a common.
    I believe an adjustable constant-current source is a safer way to control each color of these higher wattage LED's. I think these can suffer from thermal runaway if resistors are used.
     
    Last edited: Aug 12, 2013
  7. Zenock

    Active Member

    Jun 1, 2009
    36
    1
    How embarrassing. I should have actually looked at it. :p

    Parallel it is then. I stand by driving them with a PWM signal though. ;-)
     
  8. Myedroid

    Thread Starter New Member

    Aug 12, 2013
    6
    0
    Wow guys, thanks for jumping on this!

    Sorry for not being more clear, they are common anode (the pads have their +'s and -'s mixed up) This means I cannot wire them in series and would have to wire them in parallel?

    At that point my pot would change, how do I even begin to know what i would need in a pot? Anyone got a formula handy?

    I will have a driver that will supply a constant current for whatever the led pulls, up to ~2A.

    Thanks again for all the input!
     
  9. wayneh

    Expert

    Sep 9, 2010
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    I'll say it again - ditch the pot idea.

    If you want brightness control, you need a PWM controller, actually 3 of them, not a simple pot. A ghetto version would be something like a LM317 constant current circuit for each LED, but that will make quite a bit of heat at 300mA. These circuits will use pots for control, but they will carry very little power and can be very inexpensive.
     
  10. Myedroid

    Thread Starter New Member

    Aug 12, 2013
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    I will have a soundboard with components that will drive the LED. I do not have to worry about current regulation, only mitigating my voltage and varying it to change the shade of purple. That is why I was using the pot. Am I misunderstanding what you are saying?

    At the risk of sounding terribly geeky, this will be used for a hobby prop replication.
     
  11. wayneh

    Expert

    Sep 9, 2010
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    Hard to say. The LEDs MUST be controlled by the current through them, not the voltage. If your controller uses a voltage input to deliver a current output, then I understand what you are saying about voltage. It's just that we see so many folks here hoping to control LEDs by voltage. That's possible, but very difficult compared to control by current.
     
  12. Myedroid

    Thread Starter New Member

    Aug 12, 2013
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    In my limited understanding of the actual electronics behind it, the board supplies the amount of current the led wants, it just doesnt control the voltage. Basically, i get voltage in = voltage out, but a varying mAh depending on the led? Does that make sense?

    i appreciate your help!
     
  13. wayneh

    Expert

    Sep 9, 2010
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    Not really. Something has to act to limit current to a limit that YOU set, or the LEDs will quickly be destroyed. They cannot help themselves and will overeat like a fat man at a buffet. The limit can be as simple as a resistor or as complex as a PWM controller. But there needs to be something.
     
  14. Myedroid

    Thread Starter New Member

    Aug 12, 2013
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    Is it possible i have it backwards? That it will feed it the voltage that it wants but not limit the current?
     
  15. wayneh

    Expert

    Sep 9, 2010
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    Well, as Bill Clinton taught us, it depends what "it" means. We need to understand what components you're working with.
     
  16. Myedroid

    Thread Starter New Member

    Aug 12, 2013
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    Haha, well the board can control the current on second look, but possibly only down to 1A.
     
  17. wayneh

    Expert

    Sep 9, 2010
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    Heading outside to watch the meteor shower. Back tomorrow. :D
     
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