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Voltage regulator for High power LED's

Discussion in 'LEDs and Optoelectronics' started by flashdom, Aug 3, 2011.

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  1. flashdom

    flashdom

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    May 21, 2010
    Hi guys,

    I am trying to build a mood light but I'm having big trouble in powering the led's. I am using 3x 3W RGB star Led's. 'Ive been stuck on this for 3 days and so far i've come up with this:

    Use constant current power supply with 10-12V rating(possibly up to 16V with no load).
    Use some sort of voltage regulator to drop it down to 10v (this is where i'm stuck). After that I'm planning to connect each colour of the led's in series for a total of 9.6-12.4V (3.2-3.8 each) and 7.5-9V (2.5-3) for red colour led modules. This voltage difference can be dropped with resistors. I'm not going to worry about the rest for now, I need to get the led's lit first :)

    Can anyone suggest a good compromise between efficiency and complexity? Voltage regulator seems to be what I need but I don't know enough to build one so if you can give the list of parts I would need that would be fantastic! Any other alternatives welcome too of course.

    Any suggestions where to get these parts? Now I am using radionics online. (i'm in europe btw).

    Any help really appreciated!
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Please read this and post any questions you still have in this thread.
     
  3. TBennettcc

    TBennettcc

    292
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    Dec 4, 2010
    Boy, oh boy, Steve... I'm starting to love that LED sticky more and more every day... :)

    ...Yikes. I just looked up your "3x 3W RGB star LED's". Those are quite the big boys...

    Would something like this work?

    http://www.national.com/pf/LM/LM3407.html#Overview

    You're going to need a constant-current supply capable of feeding however many LEDs you want on at any single time, and the voltage should be greater than all of the forward voltage drops of all your LEDs, however you decide to wire them (series/parallel/combination).

    You shouldn't need to drop the voltage any (by using a 10V voltage regulator). Not sure where you read that. Do you have a reference web page for that? I'm not used to working with these 'super-LEDs'...
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Generally speaking, you can typically count on 350mA == 1W for a single LED. This fails for red LEDs (unless there are 2 chips in series in the LED). So 3W is typically around 1A.

    Where Vf is significantly different to 3.3V then this relationship won't hold.

    The sticky doesn't have an awful lot to say about constant current drivers because that would be a whole new sticky -- it's a big subject.
     
  5. TBennettcc

    TBennettcc

    292
    2
    Dec 4, 2010
    Think it would be possible / appropriate to start a sticky on constant-current drivers? Even just relating to LEDs? Put it as a sticky under the current LED sticky?

    I won't even pretend to know what I'm talking about, but I guess it's an idea...

    The first constant-current supply I saw was based off an LM317:

    http://users.telenet.be/davshomepage/current-source.htm

    The above site claims 1.5A max (I'm assuming with proper heatsinking)... I'm guessing this might work for a majority of the projects looking for constant-current supplies?

    Just a thought...
     
  6. flashdom

    flashdom

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    May 21, 2010
    Yeah I saw that sticky after posting this. It's helpful and a good read but doesn't go deep enough.
     
  7. flashdom

    flashdom

    18
    0
    May 21, 2010
    Since the leds(each color of the led's) will be in series, we can just assume it's 1 led with each color being 1W.
    I am confused about the current/voltage requirements of led's in general.I mean if I have constant current power supply with 10-12 Volts, do I need to worry about this kind of voltage change(considering it's constant current and ignoring red led for now)? Or do I need constant current power supply AND voltage regulator?

    Also if anyone can give me a few links to good articles/tutorials/books about constant current and voltage regulators that would be great.

    @TBennettcc
    I only wanted to drop voltage to 10v so that it could work with red colour led also. (red = 3x 2.5-3.0 or 7.5-9 green/blue = 3x 3.2-3.8 or 9.6-12.2)
     
    Last edited: Aug 3, 2011
  8. duke37

    duke37

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    Jan 9, 2011
    Look up the data sheet for the LM317T regulator, this can be configured for constant voltage output or constant current output. It is a linear regulator so dissipates some power. Swiching regulators will dissipate less power but need more designing.
     
  9. BobK

    BobK

    7,682
    1,686
    Jan 5, 2010
    You cannot regulate both current and voltage. A constant current supply will output whatever voltage is required to maintain the regulated current (within limits). So if you have a constant current supply capable of the max voltage needed, you should be all set.

    Bob
     
  10. flashdom

    flashdom

    18
    0
    May 21, 2010
    That's a lot clearer now. So if I have 350 mA constant current supply at 10-12V will that work? The voltage would be within limits but not at max voltage, will this not prevent the led form using the whole 350 mA? The red led will be out of the maximum range, will it damage the led/not light up or it doesn't matter since current is constant at 350 mA?


    @duke37 LM317T seems exactly what I need. Is there a tutorial on building the regulator with LM317T? (I have no idea what other parts I need) :)
     
    Last edited: Aug 4, 2011
  11. duke37

    duke37

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    Jan 9, 2011
    Get a data sheet on the LM317T, this will give details on how to use it.

    Controlled voltage.
    The 317 is designed to produce a controlled voltage, it raises the output voltage until there is 1.25V between the output and the sense pin and controls it there. The resistors are chosen to give this 1.25V difference at the output voltage required. There needs to be at least 3V more going into the 317 than coming out for it to work (known as the dropout voltage).

    Controlled current.
    This is similar to the controlled voltage circuit but here the voltage between the output and sense pin is generated by the output current, so, with a 10 ohm resistor the current will be controlled at (1.25/10) A = 125mA
    In this case, you need 3V to run the 317 and 1.25V across the resistor, so needing a supply at least 4.25V above the output voltage.

    Some additional components are often added to get stability and to protect the 317 from reversed polarity. The 317 contains a highly stable voltage reference, current limit to protect itself and will shut down if it gets too hot but, nevertheless, it should not be abused.
     

    Attached Files:

  12. TBennettcc

    TBennettcc

    292
    2
    Dec 4, 2010
    Please see the link I posted on the first page for your LM317T constant-current source.

    The V(f) specified for an LED is actually called the forward voltage drop. This means that if you attach the LED anode to the positive terminal of a perfect battery reading exactly 12 volts, the voltage between the LED cathode and the negative terminal of the battery will be 10 volts (i.e., two volts of potential have been dropped across the LED). Let's say you do this four more times, so there are now five LEDs connected in series. The total voltage drop across all of the LEDs totals 10 volts. This is less than the battery voltage, so (with a proper current-limiting resistor), they will still light up. However, add another LED (for a total of 12 volts dropped across the LEDs), and the LEDs will no longer light, because there is no potential once you reach the last LED of the series.

    An LED will light with any voltage across it, as long as the voltage across it is greater than the forward voltage drop. The thing that counts to power an LED is the current. That is why you should always use a constant-current source for your LEDs, providing as close to the rated current as possible without going over.

    To calculate a current-limiting resistor, you need to add up all of the voltage drops for any LEDs you want to put in series. Make sure you can supply a voltage above the total voltage drop for your longest series chain. Once you have that, calculate the difference between your supply voltage, and the voltage drop of the LEDs. Divide the difference by the current you want, and you will get the resistance of the resistor you need to use (in ohms) to achieve the desired current.

    Of course, if you're using a constant-current source, you don't need current-limiting resistors.

    Hope this helps.
     
  13. flashdom

    flashdom

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    May 21, 2010
    @duke37 thanks, LM317T looks like exactly what I need!

    @TBennettcc great explanation! It makes sense now. Looks like I will need a new psu as 12v woun't be enough :)
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Duke37, and TBennettcc, take a look at the LED sticky. It has been suggested I add a section on constant current sources.

    To my surprise (I really had forgotten it) the section on driving high power LEDs is pretty much a (brief) section on constant current sources.

    If you guys (and flashdom too!) would like to take another quick look and make recommendations as to how I can improve the sticky to better answer flashdom's question, I would really appreciate it.

    The sticky is under my name, but is the result of collaborative work. I welcome any suggestions that could help improve it.

    Please post your responses and proposed changes in the sticky thread. I'll then incorporate changes.

    Thanks :)
     
  15. duke37

    duke37

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    Jan 9, 2011
    Flashdom
    Early on you made some remark about complexity. The constant current source will give a very consistent light output but I doubt if this is what you require. As I pointed out earlier, with a 317 you need at least 4.25V across the regulator circuit. With this amount of voltage drop you will get quite good control with a simple resistor and probably enough control with 2V, easing your power supply problem.
    It should be possible to get a constant current circuit with lower voltage drop but this is likely to cost more.
     
  16. flashdom

    flashdom

    18
    0
    May 21, 2010
    Can you clarify this a bit, did you mean to say just use resistors in series with the led? If so this would be very inefficient and require high power resistors but I'm probably not getting what you are saying... The whole thing will be controlled by arduino mcu so i'm not sure how constant current or the method ur suggesting would work since it would have to be dimmed(varying current).
     
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Flashdom, you are correct. Resistors for High current LEDs are generally not the way to go.

    If you are using a constant current source, you must put your LEDs in series. If you need more than one string, then you need more than one constant current source.

    A non-switchmode constant current source is going to dissipate jut as much heat as a resistor, but will be somewhat kinder to the LED (you actually get lower Pd in the LED at reduced Vf).
     
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