PWM for power led

Discussion in 'The Projects Forum' started by BG79, Apr 19, 2011.

  1. BG79

    Thread Starter New Member

    Apr 19, 2011
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    Hi all,

    I´m working on a project for my university (industrial design) and I need to use PWM. The project is a torch with a Cree high power led (XPG), using a 6V battery. I already made the constant current regulator and it works fine.

    My issue is with the PWM. The circuits I could find (most with the 555) have current output of 200mA, and I need at least 500mA for the light I want. Is there any PWM circuit I can use for these specs (~6V in, ~500mA out)?

    Already thanks.
     
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Welcome to AAC.

    You can amplify the current using a logic level Mosfet.
     
  3. BG79

    Thread Starter New Member

    Apr 19, 2011
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  4. R!f@@

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    Show me ur schematic and I will add the FET to it and repost it.

    And no..No energy will be lost as latest fet's have low RSDON. that is the on resistance in the range of miliohms
     
  5. John P

    AAC Fanatic!

    Oct 14, 2008
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    Well--is this a circuit with an inductor that forms a (more or less) constant current setup, or is there going to be a resistor in series with the LED? If you have a resistor, there's limited use in having a low-resistance FET, as you'll just be moving the place where the power loss occurs.
     
  6. BG79

    Thread Starter New Member

    Apr 19, 2011
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    Here´s the schematic of PWM (I´ve got it from instructables.com):

    It´s a linear current regulator made with IRFZ44N and BC548. I really have loss there, but I couldn´t find a IC driver to buy. If the PWM goes to 500mA I´m planning to forget the current regulator and try the led directly to the PWM.
     
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  7. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Funny...
    Why don't you try what I did...
    Check post 5, I have attached a diagram. I think It would suite ur needs.
    The transistor can handle 500mA quite easily
    {ed}
    Hold on...let me clear up that diagram for u. Gimme a few minutes.
     
  8. R!f@@

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    That diagram has a draw back. I think that config is likely to damage the 555 pretty quickly.

    What I did is quite safe and reliable.
    ( much thanks to Bill )
     
    Last edited: Apr 19, 2011
  9. Wendy

    Moderator

    Mar 24, 2008
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    I was about to comment on that. The 555 has quite low drive, but the circuit is very similar to one of my articles.

    555 PWM Oscillator
     
  10. R!f@@

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    Apr 2, 2009
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    Or u can give this diagram a go.

    You can change the freq. by changing C1 Value
    The transistor can handle loads up to 1A safely.
    But it is a good idea to have small heat sink if u plan to go more than 500mA
     
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  11. BG79

    Thread Starter New Member

    Apr 19, 2011
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    Pretty nice.

    I´ll try both R!f@@ and Bill_Marsden´s circuit.

    Bill, I only need one led. May I just take anyone of those off?
    What is this circuit´s max. current output?
     
  12. R!f@@

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    The load is stated in post #10
     
  13. BG79

    Thread Starter New Member

    Apr 19, 2011
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    R!f@@,

    Where would the FET you said in post #4 go in my first circuit? I´m having difficulties finding the ICM7555 to build your circuit.
     
  14. Audioguru

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    Dec 20, 2007
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    An ICM7555 is a Cmos 555 made by Intersil. A TLC555 is the same made by Texas Instruments and an LMC555 is also the same made by National Semiconductor.
     
  15. BG79

    Thread Starter New Member

    Apr 19, 2011
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    I really can´t find any cmos here.

    I have a new doubt. I always tought that if current was ok, voltage wouldn´t matter, but today (looking for the cmos), the seller told me that voltage must be controlled too. Is it right? What about linear drivers like this: http://www.instructables.com/id/Circuits-for-using-High-Power-LED-s/ ? I made one of this and feeding it with 6V I measure 6V at the end.

    Already thanks
     
  16. Wendy

    Moderator

    Mar 24, 2008
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    Where is here? What part of the world are you?
     
  17. BG79

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    Apr 19, 2011
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    I´m in a small town in Brazil. I´ll order them from the capital city, but I´d like to use what I can buy in the shops close to me, to begin the work at once. At the worst I´ll learn something more :)

    I´m even looking for PIC´s tutorials (funny is that I can get PICs here. Maybe people here just don´t like cmos...).
     
  18. Audioguru

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    Dec 20, 2007
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    An LED controls its own voltage because it is a forward-biased diode. You must limit its current. The transistor in the instructables circuits limit the current in the Mosfet and LEDs.
     
  19. BG79

    Thread Starter New Member

    Apr 19, 2011
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    So (to be sure), if I send 12V, 350mAh to one high power LED it will be ok?
     
  20. Audioguru

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    Dec 20, 2007
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    If an LED is rated for 350mA then you can send a million volts and limit the current to 350mA and it will work fine (if the voltage doesn't jump over everything). The "thing" that limits the current will heat with (0.35A x 1 million V)= 350 thousand Watts.
     
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