stepping down PWM voltage efficiently (LED use)

Discussion in 'General Electronics Chat' started by NuNDoe, Dec 25, 2010.

  1. NuNDoe

    Thread Starter New Member

    Dec 25, 2010
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    Hi everyone,

    I have a 12V PWM (500Hz) dimmer for LEDs and have the following problem:

    1. I want to power (and dim) a SINGLE 350mA ~ 4V High Brightness LED (approx a 1W LED)
    2. I don't want to simply use a resistor or diode to "burn" the extra voltage down to 4V because that will be consuming more power than the LED itself --- I'll be roughly using up 2W in the circuit and 1W in the LED.
    3. Ground and the 12V PWM is all I have access to, I don't have access a constant voltage source.

    Is there an alternative way to EFFICIENTLY shift the 12V down to 5V? I was considering a buck converter however most of these have a 2ms startup time, which is getting close to the frequency of the PWM.

    Any help would be much appreciated.

    Quan
     
  2. iONic

    AAC Fanatic!

    Nov 16, 2007
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  3. #12

    Expert

    Nov 30, 2010
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    Would it be possible to adjust the PWM to 41.7% duty cycle to get what is effectively 5 volts?
     
  4. NuNDoe

    Thread Starter New Member

    Dec 25, 2010
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    I'm looking for a chip/discrete level solution as this module I'm making needs to be very compact (.5"x.5"x.5" max)
     
  5. NuNDoe

    Thread Starter New Member

    Dec 25, 2010
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    The PWM is adjustable but the LED modules needs to be operable 0-100% Duty Cycle.


    Traditionally, for 12V, LEDs are usually connected in series, requiring around total 10-11V forward voltage --- then you use a smaller resistor to drop 12V to the required forward voltage. In that case, the small resistor isn't burning up too much extra voltage.

    However, if I remove 2 LEDs from that circuit, I need a resistor to drop 12V all the way down to 3.7 ~ 4V... an 8V drop at 350mA is wasting a lot of energy compared to the LED it's driving.
     
  6. #12

    Expert

    Nov 30, 2010
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    Are you going to dim the LEDs by adjusting the PWM, possibly with a potentiometer?
     
  7. NuNDoe

    Thread Starter New Member

    Dec 25, 2010
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    the PWM is coming from a control board --- the signal is 0-12V, 0-100% duty cycle and can provide ample current (more than 3A if need), only the duty cycle of the PWM is adjustable, the amplitude is set.

    The LED module needs to be able to handle the PWM 0-100% duty cycle.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    If you got rid of the 500Hz PWM circuit, you'd have a lot more space for a proper LED dimmer circuit.
     
  9. iONic

    AAC Fanatic!

    Nov 16, 2007
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    The BuckToot is .75 long by .38 diameter. No step-down circuitry or current regulating circuitry needed.
     
  10. #12

    Expert

    Nov 30, 2010
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    I was going to say, "Just add a resistor so the pot on the PWM can't go higher than 42% duty cycle", but I can't get a straight answer. Now I've said it, so no need to reply to this.
     
  11. NuNDoe

    Thread Starter New Member

    Dec 25, 2010
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    it is a system requirement for the PWM to output 0-100% DC, I have other devices that are dependent on that standard signal.
     
  12. NuNDoe

    Thread Starter New Member

    Dec 25, 2010
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    This is currently the type of device I already have, it is designed for a DC voltage input and it will give you a constant output. However, I need to be able to give it a PWM signal and have it dim the LED for me. If i simply pass my 12V PWM as the power input of this device, it will eventually fry it.
     
  13. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Possibly a medium-large cap, or other filter to provide a steady average voltage from the incoming PWM.

    The choices are essentially:

    1) limit PWM to be a max of 40% or so
    2) use 2nd PWM driver inline with the first, filtering between the two
    3) use 3 1 W LEDs in series
    4) Use a resistor to limit current

    Is there a reason that a simple BuckPuck can't be used from the 12V line before the first PWM?
     
  14. SgtWookie

    Expert

    Jul 17, 2007
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    Why does this sound like an attempt to retrofit LEDs to automotive tail lamps?
     
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