To PWM or not to PWM

Discussion in 'General Electronics Chat' started by iONic, Nov 6, 2010.

  1. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
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    I'm looking at the datasheet for LT3760 - 8-Channel × 100mA LED Driver. My goal is to build a Ring light for my DSLR camera to use in macro work as well as act as a indoor fill light solution. The chip has the capability of using PWM to carry the avg LED current and a Iset pin to set the Io. My thought was to simple use a fixed 5k resistor in series with a 25K vaariable resistor to attain the full current adjustability of the chip from 20mA - 100mA.

    The question I have, and I'm pretty sure that it is the case with most LED, is that the LED lolor will change with the current suplied to it. If this is the case, then PWM would be the optimal method to obtain a consistent color from the LED's.

    Can anyone back up my thought on this?


    http://www.linear.com/pc/productDetail.jsp?navId=H0,C1,C1094,P90321
     
  2. bertus

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    Apr 5, 2008
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  3. jpanhalt

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    Jan 18, 2008
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    Be sure to consider the PWM frequency and exposure times. LED's actually go on and off; incandescents do not. For example, with an exposure of 1/125 and a frequency of 120 Hz, you could get some odd results. Obviously, it is easy to work around, but just be aware of it.

    John
     
  4. iONic

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    Nov 16, 2007
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    @ Bertus,

    Good article that does indeed comfirm my fears about the simple analog current control.
    PWM would be the way to go here.

    @ jpanhalt,

    Yes indeed! The chip offers a range of switching frequencies, the lower of which is in the kHz range. Havent read how PWM interacts with this switching freq, but be asured that the PWM freq. will be well past the range of "oddness." ...maybe!?


    Question 2:
    If I set the PWM duty-cycle to 50%, the Avg. LED current would be 50% of the current set, yes. Thus 75% duty-cycle would result in an avg. current of 75% of the set current?
     
    Last edited: Nov 6, 2010
  5. bertus

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    Hello,

    With PWM you are setting the on / off relation.
    With a duty cycle of 50 % the led will have half the time full power and half the time no power.
    (the effective power is 50 %)
    With a duty cycle of 75 % the led will have 3/4 of the time full power and 1/4 of the time no power.
    (the effective power is 75%)

    You could take a look at post 6 of the thread made by Bill_Marsden for a PWM circuit with an 555 and LM339:
    LEDs, 555s, Flashers, and Light Chasers

    Bertus
     
    Last edited: Nov 6, 2010
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