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led driver circuit

terrenone

I need some help with a circuit I need to drive 10 white leds from a 3.7v 2200mAh lipo battery it sounds simple but it has to be able to fit in a small container aproximity 1inch square and not produce a lot of heat,  its for my son's night im making.


Any help is welcome

Thanks

Dave

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  • 0

    The problem in driving LEDs is that they don't have the linear I/V that resistors do:  Let's imagine a one-volt potential driving a one Ohm resistor or one Siemens conductor.  Current is an ampere and power dissipation is one watt.  Double the potential, Current is two amperes and power dissipation is four watts.  If we double the potential across a diode, current increases by about a factor of ten.  This rules out voltage control and necessitates current control.  The chopper mentioned by a prior poster is the way to go because it is the alternative to simply putting a ballast resistor in series, which often draws more power than the LED.  Indicator LEDs are fine with just the ballast resistor, otherwise, 'get to the choppah.'

     

    MK is correct (there I said it!) in observing that the canonical op-amp/sense resistor/transistor CCS has stability (and outright slowness) issues  It has been found that a unity-gain buffer in the feedback path makes life much better for this circuit.  Using it in this application makes the power Q get hot rather than the ballast resistor, for what that is worth.  Chopping with a current integrator makes for a situation in which most of the power actually goes into the LED.

  • 0 in reply to D_Hersey

    Thanks Don, I always learn from your explanations. As soon as I started testing I could see that the LED wasn't acting like a resistive load. I will look into the Chopping current regulator. This wasn't my thread but I seem to have hijacked it.

    John

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