LED dimming

[jbwiden]

I am making a bike light using 3 Cree XR-E LED's and a Buckpuck. The Buckpuck can be dimmed using PWM or lowering the current with the use of a audio taper pot. Is there any reason to do one method over the other? I am already going to be using a microcontroler so the PWM adds no complexity.

Thanks,
Josh

 

[Martini]

As far as efficiency, etc. goes, it should be about the same. For convenience, I would go with the pot. Actually, I would use a three-way switch and two resistors to get three preset levels (moon mode, normal, burst).

 

[jbwiden]

Maybe I am wrong but it seems to me that the pot would actually be more efficient than the microcontroller. if I used the microcontroller only, then i would set the drive current to one amp so i could get the full range of light. but according to the cree data sheet the led will be less efficient when driven at this current. What I have heard though is that by limiting the current the color of the led will drift(by how much I do not know). does any of this have any truth or am i just misguided?


Josh

 

[evan9162]

PWM dimming is less efficient than constant current dimming. The more you dim from 100% brightness, the larger the gap between PWM and constant current.


PWM dimming will prevent color shifting associated with running at lower drive currents. If you're absolutely picky about color shifting, than PWM dimming is the way to go. However, if a bit of color shifting is acceptable, and you can accomplish constant current dimming by just adding a pot to the buckpuck, then that's the best way to go.

 

[Chriss]

How can i make easily a constant current dimmed output from a PWM dimmed output?is a paralel connected capacitor enough?

 

[VidPro]

the color shift aint doodley worth worrying about (to the human eyes), compared to the efficiency gains when running the led at lower powers.