This is my first post here. I've been lurking for a while now, searching and reading whatever I can get my hands on that's useful. My intention is to build a headlamp for caving use and I have a few technical requirements:
- Needs to have the ability to drive LED's at both high and quite low levels while remaining as efficient as possible.
- Needs to have separate very wide and more medium beam, preferably independently controlled. This will require a minimum of 2 LED's obviously.
- Single, li-ion battery source of at least 4000mAh and 7V
I have more ideas in mind but that is the general idea.
My main concern at the moment has to do with efficiency of drivers while dimming to low current levels. I was looking at a number of cost effective and well regarded drivers on DealExtreme and Kai until I found out that at least some of them have efficiencies as low as <50% when driving at their low settings. This is simply not acceptable for a light that will probably mostly be powered at low levels. And so I come to you, the knowledgable. Humble and waiting to be enlightened by your extensive knowledge.
From what I have gathered PWM dimming has very poor efficiency at low levels due to the fact that it is driving the LED at it's full rated current, at a certain frequency. Is this the sole cause of the abysmal low level efficiency of the cheaper drivers I'm looking at or is there more to it? Is it simply impossible to dim an LED to low levels (with a driver) and still remain reasonably efficient? From what I've read CC or constant current/current control dimming is much better at low levels. (Newbie's test results have confirmed this in the past)
What drivers out there use this dimming method and why is it so poorly documented whether certain dimming drivers are PWM or otherwise? I noticed that TaskLED only publishes efficiency data for their drivers while in full current mode, not while dimming. This seems to be a big lack of info if you're at all interested in it.
Anyway, thanks for reading and hopefully someone has some good suggestions!