On Mon, Feb 8, 2010 at 3:27 PM, M. Adam Davis wrote: > For instance, if an LED is rated for 10mA you may be able to drive it > with 2A pulses at a 5 or 10% duty cycle, and it will appear brighter > than simply running it steady state at 10mA. Not for very long you won't. See http://www.gardasoft.com/pdf/APP930%20Overdriving%20LEDs.pdf - the recommendation is overdriving at no more than 10 times for 5% duty cycle, or 5 times for 10% duty cycle. They don't give a figure for the required duty cycle for 200 times overdriving (my suspicion is that it would last very little time if you even switch on only for long enough for it to ramp up to full power with 0.1% duty!) Note that none of the figures there allow you to drive the LED with higher mean current than it is rated for, they are all significantly less, in order to keep the LED temperature down. It has to be significantly lower than the steady state mean since the junction will heat up whilst it is being driven, so the peak temperature will be higher than the mean. As discussed before, for a given mean current any LED will appear brighter if that current is constant rather than PWM. You simply can't make an LED brighter than by driving it with constant current at its maximum rated current. > Whether it's energy efficient or not is irrelevant. =A0The question for > this particular press release is how do they measure the lumens, and > under what drive conditions does the LED emit that power. As I understand it, the standard lab testing method is using a very short pulse at the rated current and measuring the output using an integrating sphere. Note that it's a single very short pulse, the point of which is to keep the junction temperature at the ambient measured temperature, not a series of short PWM pulses. Obviously not particularly useful in the real world, but that's how they measure. Out in the real world hobbyists seem to use a very large heatsink to keep the temperature down when testing, but in that case you still have the temperature gradient between the junction and the heatsink pad - strangely no figures for that LED cluster on the datasheet, though the single die LED I've used most (which actually has an industry leading thermal resistance!) you get a ~30K difference between the two when driving at full power. Chris -- = http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist