At 04:33 PM 08/02/2010, you wrote: >On Mon, Feb 8, 2010 at 4:16 PM, Chris McSweeny wrote: > > for a given mean current any LED will appear brighter if that > current is constant rather than PWM. > >So it sounds like all those people who are pulsing their LEDs for >increased brightness at a given power input are doing so for no >reason? > > > You simply can't make an LED brighter than by driving it with > constant current at its maximum rated current. > >Interesting! I've heard differently, but of course cannot easily find >out where, so I may have simply misunderstood. Early LEDs had a kind of threshold (maybe a good part of 1mA) below which no light came out. Pulsing them gave significantly higher visual brightness, for low average current. Modern LEDs are visible at uA in a dark room. There is typically some nonlinearity to apparent brightness vs. current, but it's not very significant. Might be 20-30%, which is barely discernable. Some LEDs have curves that are convex, some concave, depending on the type. But heating goes up as I^2*R so you don't gain hardly anything since your maximum *average* current must decrease to keep the LED chip temperature the same if you want to do a apples-apples comparison. >I suppose I'm going to have to do some experiments and find out what >my own visual perception is. Try comparing two closely-matched LEDs (when connected in series they look to be the same brightness at a reasonable current). One with DC current, the other with pulsed current. Adjust the DC current until they look to be the same brightness. Put a switch in there to swap the LEDs to be sure. Measure the average power in each case and compare. The exception is that if the pulsing is slower than the rate at which the pulses visually fuse together in your eye, then you will get higher apparent brightness at lower average power with the pulsed LED. But then you see flashing rather than an apparently continuously lit LED. >Too bad Russell is in China, I bet he'd have a field day on this topic...! Probably. ;-) >http://www.piclist.com PIC/SX FAQ & list archive >View/change your membership options at >http://mailman.mit.edu/mailman/listinfo/piclist Spehro Pefhany --"it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist