Chris.Elston Posted September 19, 2006 Report Share Posted September 19, 2006 I guess that people here are probably tired of LED questions, but I'll give these a shot anyway.1) LED forward drops are specified over a smallish range, e.g.: 2.0 - 2.2V. Does this indicate a dynamic range, or is it that any one specimen of that type of LED will have a fixed drop somewhere in that range.2) I want to drive a high brightness red LED @ 350mA from a 5V source, assuming a 2.2V forward drop across the LED. I calculate the current limiting resistor as 8R, power dissipation across it as ~1W. So far, so good (I hope :) ). The nearest preferred value resistor available is 8R2, but not many vendors carry 1W 8R2. So basically, the question is, what are my options beyond ordering the aforementioned resistor? I know that there are some ICs out there that do constant current, but I was wondering if there was anything I could do with good old 0.25W resistors to spread the power dissipation across multiple ones.3) How critical is the current anyway? I know that 350mA is the recommended current, and that they will do 400mA pulsed. I plan on controlling the brightness via PWM switching, so how does, for example a 50% duty cycle affect the current through the LED. My guess would be that you get 50% at full current, then 50% at none - and that the off periods allow the LED to cool. This effectively cooler running means that you can drive it at 400mA in the on phase?Cheers,Chris. Quote Link to comment Share on other sites More sharing options...
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