# How to get the maximum luminance out of a LED?

What is the formula to obtain the resistor value I should use to drive a LED to its maximum luminance for a given input voltage?

And for the maximum power effectiveness (luminance / watt)?

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A resistor value will not give you the maximum luminance for a given amount of power. You need to pulse a constant current driver at a few hundred hertz instead. By using a small duty cycle, you can run the LED at much more than its maximum sustainable power level, which will make it appear brighter on average. I'd recommend editing the title to reflect this, something like "Calculating resistor values for maximum LED luminance". See this Intersil app note for an example of a more complex, efficient, and bright driver. –  Kevin Vermeer Dec 13 '10 at 17:37
@reemrevnivek: Pulsing LEDs doesn't increase the apparent brightness. members.misty.com/don/ledp.html The eye perceives the average brightness over time. –  endolith Dec 13 '10 at 17:47
@reemrevnivek in particular, LEDs have a peak current rating which is typically higher than the maximum DC current rating - however, this limit is often caveated by a maximum duty cycle (often <=30%). So it's a tradeoff and really depends on the particular device parameters. –  vicatcu Dec 13 '10 at 18:49
Also note that pulse width regulation is a lot more power efficient than using a resistor, so it's going to be preferred when trying to get a lot of light from something battery powered like a bicycle safety light. –  Chris Stratton Dec 13 '10 at 19:00

I assume you're talking about the basic resistor voltage regulator. A basic equation for the resistor's value would involve the LED minimum forward voltage (Vf), LED maximum current (If), and power supply maximum voltage (Vs).

R = (Vs - Vf) / If

With supply voltages nearly equal to the LED voltage, it is somewhat hairy as the current may become overly sensitive to the resistor, so I would avoid going below 100 ohms. A very conservative equation would be as above, but set Vf to zero.

The maximum luminous efficiency of the LED alone would obviously depend on the LED itself, so you would have to look at the datasheet. For the maximum efficiency of the system as a whole, if driving more than a few LEDs (or any >100 mA LEDs) it would be best to use a switching regulator with current sensing.

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R = (Vcc - Vf) / If