Knowing the forward voltage voltage and current, you can now easily calculate the resistor required. Then looking at the other graph, 15mA responds to 1.8V as opposed to 30mA which is around 1.9V. You can see that 100% intensity is at 30mA, and 50% at 15mA. Plus another graph showing the forward voltage veruss forward current.Īn example is this datasheet: at the bottom of page 2. Most LED's will have a datasheet showing you two important graphs: the forward current vs luminous intensity. Remember the relationship will not be that much linear, and only a good LED will give you all the parameters to calculate half the brightness, but in practice, assuming half the brightness for half the maximum current is usually a good approximation.Īlso get in mind that the human eye doesn't respond to all colors in the same way, some colors will appear brighter than others for the same lumens, that's why there are several color correction curves and calibrations for light related equipments. Get the maximum current the LED can handle and with it you get the maximum lumen, than you can calculate easily the resistor, assuming half this maximum current, using V = R*I. In any case, the datasheet should show you the relationship between lumens and current and you can get some estimative about it: Good LEDs have in their datasheet a drop like graph that shows the spread of light, so the brightness also depends on the angle that you look at it. You will never be able to get 50%.the properties of resistors and other semiconductors might change with temperature and every resistor has a tolerance of how much not exact is it's resistance.
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