I have a project that is powered by a 12v power supply. I need as bright of an LED as possible and decided to use this 9v LED. According to the datasheet, its max current is 1A, which is what I would like to get it to. In order to do this, I am using this 3 Ohms resistor
According to my calculations, (12v-9v)/1A = 3 Ohms. This should get my LED to the correct current.
To test the current, I placed a multimeter in series with the 12v power supply, LED, and resistor. However, I am only reading 0.639A. I tried several power supplies and meters and it's all the same.
What am I doing wrong? Is my test setup providing too much resistance, thus decreasing the current?
Best Answer
According to the chart on page 17 of the datasheet, the terminal voltage of a nominal-9V LED rises to between 10.25 and 10.5 V (depending on temperature) at 1 A. You need to size your resistor accordingly.
But it would be far better to use an active current regulator to feed this kind of LED. Then, the current wouldn't depend on temperature. Or on your actual source voltage, for that matter.