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Watt of LED


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You need the part number and detailed datasheet from the manufacturer of the LED to determine its continuous and pulsed absolute maximum allowed currents. The datasheet tells you the range of forward voltage or you can measure it. Then the power (Watts) is the current times the voltage.

Or you can try one at higher and higher currents until it fails or smokes then you can guess that the other LEDs are the same or better.


Most 5mm diameter LEDs have their voltage and brightness rated at 20mA. Their maximum continuous current might be 30mA or 40mA.


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An LED sets its own voltage. A red one is about 1.8V to about 2.0V. A blue or white one is about 3.2V to about 3.6V and they are all different unless they were all measured and grouped.


The brightness of an LED is controlled by adjusting its current, not its voltage. To avoid burning out an LED it must have a series current-limiting resistor. The series resistor and LED can be fed a variable voltage that adjusts the current which controls the brightness.


You said you have a transformer. A transformer has an AC output which might burn out an LED that needs a DC current. +15V at 5A will heat an LED with 15V x 5A= 75W which will instantly fry an LED designed for 2V x 20mA= 0.04W.

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