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# Watt of LED

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How do I measure how many watt LED diode is on. Have some different that I do not know the value of.

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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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Is there a device that can be put on + wire  of a transformer, and I manually adjust up or down to find the correct voltage?

Example
If I have a 15 volt - 5 A adapter - sets a variable device on the + wire  - and the more I turn up from 0 the more power  to the LED

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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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That's right! Watt is Current  x voltage => [w] = [a] x [V]

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