# LED - Wattage Calculation

Discussion in 'The Projects Forum' started by krishnan_hari83, Feb 13, 2012.

1. ### krishnan_hari83 Thread Starter New Member

Feb 11, 2012
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I am a new bee to electronics field.. Kindly let me know how to calculate wattage of an LED from the specification sheet.

I am attaching the specification of an LED. Kindly advice me.

Best Regards,
Krishnan.H

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2. ### DickCappels Moderator

Aug 21, 2008
2,653
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The simple answer is that its the current through the LED times the voltage drop across the LED. For the case of maximum average current, that would be 30 milliamps at nominally 3.6 volts = 108 milliwatts. The actual voltage depends on the forward voltage rank of your particular diode, found by matching the part number for your diode with the codes in the table of electro-optical characteristics table on the second page of the data sheet.

The data sheet is missing the chart showing how forward voltage varies with temperature, so just use the numbers given in the table on the second page and assume the maximum room temperature current of 30 milliamps.

From the derating curve on the third page of the data sheet, when operating at 30 milliamps, the temperature at the LED mounting point, which is the solder terminals, cannot exceed 50 degrees. If you are going to operate at a maximum air temperature of 40 degrees, you will need a heatsink with a thermal resistance with less than (50°-40°)/0.108 watts = 93 degrees per watt.

If you have multiple LEDs per heatsink, the thermal resistance of the heatsink to ambient must be proportionally lower. (10 LEDs need 9.3 degrees per watt, etc.).

It is prudent to run the LEDs at a slightly lower current than the specified maximum.