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Electronics => Beginners => Topic started by: dark_hawk on September 08, 2013, 09:05:02 pm

Title: Wattage of current limiting Resistor running on AC
Post by: dark_hawk on September 08, 2013, 09:05:02 pm

Hi,

Got a 12.6-13.3 AC power source. Have put two LEDs anti-parallel to each other like this figure:

(http://sub.allaboutcircuits.com/images/03298.png)

Voltage drop for leds is 3.3V and Current is 20ma. Used a 270ohm 1/2W resistor.
Question is, Does calculating wattage of the resistor differ from DC to AC or is it the same?

Thank you.

Title: Re: Wattage of current limiting Resistor running on AC
Post by: mikeselectricstuff on September 08, 2013, 09:15:28 pm

In principle if you use the RMS value of the AC, it will be equivalent to DC, but the peak current will be higher (probably won't be an issue).
However the Vf of the LEDs will make the current slightly non-sinusoidal, so the RMS current will be slightly different to what you may expect, but not enough to worry about.