Resistor Question

Discussion in 'General Electronics Chat' started by BrandonNC, Jun 12, 2012.

  1. BrandonNC

    Thread Starter New Member

    Jun 10, 2012
    7
    0
    Hello all,

    Regarding resistor wattage; the LED calculators out there recommend that for a source voltage of 5V, a LED forward voltage of 2.6V, and a forward current of 28mA that I use a 1/8 W resistor. I am curious how they get this number? I thought it would be 0.14W, so rounding up to a 1/4 watt resistor?:

    5V * 28mA = 0.14 W

    Or am I suppose to use the voltage drop instead of the source voltage?:

    (5V - 2.6V) * 28mA = 2.4 * 28mA = 0.06 W?

    Thanks!
     
  2. tracecom

    AAC Fanatic!

    Apr 16, 2010
    3,871
    1,394
    The total power consumed is 140mW, but the resistor is only dissipating 67.2 mW. The rest is consumed by the LED.
     
  3. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    When calculating the power of a resistor you have to use the voltage drop across the resistor itself. Your second calculation is correct for the resistor power.

    Your first calculation show the power supplied by the supply for this portion of the circuit, which is the power dissapated by the resistor as well as the power dissapated by the led.
     
  4. BrandonNC

    Thread Starter New Member

    Jun 10, 2012
    7
    0
    Understood, thank you both!
     
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