LED Current Draw?

  1. Simple question for you guys. I am trying to drive an LED with a 5V power source and a resistor in series.

    I understand how to calculate the minimum resistance needed to drive the LED with maximum current ((voltage supply - forward voltage) / maximum current) but I can't for the life of me figure out how to calculate the current through an LED with a given resistor.

    Say, we have our 5V source --> 330 ohm resistor --> LED with forward voltage of 3.15V --> ground. What's the current flowing through the circuit?
     
  2. jcsd
  3. By applying Kirchoff's Voltage Law, you know the voltage across the resistance (supply voltage - forward voltage).

    With this, you can apply Ohm's Law to calculate the current through the resistor (you know the voltage across it and the resistance). Since it is a series circuit, the current through the resistor = the current through the LED.
     
  4. Ugh, of course. Too simple. I knew I had enough information to back that number out, I suppose it has just been a long day and my brain already went into weekend mode. Thanks!
     
  5. You'll want to check the LED datasheet for typical operating voltage @ specified current.

    Assuming a standard 5mm LED: White @ 20mA

    (Vin - Vf) / R = I

    Vin = 5 V
    Vf = 3.2V (found from LED datasheet @ 20mA)
    I = 0.02A

    5V - 3.2 / R = 0.02A

    R = 90 Ohm

    Dont burn up your resistor! Calculate power in R:
    P = I^2 * R
    P = (0.02 * .02) * 90
    P = .036W
     
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