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-   -   LED Current Draw? (http://www.physicsforums.com/showthread.php?t=476156)

 tangodirt Feb25-11 03:57 PM

LED Current Draw?

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?

 mdjensen22 Feb25-11 04:05 PM

Re: LED Current Draw?

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.

 tangodirt Feb25-11 04:11 PM

Re: LED Current Draw?

Quote:
 Quote by mdjensen22 (Post 3158013) 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.
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!

 jstamour802 Feb27-11 02:30 PM

Re: LED Current Draw?

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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