LED Current Draw?


by tangodirt
Tags: current, draw
tangodirt
tangodirt is offline
#1
Feb25-11, 03:57 PM
P: 47
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?
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mdjensen22
mdjensen22 is offline
#2
Feb25-11, 04:05 PM
P: 161
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
tangodirt is offline
#3
Feb25-11, 04:11 PM
P: 47
Quote Quote by mdjensen22 View Post
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
jstamour802 is offline
#4
Feb27-11, 02:30 PM
P: 4

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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