
#1
Feb2511, 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? 



#2
Feb2511, 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. 



#3
Feb2511, 04:11 PM

P: 47





#4
Feb2711, 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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