# EEVblog Electronics Community Forum

## Electronics => Beginners => Topic started by: EEBlackSheep on February 01, 2012, 05:37:11 am

Title: Calculating resistor for LED
Post by: EEBlackSheep on February 01, 2012, 05:37:11 am
OK, one of the simple ohms law calculations is to calculate the resistor size for an LED.  I ran across this web site describing how to do it:  http://www.seattlerobotics.org/guide/electronics.html (http://www.seattlerobotics.org/guide/electronics.html)

He has a 6VDC power source, with a 330R resistor and a red LED all in series.  He calculates the theoretical current by ignoring the LED and calculating the value for the resistor alone using the total voltage of the circuit.   I have seen this before and it make no sense to me.  Once the circuit is on, there is a voltage drop across the LED and therefore there will *not* be a full 6V across the resistor.  So the current in the circuit will be less than the his calculation.

So by my calculation, the red LED will have a 1.9V voltage drop, so 6V-1.9V=4.1V is across the 330R resistor, giving 12.4mA  I hooked up a circuit and the experiment agrees.

So... am I correct in that his method for calculating the current limiting resistor is flawed?

Title: Re: Calculating resistor for LED
Post by: amspire on February 01, 2012, 05:40:50 am
Both your reasoning and your calculations are perfect.
Title: Re: Calculating resistor for LED
Post by: IanB on February 01, 2012, 06:25:33 am
He is certainly wrong in that article when he says, "The current then flows through the LED, which doesn't have resistance"  :o

However, the LED will appear quite bright with 12 mA, hardly less than with 18 mA. So if the LED is just for indication it is quite OK in this case to simplify the calculation and ignore the voltage drop over the LED. The LED will last a bit longer and the battery consumption will be less. Sometimes "good enough" is good enough. The trick is knowing when...  :)
Title: Re: Calculating resistor for LED
Post by: Psi on February 01, 2012, 06:31:32 am
Yeah, you will see people ignoring the LED drop sometimes.
It only becomes an issue at low voltages.

I guess it's an easier way to explain it to someone with no electronic background.
Title: Re: Calculating resistor for LED
Post by: vk6zgo on February 01, 2012, 06:33:37 am
He is certainly wrong in that article when he says, "The current then flows through the LED, which doesn't have resistance"  :o

However, the LED will appear quite bright with 12 mA, hardly less than with 18 mA. So if the LED is just for indication it is quite OK in this case to simplify the calculation and ignore the voltage drop over the LED. The LED will last a bit longer and the battery consumption will be less. Sometimes "good enough" is good enough. The trick is knowing when...  :)

Exactly what I was going to say!
I must admit I usually ignore the LED voltage drop as well.
Because the human eye is not linear,there is very little difference in brightness.
It gets a bit messier with High Intensity LEDs,though.
VK6ZGO
Title: Re: Calculating resistor for LED
Post by: Jon Chandler on February 01, 2012, 11:43:19 am
Here are a  couple articles I wrote for Digital-DIY after reading a couple horrendous explanations of how to do this.

LED Calculations (http://www.clever4hire.com/special-articles/home/led-calculations)

LED Calculations - The Lab Section (http://www.clever4hire.com/special-articles/home/led-calculations---the-lab-section)