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Everything Else » Missing Something about Resistors and LEDs

January 21, 2010
by ecornwell
ecornwell's Avatar

Hello Everyone...

I'm missing something about resistors and LEDs and I can't seem to wrap my head around it. So here goes...

If I have a LED in a circuit, I should have a current limiting resistor in series with it. I understand why... but what I don't understand is how everything works in the circuit once you do that. I found the following site http://www.seattlerobotics.org/encoder/mar97/basics.html and at the bottom it has some info about LEDs.

"Most LED's seem to handle at least 15mA. If you are using a 5 volt circuit, then Ohms law tells you what resistor value to use. R = V / I, so R = 5v / .015A = 333 ohms."

So that would allow the 15ma of current to flow through the LED but how is the voltage affected? Is it just that we assume there is no voltage drop across the LED? What happens if you have multiple LED's in series? How many can you have in series?

I hope this doesn't sound silly because I feel like I'm missing something small that will just make this work!

Thanks!

January 22, 2010
by Rick_S
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Hello ecornwell,

There have been several questions on this along the way. Here is a link to an excellent writeup by BobaMosfet regarding this exact subject. Scroll down about 1/2 way and you will see all the formulas related to calculating the resistor for any given LED. You are correct to use Ohm's law, however, the voltage drop across the LED DOES come into full play in the calculation.

Here's the Link to the thread.

Rick

January 22, 2010
by ecornwell
ecornwell's Avatar

Hi Rick!

Thanks for the response, that's exactly what I was looking for!

Eric

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