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Formula for figuring series resistor values for LEDs

Don't ask me what prompted me to post this. Guess it's because the question often comes up here, what value of resistor do I need to run my LEDs off my 12-volt power supply?

So here's the generalized formula which should work for every situation. Just plug the numbers in and turn the crank.

Assuming you have a DC power supply, a single LED with a single resistor in series with it:

Vt - Vled R = ----------- Iled

where

Vt = total voltage (power supply voltage) Vled = operating voltage of LED Iled = operating current of LED (in amperes)

For example, with a 12-volt power supply and a 2-volt, 20mA LED, you'd get

R = (12 - 2) / 0.02 = 500 ohms.

Reply to
David Nebenzahl
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The Model Railroad System includes a program, Resistor, that will do this computation (you can select the supply voltage, the LED voltage, and the LED current). It will also let you know what wattage resistor you need and will also find the nearest stocked value and display the color bands for such a resistor.

Reply to
Robert Heller

On 8/28/2008 11:13 AM Robert Heller spake thus:

wrote:

That's nice.

The little formula above requires only paper and pencil (or a calculator to those who prefer automation), and will work in a power outage without a UPS.

Reply to
David Nebenzahl

How are you going to light the LED when the power is out?

---john.

Reply to
John Haskey

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These might help...

Resistors:

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LEDs:

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Bill Bill's Railroad Empire N Scale Model Railroad:

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History of N Scale:
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's Store--Books, Trains, and Toys:
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to 1,200 sites:
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Reply to
Bill

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me president, and I

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This calculator even shows you the colors on the resistor. Very neat.

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Reply to
video guy - www.locoworks.com

snipped-for-privacy@panix.com (John Haskey) wrote in news:g96v0f$frv$ snipped-for-privacy@reader1.panix.com:

Well, I don't know about you, but my truck could easily provide enough power to keep my Digitrax system happy for MONTHs. If the system's got power, then my White LED headlights can light. :-)

The mains are out, and the residents of my model railroad are happy because they've still got electricity. Imagine that. lol

Puckdropper

Reply to
Puckdropper

:

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If one loses power during a thunderstorm, they could always send up kites to get the power they need for their model railroad.

Bill

Reply to
Bill

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