### Author Topic: simple resistor calculations  (Read 1788 times)

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#### BANE

• Supreme Robot
• Posts: 639
• E=roboticsC^2
##### simple resistor calculations
« on: January 14, 2009, 06:51:34 PM »
Hello, I was trying to do some resistance calculations on some simple circuity, but keep getting unrealistic figures.
If there was a resistor and a standard LED hooked to a battery 5V, the LED was rated for 3v at 150mA; how many oms would the resistor need to have?

I know they have a lot of LED calculators, but I'm looking for the equation.

Bane

#### HDL_CinC_Dragon

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##### Re: simple resistor calculations
« Reply #1 on: January 14, 2009, 06:57:24 PM »
13 ohms
« Last Edit: January 14, 2009, 07:05:44 PM by HDL_CinC_Dragon »
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#### krich

• Robot Overlord
• Posts: 165
##### Re: simple resistor calculations
« Reply #2 on: January 14, 2009, 07:03:20 PM »
It's right there in the LED tutorial:

Equation:

(voltage applied - forward voltage drop) / forward current = resistor value

Example with 6V power source, LED with 1.85V forward voltage and 20mA current:

(6V - 1.85V) / .02A = 207.5 ohms

#### BANE

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• Posts: 639