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Electronics => Electronics => Topic started by: BANE on January 14, 2009, 06:51:34 PM

Title: simple resistor calculations
Post by: BANE 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

Title: Re: simple resistor calculations
Post by: HDL_CinC_Dragon on January 14, 2009, 06:57:24 PM

13 ohms

Title: Re: simple resistor calculations
Post by: krich 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

Title: Re: simple resistor calculations
Post by: BANE on January 14, 2009, 07:12:38 PM

oh, i knew it was something simple :P.  Thanks guys

bane