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Old 03-29-2009, 10:57 AM
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ZnsaneRyder ZnsaneRyder is offline
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Join Date: Nov 2008
Location: FLORIDA
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Default Re: 12 volt fog lamp as headlight

Originally Posted by Prasinos View Post
Thanks a lot for the help.

If the leds are not voltage sensitive that would mean they have negligible resistance? The way i thought of it if the led is designed for 20 ma 3.6 volts that would mean that it has a resistance of 180 ohms. do i take that into account or ignore it when choosing the correct resistance?
Just take the voltage and mA into account. The LED actually does not have very much internal resistance. You can't calculate LED resistance like that because it's non-linear.

LED's are strange. It's because they are semiconductors, they don't have a linear resistance, that's why you have to add a regular resistor in series with the LED to run from a voltage source. For example, white LED conducts almost no electricity under 3V, but conducts well at 3.3V and over.

Because the LED is usually rated at 3.3V, it may use less then 5mA at 3V, but use over 80mA at 4V, because the LED takes a certain voltage to turn it on, but once over that turn-on voltage, the LED shows a very low resistance. The current rise curve is very rapid once the LED starts to conduct electricity above its turn-on voltage, so you need a resistor to limit the current properly.

Also because of that turn-on voltage, the voltage of an LED is pretty regulated, meaning that regardless of the input voltage you use with a resistor, the LED will always have about 3.3-3.6V across it. By changing the value of the resistor in series with the LED, you change the current, and as a result, the brightness of the LED.
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