Why does my white LED voltage drop change with potentiometer resistance?

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asa!!
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hi i have fixed a simple circuit for lighting up a led.i have included a potentiometer. as the led is white i know that the voltage drop of the led is around 4v.when the potentiometer is at min resistance the voltege drop is actuallu around 3.8v.the problem is that when i turn the potentiometer at maximum resistance the drop is around 2.5v.I thought that the voltage drop is steady for each led and does not fluctuate.could anyone explain me why that happens?thank you
 

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asa! said:
hi i have fixed a simple circuit for lighting up a led.i have included a potentiometer. as the led is white i know that the voltage drop of the led is around 4v.when the potentiometer is at min resistance the voltege drop is actuallu around 3.8v.the problem is that when i turn the potentiometer at maximum resistance the drop is around 2.5v.I thought that the voltage drop is steady for each led and does not fluctuate.could anyone explain me why that happens?thank you
Please post a link to the datasheet for your LED. If you can't figure out the Vf specs, we can help you do that.