How much current and voltage has my LED + resistor?

Other Hardware General Electronics
1

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2

Voltage across the resistor is going to be supply voltage - LED forward voltage drop
So in your case 5 - 2 = 3V

This will be the case for all values of resistor.

What the resistor does is determine the current. So with 3V across the resistor and a 1K resistor the current will be:-
3 / 1000 = 3mA

If you want a current of 20mA then in order to have 20mA across a resistor with 3V across it you need the resistor to be:-
3 / 0.02 = 150 Ohms

The current through the resistor is the same as the current through the LED.

3

f I use a higher resistor will always my LED use 2 V? Always?

YES -I did say in that answer:-

So in your case 5 - 2 = 3V
This will be the case for all values of resistor.

Even if my source voltage is also changed?

The resistor will always have the source voltage minus the LED's forward voltage across it.

4

What I said only applies if your power supply is greater than the forward voltage of the LED. If it is the same are very close you are in a region where things are very difficult to calculate because of the non linear nature of the LED.

Electronics is like an onion, there are many layers. Most of the time you only need the top few layers but you can get into situations where you need to dig deeper. I was giving you the rule of thumb because if you pile on too much detail you can confuse a beginner and they end up learning nothing.

Most of physics is like that, we still teach Newtonian motion and it can land a probe on a comet, but it is only an approximation to the laws of motion. It only works under certain conditions. The same is true of resistor / LED calculations.

Basically if your voltage is close to the LED then you can not use it reliably, and calculation what happens needs a detailed knowledge of the temperature and characteristics of that individual LED not just the part number. Also if you calculate a resistor less than about 20 ohms, then you should not use a resistor but a constant current driver.

Have a read of this:-
http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

5

I read another expert user in another forum and he said that the forward voltage of an LED will always change with the current.

Yes it will but only slightly and not in accordance with ohms law. For what you are doing you can ignore this effect. I told you about onions.