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A two volt power supply will not work properly with your two volt LED.
If your 2V power supply is regulated, you could use a super bright LED.
This LED will be very bright at only 3.5mA and will drop about 1.65V. In this case, the series resistor will be 100Ω.
A 2V LED is like a 2V zener (in the forward biased direction). Both will not start conducting unless more voltage is available to push the current through. A resistor would limit the current, but in this case, there's nothing to limit.
Led current = V / R
= (VCC - LED Fwd V) / R
= (2 - 2) / R
= 0 / R
= [color=red]0 mA[/color]
Super Bright LED with 1.65V Fwd voltage and 100Ω series resistor:
Led current = V / R
= (VCC - LED Fwd V) / R
= (2 - 1.65) / 100
= 0.35 / 100
= [color=red]3.5 mA[/color]
resistor should be 0 ohm
This is not a resistor its a wire ( may be .0001 ohms).
Lets say your power supply was 2.1 V
.1 V / 0.0001 Ohms = 1000 amps.
You can only supply 100 ma, and yes this would probably damage the LED.
Your LED may not be 2 volts it might be 1.93 V etc.
gilperon:
Thank you so much! But isnt that strange? I can provide the voltage required by my circuit and also a current above what it's necessary but still I cant make it work with a resistor? How is that?
What part of:-
LarryD:
A two volt power supply will not work properly with your two volt LED.
Are you having trouble understanding?
I don't mind you asking questions, however asking the same questions in different threads is called cross posting and pisses people off here.
But I dont understand this: if I need 2V to run a LED and my source power provides that 2 V source power why my led should not work?
Because you do not need 2V to run the LED the 2V is only the forward voltage drop. The forward voltage of an LED changes from device to device, over the age of any one individual device and the temperature.
It is nominally 2V but changes. Even if it is exactly 2V and your power supply is exactly 2V you still have no means of CONTROLLING the current. Therefore you can not drive an LED with simply the voltage it has as a forward drop.
Sorry but I disagree with dlloyd. See, your formula is wrong: the current the flows in the led should be:
voltage across the led/R. It should not be 2 - 2 but only 2.
Use Ohm's Law to calculate the voltage across the resistor, not the LED.
OK, I've taken your 2V LED, 2V power source, added a 100Ω resistor, then created an analogy for it:
Water/pressure analogy...
Increase pressure to 2.5 psi...
Inserting your components...
Using 2.5V power source...