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I have a source power with 2 V and 100mA. I have an LED that has a forward voltage of 2 V and a max current of 20mA.

Should I use a resistor to reduce the current? I see many formulas in the internet saying the the resistor value should be:

(Source Voltage - LED forwared voltage)/LED max current

This formula always worked fine for me, but now that I have a 2 V source power this formula tells me that the resistor should be 0 ohm. Of course this would damage my LED cause the 100mA provided by my source power is way above the 20mA current limit of the lED.

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No, it would not damage your led. The forward voltage of a led is, the voltage at which the led 'conducts' the maximum current it can handle. Since diodes have a (exponential) curve, the current though a diode depends on the voltage across it.

So if you give the led 2V, it will use only 20mA. However if the voltage goes to e.g. 2.1V the current might be 25mA (since the curve is exponential), which is a bit too high, and will reduce the leds lifespan.

So for safely I'd add some resistors. E.g. a 10Ohm resistor will limit the current to 20mA max, as long as the voltage doesn't go over 2.2V

PS note that led Christmas lights use the same method to limit current, without a single resistor. I have here a 24v string of leds, grouped into 10 leds connected in series. So the voltage across each led is 2.4V. They are white leds, so they probably have a forward voltage of around 3.2v. But since they only get 2.4v, they will probably use only 5mA or less.

  • Thank you so much but if I add a 10 ohm resistor it will consume voltage, isnt? If it consume voltage the LED will not have it's forward voltage required in the specs ov 2V cause some of the voltage will be lost to the resistor. Will it work anyway? – Samul Nov 22 '14 at 20:11
  • The led will be a slightly less bright. – Gerben Nov 23 '14 at 13:07
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You most likely will not be able to use that power supply with that LED properly. The resistance graph of an diode is quite peculiar.

As you can see, there isn't a "hard" forward voltage, but rather a smooth curve that changes depending on what current is passing through the diode. You can try to use a value of, say 1.95V for the LED, but then you won't pass the same amount of current that you used in your calculations.

Extended reading: Load line (Wikipedia)

  • Thank you so much I read the wikipedia and understood well the non linear with the linear part in a circuit and the exponential behaviour of the LED with current/voltage. But if I use a 100 ohm resistor, there will be a voltage drop across the resistor right? So the LED will have not the 2 V to operate, but will it work? Can I calculate the voltage across the LED with the 100 ohm resistor in series? – Samul Nov 22 '14 at 20:16
  • The LED will still drop some voltage. Use load line analysis to figure out how much. – Ignacio Vazquez-Abrams Nov 22 '14 at 20:49

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