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I am conecting a green LED to 5V and I need to calculate resisitor value. Those are parameters from the datasheet:

  • Vf=3.6-4V
  • If=20mA.

According to Ohm's law: R=(Vi-Vf)/If, so when I change values I have R=1/0.02=50 ohms.

But it seems to me that this is too small. I read in several sites that the resistor shoul be 1k or at least 330 ohm. So I don't know which resistance is right.

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  • \$\begingroup\$ The maximum Vf for this LED is 20 mA. That doesn't mean you have to operate it at 20 mA. \$\endgroup\$ – The Photon Jul 4 at 16:57
  • \$\begingroup\$ Also 3.6V to 4V is exceptionally high for a green LED. It might be one of those new technologies. Typically, a green LED would have approximately 2V over it. So that might explain why you typically have seen larger resistors before for a green LED, but your LED might need lower. \$\endgroup\$ – Justme Jul 4 at 17:10
  • \$\begingroup\$ @ThePhoton: That should be the max If, not Vf. The Vf the OP mentioned sounds very high for a green LED - he probably took values from the Absolute Maximum Rating table, rather than the more sensible operating values. \$\endgroup\$ – Peter Bennett Jul 4 at 17:11
  • \$\begingroup\$ LEDs are not at all fussy about the operating current, as long as you don't exceed the Absolute Maximum rating. I've had some green LEDs that I had to run at under 1 mA to get the brightness down to an acceptable level for my use. \$\endgroup\$ – Peter Bennett Jul 4 at 17:13
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    \$\begingroup\$ @cerouno Please provide a link to the actual data sheet. \$\endgroup\$ – Russell Borogove Jul 4 at 18:17
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There is no one "right" resistor value to limit current for a LED.

Using the Absolute Maximum current and minimum specified forward voltage in your formula will give you the minimum resistance that will not result in excessive current in the LED. You may (and probably should) use a higher resistance to ajdust the LED brightness to a suitable value - modern LEDs are quite bright at much less than the Absolute Maximum current.

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