Why is a 220 Ω resistor for this LED suggested when Ohm’s law seems to say much less is required


I have been interested in this particular switch with a built-in LED.

According to the website, the forward voltage for the LED inside the switch is "about 3 V". It then goes on to recommend a resistor of ateast 220 Ω resistor as do some other sources. I plan on powering the LED through the Pi Pico VBUS pin which, if I understand correctly, is roughly 5 V. Also, after some research I came to the conclusion that the blue LED is most likely in the 25 mA current draw ballpark. So with that info in mind…

5 V – 3 V = 2 V

2 V / 0.025 A = 80 Ω

Now, although the current draw was a guess on my part, I figured that even if it is slightly different than the number I gave it wouldn't be enough to change what I am trying to convey. I don't quite understand the disparity between what articles say and what Ohm's law seems to say.

Ohm's law, at least on paper, seems to indicate a resistor in the ballpark quite a bit lower than the 220 Ω I see on Adafruit as well as other articles. Is it just because 220 Ω is much more standard and easier to find? If so, will the decreased current resulting from the 220 Ω resistor cause enough of a difference in brightness in the LED for it to matter, or can I just use 220 Ω and be done with it?

Best Answer

LEDs are not very fussy about the current you apply, but will be dimmer with lower currents.

The 25 mA you mention is around the Absolute Maximum rating for common LEDs. If you exceed the Absolute Maximum rating for a part, the part may be damaged.

I find most common LEDs are bright enough for indicator use with 10 mA or less. I once had to reduce the current for a green LED to under 1 mA to get it dim enough for my application!

If you have a few resistors on hand, try various values in series with different LEDs to see what effects they have on the brightness.

If you are just using the LEDs as indicators, you will probably find quite low currents are adequate.

If you are using the LEDs for general lighting, you will want to operate the LED near the Absolute Maximum rating to get most brightness.