USB to power LED - voltage drop, resisters, oh my

I've repurposed a stereo console with all new electronics inside. It has a small light on the front that indicated when the old stereo was on. Using a USB cable I wired an LED to one end while the other plugs into a USB power outlet on the back of my new stereo. When the stereo is on it powers the USB port, turning on the LED light. I didn't take voltage into account, and while it's working now I believe it will eventually burn out because I did not put in a resistor.

I'm trying to figure this out but I'm having trouble. The USB power out is 5v. I want to use a blue LED (Parameters: DC 3V - 3.2V Volt 20 mA). So I've come up with either a 68ohm or 51ohm resistor. Can anyone confirm which is correct, or if neither?

The light is in the center panel at the bottom.

 

based on 3.2 forward volts my calculation for 1 LED is 90 - 100 ohms. Well, not "my" calculation.

https://ledcalculator.net/#p=5&v=3.2&c=20&n=1&o=w
https://www.petervis.com/electronics/led/led-resistor-calculator.html

If I bump the forward voltage to 3.8 I get 61 ohms. The thing to remember is with a lot of electronics close enough is good enough. If you put in the bigger resistor (100 ohm) the only downside is the LED will be dimmer. If you put in the smaller resistor (51 ohm) you might get a marginally shorter life. But, LED's last for close to forever, so shorter could still be a very long life. I'd experiment with different resistors to get the amount of illumination you like.

 

I'm not concerned with a slightly shorter life, just don't want to burn one out every couple weeks. LEDs are cheap and it's in a spot that's easy to change. I think I'll go with the 51ohm as brighter is what I need. Thanks!

 

I ended up with the 51ohm resistor. I did a quick, messy solder job just to see how it works. It's much brighter than I anticipated! I looked into the light and I'm still seeing spots 5 minutes later. I went overboard and got super bright, clear lens LEDs. In the middle of the day it puts a blue spotlight on the wall 8 feet away. I sanded the lens to soften the light a bit, but it's still very bright. I may order some opaque lens LEDs or bigger resistors.

I also discovered my soldering skills suck so much. I've only done it a few times over the years. I really need to watch a video and practice.

Thanks for the recommendation on the resistor!

 

LOL - I feel your pain. You can solder another resistor in line with the first and have the equivalent of a 102 ohm resistor. I do it all the time when I don't have the exact value I'd like.

power > resistor > resistor > as many resistors as you want > LED