Electronics > Beginners

How can I determine the resistor value for an unknown type of LED?

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Mr.B:
I once had a similar application where I wanted to use the original LEDs in the original case of an old wyse terminal.
Rather than powering the original LEDs, I glued a surface mount LED onto the back of each of the original packages.
The light emitted from a modern surface mount LED was more than sufficient to make it appear that the original LED was working.

tooki:

--- Quote from: rdl on January 29, 2020, 06:39:16 pm ---I bought an 525nm green LED last year planning to use it as a power indicator for a PC. At 20 mA it will cause temporary blindness. At 5 mA it's almost too bright to look at directly. At 300 uA (yes, microamps) it's still much brighter than the average 1980s era green LED.

--- End quote ---
Absolutely! I have modern green LEDs like that and I also run them at half a milliamp or less, when used as an indicator. (Another thing i do is to sand down the front of the LED, since the frosted surface diffuses it a fair bit.)

Fungus:

--- Quote from: tooki on January 29, 2020, 08:58:54 pm ---Another thing i do is to sand down the front of the LED, since the frosted surface diffuses it a fair bit.

--- End quote ---

I thought I was the only one who did that.

(making super bright, diffuse LEDs by hand)

Brumby:
In reading through these posts, I admit I overlooked one point... not finding out the measurements of your test circuit.

I think we would all like to see the actual figures you got from taking the measurements I suggested.  That will tell us a lot - and we will be able to make better recommendations!

LED rule #1: The lower the current, the longer the life you can expect.

iMo:
20mA advice is a BS.
All modern LEDs work fine at <1mA, any color (as said above).
The far east vendors recognized that a couple years back and started to deploy 1k resistors on newer boards.
I did a measurement with about 20 different LEDs I got at local store and most of them lit fine at 3k6 (3.3V) and an orange or white one even at 10k..

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