Electronics > Beginners

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

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rdl:
Keep in mind he is re-using LEDS from some 1980s vintage equipment. 20 mA is not out of line for LEDs of that era.

iMo:
Ok, LEDs from 70ties and 80ties are 10-20mA, sure.

james_s:
LEDs from the 70s-80s is exactly what this thread is about.

There are reasons to run modern LEDs at 10-20mA, but not if you're using them as indicators. I can't count the number of times I've removed LEDs or changed resistors on various boards because they came with blinding bright SMT LEDs on them.

tooki:

--- Quote from: imo on January 29, 2020, 11:46:15 pm ---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..

--- End quote ---
I think it would behoove you to actually read the original post before calling BS... If you had, you'd know that this entire thread exists because the OP needs to know how to determine the Vf of the original, vintage LEDs.  :palm:

tooki:

--- Quote from: Fungus on January 29, 2020, 09:34:10 pm ---
--- 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)

--- End quote ---
It baffles me that most THT indicator LEDs now come in water-clear packages, instead of a diffused package.

I've been experimenting with all sorts of things to make good indicator LEDs. Currently, the best bet has been to start with a  2x3x4mm LED, paint it all over with either white or metallic silver paint (to reduce leakage out the sides, as well as give a more uniform appearance from the top) and then sand the top matte. White heat shrink works, too, but I need to find a thinner tube that fits more snugly before shrinking, and it's more finicky to get flush. I've been considering trying this clear spray paint I saw in a shop that is specifically for making glass/acrylic look frosted.

(If this sounds reminiscent of the 80s-style rectangular LEDs that had a white plastic frame and diffused epoxy inside, that's basically the effect I'm going for. Those LEDs aren't available at all any more, never mind in modern colors...)

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