unmatched LEDs

[no-fi]

I needed a bunch of LEDs for my 6582 build, and decided that white LEDs would look nice. I had a bit of a look around and decided that clear bright white LEDs would work, as long as I played about and picked a decent resistor value to limit the current to a sensible amount.

so, I found someone selling them on ebay from china, and ordered 200 of them (and then 4 weeks later, they arrive..... d'oh!) Last night I finally had a chance to sit down and work out my resistor value to run these, but in the process I discovered they're really not consistent at all.

I used my 6582's 5V supply, via the GND/5V pad pair on the panel connector line, I soldered a 1K and a 2K resistor to the 2 pads to make a pair of contacts with a set resistance I could quickly test each LED with...

Then, I noticed that about only 75% of the LEDs are a decent brightness. The rest are really really dull, right down to completely dead. And even the ones that I flagged as basically OK seem to have a bit of variance across them... I think they seem OK enough looking at them one after the other, but I do notice variation between then, and think that next to each other in the matrix I am going to be very not happy with them! and with 25% being no good, I'm also a little worried about what will happen in the future - will the good ones fail too? So... I guess I need to get a whole new set of LEDs now.

Anyway - is this a normal thing to expect with white LEDs? (I never used them before) or did I just waste 4 weeks of my life waiting for garbage to be shipped to me? I've never seen anything like this with any other LEDs I've ever used.

I think I'm looking for an excuse to dispute the ebay transaction at the moment, and just move on and find new LEDs. but any reassurances from people who bought a similar batch of white LEDs from ebay with a big variance in testing and the good 75% ended up working great and looking fine in their midibox would be appreciated too... :-)

[philetaylor]

Personally I think white (or blue) led's are a bad idea for this sort of project however to answer your question, these LEDs (in bulk from China) are unlikely to be subject to particularly stringent testing or they may even have failed quality control testing hence the variance in brightness.

As white LED's are effectively blue ones with chemicals added I think that Wilba's comments on blue LEDs apply:

Phil

[no-fi]

yeah quality control rejects come to mind for the explanation of so many bad ones. I've never had LEDs with such variance before, and these shouldn't be any different just because they are white..... and I don't trust the rest of them to have a long and useful lifespan.

I might email the seller and see what I can arrange in a refund.

From what I understand, white LEDs shouldn't have most of the problems associated with blue(which I agree are bad, hate the look of anyway, and would never use) because the "chemicals added" are there to absorb the direct LED light and then transmit a wide band of other light wavelengths (which is how they look white!) that your eyes should be able to deal with.

I guess a bit of UV is not good, but coupling this with large value resistors to keep brightness down (no 220R voltage drop resistors here!) I'm not sure this is such a no-go, unless someone can give me numbers on huge levels of UV wavelength light coming from white LEDs.

Edited December 27, 2009 by no-fi

[no-fi]

well I did a bit of reading on wikipedia... seems like white LEDs do have a fair spike of blue in them still, but they look way more spread to me than plain blue, and even more "natural" than a combination of Red, Green, and Blue LEDs.

but yeah - variable brightness has killed the white LEDs plan for me. I suspect that maybe they'd be a bit more consistent if I was driving them at their rated spec (and therefore blinding myself) because with lower resistor value testing, they all seem a lot more consistent... (or my eyes are saturating......) but part of what I was wanting to do with hi power LEDs was to get usable light out of them with a reduction in current drain on my poor old C64 PSU. Oh well.

I really wish all this info here was IN THE WIKI before I'd spend all this time waiting for my white LEDs. I mean.. reading that linked forum post, it sounds like blue LEDs (and white LEDS) are basically the work of satan - but if you read the CS construction guide and the parts list it even talks about how you **should buy** high brightness LEDS, and talks about high value resistors to manage the use of blue ones in particular... This is very frustrating. Oh well.

Updated the WIKI to warn future generations of builders.

Edited December 28, 2009 by no-fi