Sarge,
I understand what you wanted me to do now.

At 20ma, I measure 3.18 V across one of my "dim" LEDs. I am measuring 3.16 across one of the "bright" LEDs.

It's not just one single LED; it's the sum of the Vf's of all the LEDs in a string.
One or more of the LEDs in the bright string has a very low VF, and one or more of the LEDs in the dim string has a very high Vf.

And here is something curious.

With one of the "bright" LEDs disconnected, the whole panel lights starts to light around 8.7 VDC.

I'd bet I could do better if I disconnected those other "bright" LEDs. I would be willing to do that or replace them if it meant light the whole panel at a lower voltage.

The low Vf and high Vf LEDs are "problem children". What you can often do is put one low Vf and one high Vf in a string, and the two will even each other out.

For example, if you have three LEDs that have exactly 3.2v across each in series @ 20mA, then the total voltage drop is 9.6v.

However, if one Vf is quite low, like 2.9v, the total drop across the string will be 9.3v at 20mA.

If one Vf is quite high, like around 3.5v, the total Vf will be significantly higher.

But if you match up one that has a low Vf (2.9v) with the high Vf LED (3.5v), you can still get 2.9+3.5+3.2 = 9.6v.

 

In a local store I bought cheap LEDs made in India. Half did not light. The other half had different brightnesses and forward voltages. They are terrible.

I was given bags of American Lumileds as thanks for helping with a project. They are exactly the same. They are perfect.
I bought a bag of American Fairchild LEDs. They are also exactly the same. They are also perfect.

 

It's not just one single LED; it's the sum of the Vf's of all the LEDs in a string.

Do you want me to do further testing?


One thought I had was just to remove the problem children entirely.


Once I have a proof of concept with existing light I intend to replace the LEDs.

Will I find brighter and equally or more efficient LEDs than what I have?

 

Will I find brighter and equally or more efficient LEDs than what I have?

Cheap Chinese LEDs bought at E-bay are very poor quality.
American LEDs and European LEDs are very high quality and are worth their slightly more expensive cost.

 

LED technology keeps advancing. There are brighter LEDs with lower Vf's available now than their were in years past.

You can take the problem children out if you want. However, if you test them, you will find that the sum of their Vf's in one is different than the sum of the Vf's in the other.

If you were building a large LED array for higher voltage (let's say, 36v) and had 10 white LEDs in each string with current limiting resistors, the random distribution of Vf's would very likely equal out, as you would have a large enough statistical sample of the group of LEDs.

However, when you only have three LEDs in series, that is not much of a statistical sample.

AudioGuru,
LumiLEDs are made by Phillips, whom are based in the Netherlands.
Fairchild semiconductors are generally made in Asia.

 

How is the size measured? Is this the diameter of the base? I.E. I wold like something that I can replace in my existing board.


What is a decent mcd rating? Or better, could someone hazard to guess at what the mcd is of these older LEDs?


I sure wish I could get the specs on these things, it would make this a lot easier.

 

LumiLEDs are made by Phillips, whom are based in the Netherlands.
Fairchild semiconductors are generally made in Asia.

Philips of Holland (Netherlands?) bought Lumileds who are in America.
Both are made in Asia. Everything (?) is made in Asia now.