Older white LEDs (ca. 2005-2010?) had a distinct purple tinge while modern ones have a spectrum much closer to a blackbody. I rather like the look of the older style and would like to purchase some now. Does anyone know what keywords to use to find the older style in a parametric search?
Electronic – Name for old-style (purple-tinged) white LEDs
identificationledterminology
Related Solutions
I once had the same problem and could not figure it out until I measured the voltage on the other end of the strip: It was dropping whole 3V accross the entire length! You could even see the difference in brightness when comparing the first and last LED.
Consider the resistance of the 5 meter trace of copper. These strips are usually produced on cheap flexible PCB with the default (35um) copper layers and have very high resistance.
It is clear that these LED strips will, at least in one piece, never meet the spec printed on them.
You could do a sort of modified high-side PWM controlled joule thief for each of the LEDs, with each having an independent PWM controlling the power though the LED.
Simple version might look something like this...
simulate this circuit – Schematic created using CircuitLab
This is different from a normal joule thief in that...
The chopping is done via PWM rather than feedback. This lets you control the power though the LED.
The LED is connected to the high side rather than ground. This means you will need more kick in the inductor to get the LED up to minimum voltage, but has the advantage that there is no leakage current though the LED when the PWM is off.
Note that there is not a straight relationship between PWM and brightness here. When PWM is off, LED is off and no current flows. As you increase PWM duty cycle, you will hit a minimum current needed to light the LED. Then as you increase more, you will get brighter and brighter LED until you hit a maximum. Past the maximum the LED will start getting dimmer and you will start killing the battery, so stop increasing the PWM duty cycle when you get to the maximum brightness.
This circuit should have very low quiescent current- basically whatever the leakage current of the transistor is.
Best Answer
These are called "Full Conversion White LEDs" though by the early two thousands they were partial conversion so you want to look for "Partial Conversion White LEDs". They worked by producing a UV light that excited a phosphorus layer that produced the white light. Some of the purple light from the UV would seep through and cause a purple tint. Also try looking up "Phosphor Film Conversion LED".
Here is an article that talks about them and their history: https://hackaday.com/2018/10/29/history-of-white-leds/
Here is a more technical article discussing it: https://www.digikey.com/en/articles/phosphor-film-conversion-for-white-leds
To be more specific the full name would be "Phosphor conversion indium gallium nitride white LED" sometimes called InGaN, the phosphor used is called yttrium aluminum garnet or YAG. Here is a picture of it:
You can buy them here (notice the blue hue in the picture):
https://www.adafruit.com/product/754?gclid=CjwKCAjwkdL6BRAREiwA-kiczD67ahIX1q9TtjzMmoOsKO2we7xG-Lax9O1Nh0yBdo2tt5wLa0cOSRoCelgQAvD_BwE
Note: The "Cool White" modern LEDs mentioned in the other answer do not have a color spectrum that looks anything like first-gen partial conversion LEDs. Cool white will still come across as white, though towards the blue part of the spectrum. But the InGaN with YAG phosphor will actually look as though it has a distinct bluish color with a strong blue peak in its color spectrum that exceeds that of modern Cool White LED.