Hi Beaky, I read with interest your posts on electronics stuff and find them interesting and informative, but to be frank, and I mean absolutely no offence and cheerfully add that you should feel free to ignore me, I reckon your led techniques and understanding are far from current and good practise.
The Vf of a led is a function of the junction material, they are not a linear device as you are presenting them. Most have a Vf of about 2.3 volts, the technique you've described works in limited cases with small numbers of leds (in each group) between the supply voltages and where the sum of the led Vfs (in each group) roughly matches the supply output voltage.
(This is also the reason the Cointaker and Ablaze leds can be used in parallel wired strings - as everyone who buys them uses them - they have an inbuilt resistor that acts as a current limiter, or a primitive current regulator if you like, in each led. This is also the poor feature that I alluded to in my earlier post)
Good practice drives leds in series and regulates current to vary brightness. A pretty fair summary can be found here
http://www.ledsmagazine.com/features/4/8/1A string of leds like the GI in a pin with about 40 to 50 driven all in parallel (with no current limit resister as described above) will suffer variations in brightness amongst individual leds due to variations in junction temperatures and the thermal runaway that will result. This would be made worse if you tried to use different coloured ones. I strongly suspect driving it this way will lead to failures of those that operate higher up on the Vf v If curve. This will lead to rapid increase in junction temp, lower junction resistance, higher current flow and ultimately failure.
One technique people use is to place several strings of series leds in parallel to even out these differences - this is how I would go about it if I were trying to control strings of GI leds. Take a feed of about 25V, loose a few volts headroom for the regulator and then arrange, say, 4 or 5 parallel string each with ten series leds in them.
Anyway. I'm happy for us to disagree and we can each continue to drive our leds in ways that we like, each of us secure in the knowledge that the other is wrong! Good luck to you and your leds.
