Question about current balancing in LED strings

[EasyGoing1]

I'm sure most of you are familiar with Paul Daniels ... who is someone I consider to be an expert in what he does. Not to mention he's an incredibly wonderful person and is always willing to help people find knowledge.

He used this simple design to balance the current in some LED strings



and I have a couple of questions about it if anyone can answer:

1) What would determine the values of the emitter resistors?

2) The fact that the collector of one transistor is connected to the base of all of them ... why would that not cause the transistors to saturate to their fullest possible potential which would then cause current to flow at maximum possible values through the transistor?

My first thought was that the collector served as a means of choking the current because of how it is connected to the base, but then I realized as voltage increases at the collector then so does the voltage at the base which would then open the transistor up more there-by causing an effect where it would reach max values at the base and render the transistor fully open all the time.

Am I incorrect in that thinking?

[stj]

Re: Question about current balancing in LED strings

i dont understand that circuit.
and the exact wiring makes it dependent on the left string not losing any leds!

[CapLeaker]

Re: Question about current balancing in LED strings

Use a proper led driver instead! Has even less parts (no transistor, no resistor, just the led driver) than this circuit and is current controlled.

[EasyGoing1]

Re: Question about current balancing in LED strings

i dont understand that circuit.
and the exact wiring makes it dependent on the left string not losing any leds!

Good point ... though I suppose all things being equal ... the odds of that happening are slim ... and you can just replace them easily enough. Not to mention that if the whole array goes down, you know the culprit is in the first leg.