On Tuesday, September 25, 2018 at 5:54:11 AM UTC-7, Ned Simmons wrote:
On Mon, 24 Sep 2018 19:30:37 -0700 (PDT), whit3rd
wrote:

On Monday, September 24, 2018 at 6:12:23 PM UTC-7, Ned Simmons wrote:

...you can't just connect parallel strings of LED's to a
constant current source and expect the current in the individual
strings will be equal.

Yet, it's done all the time. A 'nine-LED' flashlight with three AAA cells
has nine strings of one LED each, in parallel.

Desk lamps with three or four LED strings in parallel are common, also.
Sometimes resistors are added to each string, but not always.

So how do you insure that the current is divided equally among the
strings?

I prefer not to do it that way, but for a manufacturer, batch-matching of the
LEDs is the first step. Designing the lamp so that all stay at similar temperatures
is the second. Maybe, too, the LEDs are deliberatelyl degenerated somewhat
(aren't ideal diodes). In similar situations, I've seen fuses used, which both
add series resistance and which prevent a single shorted (failed) unit from
causing a total failure.