LED Lighting (350 mA). Is there any standard for # of LEDs is a string?

[Zero999]

You have many LEDs in series, so the variation in forward voltage drops will cancel each other out. You won't have six 2.75V LEDs, or 3.2V LEDs, but an even distribution of forward voltages, giving a total forward voltage of about 3V.

I have an engineering rule that I always stick to: never do a basic design based on probabilities, typical values or wishful thinking.
You can always downgrade afterwards based on experience.

And thinking that a reel of parts exhibit a nice Gaussian distribution is wishful thinking. You never know if a really big customer has asked the manufacturer to select parts with a certain. V_F. This happens much more often than you think (don't ask me how I know, I've been in the semiconductor industry for 40+ years). The rest is sold on the open market, but the Gaussian distribution is gone, although all parts confirm to specs.

That's exactly why I keep saying do some actual measurements, before finalising the design.

[Benta]

And why I'm saying do worst-case/best-case or minimum/maximum spec analysis first. Measurements will not cure the "wishful thinking" part. Calculations will.

[nctnico]

I don't mind running at ratings, personally; as long as they're good parts, that's what really matters.  That, and thermal management.

Thermal management is easy to underestimate. Even 60mW LEDs need like 4x4mm pads in order to have sufficient heatsinking in real life applications.

Very true, but in this case quite easy: total P_D is 19 x 0.35 A = 6.65 W. Not insurmountable. The idea is to mount the lights/LEDs/regulator on an aluminium baseplate anyway. Shape TBD.

To be sure: I'm writing about 60 milli-Watt = 0.06W!

[Benta]

To be sure: I'm writing about 60 milli-Watt = 0.06W!

I understand fully what you're writing.
But I've designed 150 W Class-AB amps and 20 W Class-A amps. Thermal management of 6.7 W is easy.
You keep on harping on this. Why? A standard 0.3 W resistor won't even get a little warm at 0.06 W.

[Zero999]

And why I'm saying do worst-case/best-case or minimum/maximum spec analysis first. Measurements will not cure the "wishful thinking" part. Calculations will.

Calculations putting the worst case into everything will often result in over-engineering, which might be acceptable in safety critical applications, but otherwise counterproductive.

[nctnico]

To be sure: I'm writing about 60 milli-Watt = 0.06W!

I understand fully what you're writing.
But I've designed 150 W Class-AB amps and 20 W Class-A amps. Thermal management of 6.7 W is easy.
You keep on harping on this. Why? A standard 0.3 W resistor won't even get a little warm at 0.06 W.

Because some LEDs will fail without proper heatsinking even at power levels that look like you don't need to care about at all. You can't compare a resistor with a ceramic substrate with a LED that is mostly plastic. All in all: it is not 'harping on', it is sharing practical experience. But my remark about thermal design was aimed at T3sl4co1l. I got the impression that you missed the m for milli.

[Benta]

Calculations putting the worst case into everything will often result in over-engineering, which might be acceptable in safety critical applications, but otherwise counterproductive.

Somehow, this is evolving a bit into a Pub discussion, which I like... but we should rather meet there
! won't rub your nose in British car manufacturing that used your approach vs. "German over engineering". It's history.

You take a Pint, I'll take a Mass.

Cheers.

[Zero999]

Calculations putting the worst case into everything will often result in over-engineering, which might be acceptable in safety critical applications, but otherwise counterproductive.

Somehow, this is evolving a bit into a Pub discussion, which I like... but we should rather meet there
! won't rub your nose into British car manufacturing that used your approach vs. "German over engineering". It's history.

You take a Pint, I'll take a Mass.

Cheers.

German over-engineering didn't win the war. 

How many of these are you going to make? Your original post says you're using second hand power supplies. I doubt it needs to meet stringent quality assurance. It's definitely feasible to just measure the forward voltages.

[Kjelt]

! won't rub your nose in British car manufacturing that used your approach vs. "German over engineering". It's history.

Uhum the most reliable cars remain the japanese cars, some german cars being very unreliable. Just google it 

[T3sl4co1l]

You keep on harping on this. Why? A standard 0.3 W resistor won't even get a little warm at 0.06 W.

Because some LEDs will fail without proper heatsinking even at power levels that look like you don't need to care about at all. You can't compare a resistor with a ceramic substrate with a LED that is mostly plastic. All in all: it is not 'harping on', it is sharing practical experience. But my remark about thermal design was aimed at T3sl4co1l. I got the impression that you missed the m for milli.

Indeed, a recent project I had to use 0402 white and green LEDs for indication, and had no room for even much trace width to keep them cool.  They ended up multiplexed at fairly modest intensity though (partly for matching intensity with the much dimmer red LEDs, partly didn't need full intensity anyway) so it's probably fine.  4-layer board as well, but that really isn't going to help as much as you'd want when the component width is barely more than the laminate thickness (well, something like that anyways; would be neat to get some measurements to see by how much!).

Tim