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LED Lighting (350 mA). Is there any standard for # of LEDs is a string?
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T3sl4co1l:
I don't mind running at ratings, personally; as long as they're good parts, that's what really matters.  That, and thermal management.  Which, it sounds like they're the pricey kind, so they ought to perform well (high lm/W).

Last pricey ones I got were, let's see... Cree XTEAW-00-0000-00000HGE3.  Far smaller than the old gull-wing-cylinder style they replaced, but their efficiency is so much greater, they still run cooler.  It's... a little insane.  And quite wonderful.

So, if it's already designed to run an acceptable temperature with older chips, or, like, resistors (i.e., assuming 100% applied power goes to heat -- in reality it's more like 50% now!), they'll run quite cool.  If it's new design, and you're accounting for dissipated power only, or don't have much opportunity to dissipate heat in the first place (compact, enclosed?), yeah, derating may be desirable.

Tim
Zero999:

--- Quote from: T3sl4co1l on August 13, 2022, 03:56:54 pm ---I don't mind running at ratings, personally; as long as they're good parts, that's what really matters.  That, and thermal management.  Which, it sounds like they're the pricey kind, so they ought to perform well (high lm/W).

Last pricey ones I got were, let's see... Cree XTEAW-00-0000-00000HGE3.  Far smaller than the old gull-wing-cylinder style they replaced, but their efficiency is so much greater, they still run cooler.  It's... a little insane.  And quite wonderful.

So, if it's already designed to run an acceptable temperature with older chips, or, like, resistors (i.e., assuming 100% applied power goes to heat -- in reality it's more like 50% now!), they'll run quite cool.  If it's new design, and you're accounting for dissipated power only, or don't have much opportunity to dissipate heat in the first place (compact, enclosed?), yeah, derating may be desirable.

Tim

--- End quote ---
I'm just going from my experience of commercial LED bulbs, which generally run the LEDs at their ratings and invariably don't last as long as they should. Then there's the Dubai lamp which is more efficient and lasts for much longer too.

If budget isn't an issue, the LT3080 can be used as a current sink with a low drop-out voltage.
nctnico:

--- Quote from: T3sl4co1l on August 13, 2022, 03:56:54 pm ---I don't mind running at ratings, personally; as long as they're good parts, that's what really matters.  That, and thermal management.

--- End quote ---
Thermal management is easy to underestimate. Even 60mW LEDs need like 4x4mm pads in order to have sufficient heatsinking in real life applications. Otherwise they will (not can, but will!) fail. A LED is a semiconductor so their operational temperature is rather low compared to passives like a resistor. It is quite similar to power transistors. Once you've got them mounted onto a heatsink and apply derating for elevated environment temperatures, you often realise you can only dissipate 25% of what the transistor is rated for.
Benta:

--- Quote from: Zero999 on August 13, 2022, 09:31:13 am ---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.

--- End quote ---
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 exhibits 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 VF. 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.
Benta:

--- Quote from: nctnico on August 13, 2022, 06:30:59 pm ---
--- Quote from: T3sl4co1l on August 13, 2022, 03:56:54 pm ---I don't mind running at ratings, personally; as long as they're good parts, that's what really matters.  That, and thermal management.

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

--- End quote ---
Very true, but in this case quite easy: total PD 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.
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