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

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Benta:
The title says it best.

My idea is reusing laptop PSUs (normally 19 V, 90 W = 4.7 A) for lighting. The landfills are full of them, and I think it's a recycling project worth considering.
But to keep it simple, 19 V would only supply 4-LED strings.
Is this a normal length? Is there even some kind of standard?

Thanks.

tom66:
Not as far as I'm aware.  Most LED power supplies are constant current, with a maximum output voltage under SELV limits (~100VDC is as high as I've seen them.)

If you want to drive LEDs with a laptop supply, you can, but be aware that it will require a power resistor to balance the current between each string.  That resistor might well drop 5V @ 350mA = 1.75W.  So not insignificant power to dissipate.

Benta:
Thank for your reply.
No, not resistor-drop, but active regulation and splitting. I'm fully aware of how to drive LEDs.

T3sl4co1l:
Good LEDs are down around 3.0V; should be able to run at least five, preferably six at exactly 19V.  Even at 3.6V, five should be easy.  If they're over some distance, one less would be acceptable of course.  Four may be about ideal if using a CC buck converter to drive them.

Yeah, not aware of a standard either, but on the other hand, laptop supplies are surprisingly well agreed on something around 19V.  Sounds like you've got your standard! ;D

Tim

Zero999:

--- Quote from: T3sl4co1l on August 09, 2022, 01:57:59 am ---Good LEDs are down around 3.0V; should be able to run at least five, preferably six at exactly 19V.  Even at 3.6V, five should be easy.  If they're over some distance, one less would be acceptable of course.  Four may be about ideal if using a CC buck converter to drive them.

Yeah, not aware of a standard either, but on the other hand, laptop supplies are surprisingly well agreed on something around 19V.  Sounds like you've got your standard! ;D

Tim

--- End quote ---
I've previously run six old LEDs off a 19V constant voltage power supply, but they had forward voltage of more than 3V at the rated current, so were been under-run.

Six modern LEDs, run at 80% of the rating, with the standard op-amp+reference+transistor+current sense resistor, is feasible off 19V.

You don't need anything fancy. The crappy old LM358, a generic NPN transistor, such as the BC337 and a reference, such as the TL431, with a potential divider to make the sense voltage in the hundreds of mV.

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