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

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Kjelt:

--- Quote from: Benta on August 11, 2022, 09:22:40 pm ---
--- Quote from: Kjelt on August 11, 2022, 09:09:32 pm ---The delta in forward Voltage drop is less than 0,1%.

--- End quote ---
Source/documentation?

--- End quote ---
Testreports of reels of leds.
With the 3.xx V types you will have max ten mV's differences on leds from the same reel ( ofcourse when in series with the same current).
If you get different reels , different production dates your mileage may vary.

Kjelt:

--- Quote from: nctnico on August 11, 2022, 09:48:01 pm ---That may be, but my experience is different (also with monitor backlights). When looking at white LED datasheets from Osram, they specify a variation in forward voltage of about 1% within a bin.
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You always keep a safe margin with your specs or you get many returns.
So lets say it is 0,05% in reality that is ten mV's difference in Vfd.
If it is evenly distributed you have no problem. If you are really worried measure each led and put it in different new bins with tighter spec. But as I said even A brands don't bother.


--- Quote --- It is possible that the lamps you have seen, have the PCB traces act as a dropping resistor just in case. LEDs from the same batch may have a much closer match but I doubt a manufacturer is going to bin LEDs within a 0.1% match.

--- End quote ---
That could be but the traces are pretty wide.  I will do a measurement on them again, measuring each led for Vfd and the voltage between the leds (trace resistance).

Kjelt:
I can't find this back in the datasheets. Most manufacturers do Lm and CRI binning.
 Only a few manufacturers do Vfd bins it seems.

nctnico:
With very large batches you can do your own verification and gamble it but if the parts start to vary wider for some reason, you have no claim at all.

A fellow engineer once told me their company needed 1% capacitors for a product. Because the 1% capacitors where expensive, they bought 5% capacitors and tested them theirselves. The out of spec capacitors where used elsewhere so nothing got lost. After some time they started to receive batches of capacitors where all capacitors where outside 1%. Turns out the manufacturer had started checking the capacitors themselves to sell as 1% tolerance.

Closer to home: I have been bitten as well by components that got worse. At some point a batch of a products started to show strange behaviour. Turned out the decoupling capacitors where not as good as they used to be. Same part number and still in spec but simply made worse.

Zero999:
Here's what I was talking about earlier. It requires just under 1V of headroom.

It depends on your requirements. Does it actually matter if the current falls a bit below the set point? You'll probably not notice if it's 300mA, rather than 350mA.

I suggest you measure the voltage of the power supply and the forward voltage of the LED strings at different currents.

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