General > General Technical Chat
LED Lighting (350 mA). Is there any standard for # of LEDs is a string?
Kjelt:
--- Quote from: Benta on August 08, 2022, 08:02:18 pm --- Is there even some kind of standard?
--- End quote ---
AFAIK only difference is between LV (upto I believe 50VDC but could be a bit higher) and HV. HV you need more isolation larger distance between traces etc.
And there is a difference in driver being mains isolated or not. Latter needs extra safety.
--- Quote from: nctnico on August 10, 2022, 12:34:06 pm ---LEDs strings in parallel don't work well. You'll see differences in brightness. I've seen current mismatches up to 25%. For the best results, you need a current source per string (even if it is a simple dropping resistor).
--- End quote ---
I disagree, I have seen many ledlight pcbs from A brands and they use 3 to 4 parallel strings even without resistor. I can take pictures for you, from pro lighting fixtures for parking garages costing €150 a piece to kitchen lighting. The trick is to use A brand leds from the same bin. For our hobby use buy a reel and use those they are also from the same bin.
The delta in forward Voltage drop is less than 0,1%. But when one led dies or is damaged then you have to replace the pcb in total ofcourse (average 25000 hours).
Benta:
--- 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?
nctnico:
--- Quote from: Kjelt on August 11, 2022, 09:09:32 pm ---
--- Quote from: nctnico on August 10, 2022, 12:34:06 pm ---LEDs strings in parallel don't work well. You'll see differences in brightness. I've seen current mismatches up to 25%. For the best results, you need a current source per string (even if it is a simple dropping resistor).
--- End quote ---
I disagree, I have seen many ledlight pcbs from A brands and they use 3 to 4 parallel strings even without resistor. I can take pictures for you, from pro lighting fixtures for parking garages costing €150 a piece to kitchen lighting.
--- End quote ---
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. 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.
Benta:
Thanks to a lot of helpful inputs and feedback here, I think I've now arrived at my final circuit. Some adjustments/tuning were indicated.
For a 19 V input, I've settled on a 5-LED string. This leads to a power dissipation in the current regulator circuit of around 1.5...2 W, which is acceptable to me.
LED current is set at 340...350 mA (pretty standard for power LEDs).
Neither voltage nor temperature stability are an issue, I attach plots of the LED current for anyone interested.
Cost is around 60 cents for the the controller plus 7.50 Euro for the LEDs. This is not a cheap China product, but uses OSRAM quality LEDs.
Feel free to copy/modify the design as you like. (KiCAD design file (.zip) is also attached).
Cheers.
mariush:
I don't know where you get that 1.5 euro per led from ... I can find Osram Oslon 150 degree leds at under $0.5 in quantity (600+)
You have the CM series with CRI 90+ , the EM one with CRI 80+ and the PM with CRI 70+
The better CM ones are 1$ each for 10 on Digikey, 0.7$ for 100 : https://www.digikey.com/en/products/detail/ams-osram-usa-inc/GW-CSHPM1-CM-KSKU-XX58-1-350-R18/6599016
Same price for the EM ones (CRI 80+) : https://www.digikey.com/en/products/detail/ams-osram-usa-inc/GW-CSHPM1-EM-LPLR-XX58-1-350-R18/6599021
Sure add VAT and shipping but we're still at much less than 1.5 euro per led.
Anyway, unless those osram are better binned or your need those lenses on the leds ... these Samsung leds seem to be just as good for much lower price, at 15 cents if you buy 100 : https://www.digikey.com/en/products/detail/samsung-semiconductor-inc/SCS9WT93HPL2WLS03F/6569400
Still CRI 90, but it's 5.9v -ish forward voltage, 150mA current ...
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version