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| Match LED Brightness |
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| Kasper:
To add, op said they are sourcing from JLCPCB smt library so I am guessing the 2 leds are not on opposite sides of the efficiency spectrum. If op chooses the most expensive of one color and the cheapest of the other color that probably won't help. |
| Kasper:
I edited my earlier post to add: efficiency and viewing angle may be so different that these quick solutions do not help. |
| tooki:
As Siwastaja said, they still make the old-style low efficiency LEDs, which was my point (I dug out an actual old one simply because it was a sure way to ensure it wasn’t an efficient chemistry). But no, your first method isn’t any help in practice. As Siwastaja also said, LED efficiency spans multiple orders of magnitude, even among modern LEDs. Choosing a similar drive current tells you nothing about whether two different LEDs will have similar light output. |
| Zero999:
--- Quote from: StillTrying on November 04, 2019, 01:47:38 am --- --- Quote from: codingwithethanol on November 04, 2019, 01:21:51 am ---@StillTrying I'm trying to spec out blue and yellow LEDs, I have some THT ones but SMD is a mystery. --- End quote --- It wouldn't surprise me if the yellow needs 50%+ more current than the blue to look similar brightness. --- End quote --- Yes, that's certainly true of old yellow LEDs. A much better alternative is to use phosphor converted LEDs which are just deep blue/violet/UV LEDs with a yellow phosphor. They have a higher forward drop, but are much more efficient than the crappy old LEDs and have a much broader spectrum, so are often specified in chromaticity, rather than dominant wavelength. https://static.rapidonline.com/pdf/55-9182.pdf |
| Kasper:
--- Quote from: tooki on November 04, 2019, 09:59:14 pm ---As Siwastaja said, they still make the old-style low efficiency LEDs, which was my point (I dug out an actual old one simply because it was a sure way to ensure it wasn’t an efficient chemistry). But no, your first method isn’t any help in practice. As Siwastaja also said, LED efficiency spans multiple orders of magnitude, even among modern LEDs. Choosing a similar drive current tells you nothing about whether two different LEDs will have similar light output. --- End quote --- --- Quote from: Psi on November 04, 2019, 10:20:40 am ---Best thing to do is look at the mcd rating and only compare leds that have the same viewing angle. That will match them up pretty well. --- End quote --- I looked up 7 different smt LEDs on JLCPCB parts list. Just used ones rated at 20mA. Did not even find 1 order of magnitude difference in efficiency. And found they all had similar viewing angles. MPN: Yellow: 19-213/Y2C- CQ2R2L/3T(CY), 17-21SUYC/TR8, FC-DA1608HYK-588J, E6C0805UYAC1UDA Blue: X L - 0 8 0 5 Q B C, E6C1206QBAC1UDA, MHT170UBCT Yellow: Intensity: Min, Typ, Max (MCD): 70, 125, 220 View angle: Min, Max: 120°, 130° Typ Vf: 2V Blue: Intensity: Min, Typ, Max (MCD): 50, 111, 225 View angle: Min, Max: 120°, 140° Typ Vf: 3V Comparing yellows to blues, there was much less difference in intensities and viewing angles than there was in Vf. Resistor voltage with 3.3V supply: Vr_yellow = 3.3V - 2V = 1.3V Vr_blue = 3.3V - 3V = 0.3V Example currents with 3.3V supply and 65 ohm resistor: If_yellow = 20mA If_blue = 5mA Typical luminous intensity at 20mA: Yellow: 125 MCD Typical luminous intensity at 5mA (based on linear approximation of current vs luminous intensity): Blue: 28 MCD Results: Difference in typical MCD based on using same value resistor for both LEDs: 125 MCD / 28 MCD 4.5 times difference Typical difference in MCD between yellow and blue: 125 MCD / 111 MCD 1.1 times difference Worst case difference in MCD (max yellow / min blue): 220 MCD vs 50 MCD 4.4 times different Conclusion: In this case difference in Vf is as impactful as difference in efficiency. If you go on digi-key and look up the extremes then you can find some big differences in efficiency but that comes with different packages and different costs. Odds are the purchaser is going to choose identical packages and similar costs and it seems like that will yield similar efficiencies. At least similar enough that there is room for current matching to cause an improvement in intensity matching. |
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