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| Match LED Brightness |
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| exe:
I had the same problem and I used my eyes and individual resistors for three different leds (red, green, and orange). BTW leds vary wildly when it comes to efficiency. I bought more than 10 different kinds to understand what I need. At the end I used leds with highest efficiency because why wasting power. |
| tooki:
--- Quote from: Psi on November 04, 2019, 10:20:40 am --- --- Quote from: Kasper on November 04, 2019, 01:16:41 am ---Short version: Pick resistors for each LED that cause them to have the same current. For example if you want 5mA in each LED: R = (Vsupply - Vf) / I R = (Vsupply - Vf) / 0.005A If they have similar current they should have somewhat similar brightness but you could go further... --- End quote --- Doesn't really work. At the same current, or even the same wattage there can be a huge difference in brightness. The efficiency of LEDs varies a lot by color and by the chemistry used. Even for one color, like yellow, there are a few different chemistry that can be used and they can have wildly different efficiency. 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 --- Exactly. When I was writing my prior reply, I dug out some old green LEDs from around 1990 and a modern green LED. At 20mA, the old green LED is far dimmer than the modern LED at 0.5mA. The modern LED at 20mA is blindingly bright. |
| Kasper:
--- Quote from: Kasper on November 04, 2019, 01:16:41 am ---Short version: Pick resistors for each LED that cause them to have the same current. For example if you want 5mA in each LED: R = (Vsupply - Vf) / I R = (Vsupply - Vf) / 0.005A If they have similar current they should have somewhat similar brightness but you could go further... Longer version: Look up the amount of light output per current input and pick a current for each LED that will yield similar outputs. Estimate Vf of each LED based on current. This should be listed in datasheet maybe in a graph. Use a different resistor for each LED: R = (Vsupply - Vf) / I --- End quote --- --- Quote from: tooki on November 04, 2019, 11:23:12 am --- --- Quote from: Psi on November 04, 2019, 10:20:40 am --- --- Quote from: Kasper on November 04, 2019, 01:16:41 am ---Short version: Pick resistors for each LED that cause them to have the same current. For example if you want 5mA in each LED: R = (Vsupply - Vf) / I R = (Vsupply - Vf) / 0.005A If they have similar current they should have somewhat similar brightness but you could go further... --- End quote --- Doesn't really work. At the same current, or even the same wattage there can be a huge difference in brightness. The efficiency of LEDs varies a lot by color and by the chemistry used. Even for one color, like yellow, there are a few different chemistry that can be used and they can have wildly different efficiency. 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 --- Exactly. When I was writing my prior reply, I dug out some old green LEDs from around 1990 and a modern green LED. At 20mA, the old green LED is far dimmer than the modern LED at 0.5mA. The modern LED at 20mA is blindingly bright. --- End quote --- Quoting people doesn't really work if you don't read their whole post or the original post. My post was clearly broken into a quick solution for someone who seems to want a quick solution and then a better solution with an explanation that the quick solution is not great. You quote the 1st half of my post, say it doesn't really work (as if I didn't just say that) and then you go on to repeat the 2nd half of my quote which you for some reason left out. Your whole post could have been better written in 1 sentence: 'viewing angle should also be matched'. OP also said parts are coming from JLCPCB's SMT library so I doubt 1 is 30 years old and the other is modern. |
| Siwastaja:
--- Quote from: Kasper on November 04, 2019, 04:28:39 pm ---My post was clearly broken into a quick solution for someone who seems to want a quick solution and then a better solution with an explanation that the quick solution is not great. --- End quote --- Doesn't matter. Sorry to say, but your "quick" "solution" is just plainly wrong, and won't work as a "first order estimate" to be improved upon. It's waste of time, and misleading. LED efficiencies easily vary by two orders of magnitude. Assuming current has anything to do with brightness is just wrong. It's not even about the age of the LED - very inefficient and dim LEDs are still being made, especially green. The quick solution is to look at mCd numbers in datasheets, if available. The better solution is finetuning by eye. Choosing the most efficient (bright) LEDs is a good start, also to save energy. Then fine-tune by adjusting resistors - this is done by eye. So following your solution, you should go in the opposite direction, from step 2 to step 1. |
| Kasper:
--- Quote from: Siwastaja on November 04, 2019, 04:46:39 pm --- --- Quote from: Kasper on November 04, 2019, 04:28:39 pm ---My post was clearly broken into a quick solution for someone who seems to want a quick solution and then a better solution with an explanation that the quick solution is not great. --- End quote --- Doesn't matter. Sorry to say, but your "quick" "solution" is just plainly wrong, and won't work as a "first order estimate" to be improved upon. It's waste of time, and misleading. LED efficiencies easily vary by two orders of magnitude. Assuming current has anything to do with brightness is just wrong. It's not even about the age of the LED - very inefficient and dim LEDs are still being made, especially green. The quick solution is to look at mCd numbers in datasheets, if available. The better solution is finetuning by eye. Choosing the most efficient (bright) LEDs is a good start, also to save energy. Then fine-tune by adjusting resistors - this is done by eye. So following your solution, you should go in the opposite direction, from step 2 to step 1. --- End quote --- Quick solution would most likely be better than using the same resistor for each LED. |
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