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Old 1st August 2010, 09:42 PM   #1
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Default How do you determine the brightness of an LED?

Here are the specs VAOL-5MSBY2 VCC Standard LED - Through Hole

The LED will be used as a power indicator on the front panel.
My power source will be 24V. I don't know how much mA I should use as this will help me determine what resistor I should use. 20mA is 1500 mcd right? So that's probably 75 mcd/mA. I don't have anything to reference it to. How bright is 1500 mcd, 1000 mcd, 500 mcd, etc. @ 60 degrees? I don't want it to be blinding when looking directly into it nor do I want it to be too dim under medium to high ambient light conditions.
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Old 1st August 2010, 09:50 PM   #2
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Hope this will be helpfull :
LED calculator for single LEDs
According to your Led specs, R will be about 1.2K/1W
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Old 1st August 2010, 10:19 PM   #3
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That's the calc I used as well, however I will not run it at 20mA; too bright and will have a shorter lifespan. In order to decrease mcd, I will need to decrease mA by increasing resistance, which is what I'm getting at.

If I wanted 750 mcd as opposed to 1500 mcd, then I would get 2.2 Kohms @ 1/2W. The problem is, how bright is 750 mcd? I can't find any references online.
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Old 2nd August 2010, 05:03 AM   #4
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This is one thing where "learning is doing," where I mean you have to use trial and error. I found that with as little as 5mA the LEDs were still very bright. Try a low voltage source like a 9V battery, use the same equation and do 3-5-10 mA to see what you like. Then use the same mA for your final install. Or just experiment with your final install in the first place using a variable resistor or something. What is stopping you from this- no LED or resistors? Just planning right now and have no parts?

With 24VAC you have to be careful about power dissipation, might need a 1/2 W resistor- but you are all over that issue.
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Old 2nd August 2010, 06:58 AM   #5
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I use a 10k trim pot and slowly lower the resistance until it's at the brightness I want. I then measure the pot setting and replace it with a suitable resistor.

Perfect brightness every time.
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Old 2nd August 2010, 10:13 PM   #6
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Quote:
Originally Posted by lgreen View Post
This is one thing where "learning is doing," where I mean you have to use trial and error. I found that with as little as 5mA the LEDs were still very bright. Try a low voltage source like a 9V battery, use the same equation and do 3-5-10 mA to see what you like. Then use the same mA for your final install. Or just experiment with your final install in the first place using a variable resistor or something. What is stopping you from this- no LED or resistors? Just planning right now and have no parts?

With 24VAC you have to be careful about power dissipation, might need a 1/2 W resistor- but you are all over that issue.
Weird. I asked my dad if 20mA is bright and blinding (if looked directly into it). He said not at all. This is a 3.5V diffused LED. I'm guessing the ones you were using were the clear type?

I'm ordering stuff from Mouser and Digikey so I would like to have everything in one package. I would like to avoid "The Shack" as much as possible.

Quote:
Originally Posted by theAnonymous1 View Post
I use a 10k trim pot and slowly lower the resistance until it's at the brightness I want. I then measure the pot setting and replace it with a suitable resistor.

Perfect brightness every time.
That's a great idea! The horror of ordering every resistors from The Shack just for testing purposes. Thank god trim pots were invented.
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