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Old 25th December 2008, 02:00 PM   #1
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Default LED cathode bias - how to pick LED?

Sorry of this info is available in another thread - I've read through as much as I can but can't find a simple description of how to select an appropriate LED.

My Spud amp has an 82ohm resistor with 470uF cap for cathode bias. There is about 2.5V over the resistor so there's about 30mA.

How do I use this info to pick an appropriate LED?

Thanks in advance - and Merry Christmas!
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Old 25th December 2008, 02:18 PM   #2
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Is it as simple as picking an LED that has a 2.5V forward voltage drop at 30mA?

Sorry if I'm being dumb.
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Old 25th December 2008, 02:25 PM   #3
Fuling is offline Fuling  Sweden
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Is it as simple as picking an LED that has a 2.5V forward voltage drop at 30mA?
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Old 25th December 2008, 02:27 PM   #4
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Type Colour IF
max. VF
typ. VF
max. VR
max. Luminous
intensity Viewing
angle Wavelength
Standard Red 30mA 1.7V 2.1V 5V 5mcd @ 10mA 60 660nm
Standard Bright red 30mA 2.0V 2.5V 5V 80mcd @ 10mA 60 625nm
Standard Yellow 30mA 2.1V 2.5V 5V 32mcd @ 10mA 60 590nm
Standard Green 25mA 2.2V 2.5V 5V 32mcd @ 10mA 60 565nm
High intensity Blue 30mA 4.5V 5.5V 5V 60mcd @ 20mA 50 430nm
Super bright Red 30mA 1.85V 2.5V 5V 500mcd @ 20mA 60 660nm
Low current Red 30mA 1.7V 2.0V 5V 5mcd @ 2mA 60 625nm

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Old 25th December 2008, 02:37 PM   #5
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Great - thanks!
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Old 25th December 2008, 05:00 PM   #6
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What? There isn't some magically optimal choice?
Does no one care to specify a dynamic resistance?

Obviously they have enough resistance to drop
the specified volts, but then what happens? How
well do they keep steady voltage as current rises?
Even the shape of the knee might be important.

They can't all be the same. Are two reds better
than one blue? The question has to be asked...
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Old 25th December 2008, 06:06 PM   #7
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I suggest you read this very good article:


Kind Regards
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Old 25th December 2008, 06:41 PM   #8
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I've ordered a bunch of these red LEDs. Page 5 of the data sheet shows that I should get about 2.35V drop with 30mA current.

LED data sheet

The graph looks pretty linear from 20-80mA, so hopefully this will be fine.

I'm assuming that dropping the bias from 2.5V to 2.35V won't be an issue. I actually ordered 15x LEDs so I can test them and use the ones with the highest V drop.
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Old 25th December 2008, 07:52 PM   #9
SY is offline SY  United States
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From the datasheet graphs, the amber look to have much lower dynamic resistance. The red are about 12 ohms where you want to run them. Amber look to be about 2 ohm, but only a 2V drop. You might consider using two IR LEDs in series which will give you 2.4-2.5V, and the better ones are in the 2 ohm range.
"It is difficult to get a man to understand something, when his salary depends upon his not understanding it." Upton Sinclair
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Old 26th December 2008, 04:03 PM   #10
Sheldon is offline Sheldon  United States
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Don't be afraid to mix and match technologies either. If you achieve most of the drop you need with a *good* LED - low dynamic resistance, or maybe even more important - linear resistance within the expected current range, you can use a series resistor for the rest of the voltage drop. This assumes that you have a little gain to burn and that the output impedance still be low enough with the added series resistance.

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