LED cathode bias - how to pick LED?

sharpi31 · 25th December 2008, 02:00 PM

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!

sharpi31 · 25th December 2008, 02:18 PM

Is it as simple as picking an LED that has a 2.5V forward voltage drop at 30mA?

Sorry if I'm being dumb.

Fuling · 25th December 2008, 02:25 PM

Quote:

Is it as simple as picking an LED that has a 2.5V forward voltage drop at 30mA?

Yes!

Original Burnedfingers · 25th December 2008, 02:27 PM

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

http://www.kpsec.freeuk.com/components/led.htm

sharpi31 · 25th December 2008, 02:37 PM

Great - thanks!

kenpeter · 25th December 2008, 05:00 PM

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...

Drewaudio · 25th December 2008, 06:06 PM

Hello,

I suggest you read this very good article:

http://syclotron.com/?page_id=3

Kind Regards
Andrew

sharpi31 · 25th December 2008, 06:41 PM

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.

SY · 25th December 2008, 07:52 PM

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.

Sheldon · 26th December 2008, 04:03 PM

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.

Sheldon

25th December 2008, 02:00 PM	#1
sharpi31 diyAudio Member Join Date: Oct 2005	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!

25th December 2008, 07:52 PM	#9
SY diyAudio Moderator Join Date: Oct 2002 Location: Chicagoland Blog Entries: 1	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. __________________ If there's a sucker born every minute, where do the rest of them come from?

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25th December 2008, 02:18 PM	#2
sharpi31 diyAudio Member Join Date: Oct 2005	Is it as simple as picking an LED that has a 2.5V forward voltage drop at 30mA? Sorry if I'm being dumb.

25th December 2008, 02:27 PM	#4
Original Burnedfingers diyAudio Member Join Date: Nov 2002	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 http://www.kpsec.freeuk.com/components/led.htm

25th December 2008, 02:37 PM	#5
sharpi31 diyAudio Member Join Date: Oct 2005	Great - thanks!

25th December 2008, 05:00 PM	#6
kenpeter diyAudio Member Join Date: Nov 2007 Location: Dallas	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...

25th December 2008, 06:06 PM	#7
Drewaudio diyAudio Member Join Date: Mar 2006 Location: New Zealand	Hello, I suggest you read this very good article: http://syclotron.com/?page_id=3 Kind Regards Andrew

25th December 2008, 06:41 PM	#8
sharpi31 diyAudio Member Join Date: Oct 2005	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.

26th December 2008, 04:03 PM	#10
Sheldon diyAudio Member Join Date: Dec 2003 Location: San Diego	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. Sheldon