LED bias for 6N1P driver

alecu7 · 20th September 2012, 02:38 PM

Hi all,

All schematics I could find about LED bias on a typical SET amplifier 6N1P + KT88 (a.k.a. Abdellah) show 2 red LEDs in series, drop in volatge being ~1.66V / LED.
My question is could I use a blue LED from Vishay part number TLHB4400 instead of 2 red LEDs? The spec shows 3.9V@20mA (from the graph ~3.5V@4mA). I know blue LEDs are different internally than red LEDs but for this application will this be ok?

Thanks,
Alex

flatheadmurre · 20th September 2012, 05:10 PM

The trick is to find a suitable voltage drop with the smallest resistance possible.

Cheap red leds seems to have the lowest resistans/voltage ratio.

Ultra bright leds and blue ones is less good because of higher internal resistance.

Voltagedrop depends on the current so check manufacturers spec sheet to find a suitable led.

tomchr · 20th September 2012, 11:51 PM

What he said!

You can find the dynamic resistance of the LED by looking in the datasheet. The dynamic resistance is the slope of tangent to the forward voltage vs forward current curve at the chosen operating current.

~Tom

alecu7 · 21st September 2012, 03:19 AM

From the datasheet the slope looks pretty much constant. I will try it.

Thanks,
Alex

20th September 2012, 02:38 PM	#1
alecu7 diyAudio Member Join Date: Mar 2010 Location: Hamilton	LED bias for 6N1P driver Hi all, All schematics I could find about LED bias on a typical SET amplifier 6N1P + KT88 (a.k.a. Abdellah) show 2 red LEDs in series, drop in volatge being ~1.66V / LED. My question is could I use a blue LED from Vishay part number TLHB4400 instead of 2 red LEDs? The spec shows 3.9V@20mA (from the graph ~3.5V@4mA). I know blue LEDs are different internally than red LEDs but for this application will this be ok? Thanks, Alex

20th September 2012, 11:51 PM	#3
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	What he said! You can find the dynamic resistance of the LED by looking in the datasheet. The dynamic resistance is the slope of tangent to the forward voltage vs forward current curve at the chosen operating current. ~Tom __________________ 21st Century Maida Regulator, Universal Filament Regulator, 300B Driver PCBs, and more... Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right

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20th September 2012, 05:10 PM	#2
flatheadmurre diyAudio Member Join Date: Mar 2011 Location: Jarna	The trick is to find a suitable voltage drop with the smallest resistance possible. Cheap red leds seems to have the lowest resistans/voltage ratio. Ultra bright leds and blue ones is less good because of higher internal resistance. Voltagedrop depends on the current so check manufacturers spec sheet to find a suitable led.

21st September 2012, 03:19 AM	#4
alecu7 diyAudio Member Join Date: Mar 2010 Location: Hamilton	From the datasheet the slope looks pretty much constant. I will try it. Thanks, Alex