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Old 4th September 2013, 03:41 PM   #1
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Default Zener vs Led as reference

I have been thinking about building a Sulzer-based regulator, using a led as a reference instead of a zener diode. It is cheaper, and I have read that it is quite low-noise too. I believe that with a classic filtering it would be excellent, biased at around 1mA.

What do you think? And a bonus question: do regulators work better when they use as reference a small or big part of the output voltage, or is this irrelevant?

EDIT: Is there any reason why certain colors are superior to others, in terms of noise/stability?

Last edited by audiostrat; 4th September 2013 at 03:45 PM.
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Old 4th September 2013, 04:35 PM   #2
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One of the bests references diodes is the popular TL431, intended specially for this use. Also ic tan do a series or shunt regulator with few some more components.

The only one time I used a led as referece, the voltage looks to vary far larger than a zener.
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Old 4th September 2013, 05:32 PM   #3
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Interesting reading: LED noise masurement

Yes, if you need low noise low voltage reference, LED is your best choice. If you need 15V or more, it is easier to use Zener than stack of LEDs.
Bias your LED with low current. Yes, dynamic impadance will be slightly greater if biasing with low current, but as you will use additional RC filtering, dynamic impedance doesn't matter.
With LED, be careful of PSRR and use CCS to privide constant current to your LED and to increase PSRR.
Your noise will be lower if you use reference closer to output voltage. You can use 10V reference and divide to 5V with resistor network. Never use 1V reference and crank up the gain (5x) to reach 5V, if you need low noise.
Green, red, yellow LEDs have lower noise than blue and white (generally speaking).

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Old 4th September 2013, 06:36 PM   #4
RJM1 is offline RJM1  United States
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You Might want to look here. Some noise measurements for LEDs and zener diodes
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Old 4th September 2013, 07:10 PM   #5
jcx is offline jcx  United States
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LM329 buried Zener are still in stock even though EOL

lots of industrial ADC/DAC Vrefs have been made over the years, some innovations like the AD XFET promise even lower noise

but for audio heavy RC low pass can give fine dynamic noise - the kind you can hear - while TC, drift, accuracy really don't matter as much for many audio circuits
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Old 4th September 2013, 07:18 PM   #6
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Zeners has been designed to stabilize voltages, led's for lighting (LED stands for Light Emitter Diode). Why to reinvent the wheel???
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Old 4th September 2013, 07:33 PM   #7
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Quote:
Originally Posted by Osvaldo de Banfield View Post
Zeners has been designed to stabilize voltages, led's for lighting (LED stands for Light Emitter Diode). Why to reinvent the wheel???
Zeners are horribly noisy as all reverse biased PN junctions. Forward biased junctions (like LEDs) are way more quiet.

LEDs are practical as they show you that current runs. Temperature wise they are not the most stable but for a lot of circuits that is not so critical.

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Old 5th September 2013, 09:40 AM   #8
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Thank you everyone for the answers!

In one of the threads you posted, I read that LED's don't have very good thermal stability. Isn't thermal drift supposed to be a property of PN junctions in general though?

EDIT: Guess I read that in the above post too!!
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Old 5th September 2013, 11:20 AM   #9
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A filtered Zener is usually low enough noise for most audio circuits.
A stack of red LEDs will have more noise than a single red LED. Filtering will reduce that noise.
300mW to 500mW Zeners are cheaper than red LEDS. expect 1p:2p when bought in bulk.
A stack of 6 red LEDs comes to about 12p compared to 1p for a 12V Zener.
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Old 5th September 2013, 03:31 PM   #10
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LM395 are a good choice, as has been said here. But it is more expensive that a normal zener. I still prefer TL431.
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