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Old 27th June 2012, 09:04 AM   #1521
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AndrewT has reason LEDs & Zener have less noise, also as per AndrewT advice a Zener can be more appropiate for you high voltage.
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Old 27th June 2012, 09:38 AM   #1522
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Question ''Lazybutt'' Fred's Excel calc

I have a question about ''Lazybutt'' Fred's Excel calc pre-filter calculator wich values in inductance for L1 & wich valkues in capacitance for C1,2,3?
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Old 27th June 2012, 12:27 PM   #1523
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Hi Andrew,

It's against my believe to use "noisy" zeners, but I'm willing to give it a try.
To reach 30V I now use led's and R's, do you have a certain type/serie/brand of zeners which is your favorite ? Minimal noise etc.

thanks in advance.
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Old 1st July 2012, 06:40 AM   #1524
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what type and value cap Is recommended if you have noisy ref components like too much leds or zener
Is there a compact alternative to black gate N - Elna bipolar or other lytic
anyone tried wima mks 5 mm pitch - they got 22 uF value, or evox MMK or SMR

Last edited by samoloko; 1st July 2012 at 06:48 AM.
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Old 2nd July 2012, 08:06 AM   #1525
RCruz is offline RCruz  Switzerland
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Quote:
Originally Posted by AndrewT View Post
A voltage reference will perform better than a resistor.
Usually a voltage reference will have a few ohms or maybe tens of ohms of dynamic impedance. A resistor must have a dynamic impedance equal to it's resistance. If you use a 1k0 resistor as your voltage Vref string then the dynamic impedance will be 1k0, If you use a 20V Zener & a 100ohm VR set to 49ohms then the dynamic impedance will be very approximately 60ohms. That is approximately 14 times lower impedance of the Vref. That makes for a better regulator, not 14 times better, but quite a bit better.

Instead of a Zener, you can use LEDs as Vref elements. Most of these Salas regulators use a short string of red LEDs just because they allow the regulator to perform better. 20V from a string of red LEDs would require 10 to 12 LEDs. That is a bit too many, a Zener would probably be better for this higher voltage.

No matter whether a resistor has a 5ppm or 50ppm or 0.5ppm tempco, it can never have a dynamic impedance as low as an appropriately selected Vref semiconductor.
Hi Andrew

Thank you for the very clear explanation.
What is your opinion about a precision reference like LM329 ? The pdf states a dynamic impedance of 0.8ohm.... Can it be used for vref in this case ? What about tjhe value of the filtering cap in this case ?
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Old 2nd July 2012, 12:52 PM   #1526
PET-240 is offline PET-240  Australia
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I concur Andrew, bloody marvellous explanation!

Though may I ask, what effect the dynamic imp has on the regulation? Does it "react" quicker to give a smoother output or holds the output voltage more stable under temperature variations?

Look forward to an informative answer. Thanks for putting up with those of us that don't know all this stuff!
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Old 3rd July 2012, 11:21 AM   #1527
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With TeaBag's GB being dispatched soon, I though it was time to start working on my BOM.
The goal is to power my DD turntable with the BiB.

The TT needs ~200mA at normal spinning and ~400mA peak when starting rotating, all @21V.
Because the load is both digital and inductive, I think that going hot-rod would be a nice idea. Maybe as high as 1A?
The input DC is going to be 30V (2x12V secondaries @120VA)
According to the excel sheet, the CSS should be 3leds and R101 0,5R at 0,5W

Since the voltage is on the high side, I guess that a zener (20V 0,5W ?) filtered with lytics should be a better choice for the reference, along with a 1K trimmer for tweaking.
Unfortunately Q106 results in dissipating almost 17W and will need a substantial size of heatsink. So I really need your opinion on if hot-rodding would really be useful in my case. Dropping the current to 600mA results in ~9W at Q106 which again is high but really more manageable.

Thanks a lot for your time
Dimitris
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Old 3rd July 2012, 11:27 AM   #1528
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I would keep it at 600ma in your situation driving a motor. 600ma is allready a bit in the hotrod zone.

Suggestion : if things get really hot just push the start button a 100 times

Last edited by acidbrain; 3rd July 2012 at 11:32 AM.
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Old 3rd July 2012, 11:29 AM   #1529
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Who knows the dymanic impedance of each 1.9V 20mA red LEDs?

How can calculate the dynamic impedance of one resistor, for example 220 ohms?

Last edited by merlin el mago; 3rd July 2012 at 11:53 AM.
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Old 3rd July 2012, 12:19 PM   #1530
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Is ok this formula:

Rd = 26mV @ 25șC / If

For example dide 20mA will be:

Rd = 26mV / 20mA = 1.3 ohms

For a resistor will be:

Rd = 26mV / (I:V/R across Resistor)

26mV is given by the formula where
k is the Boltzmann constant k=1.38e^-23 joules/Kelvin
T is the absolut temperature in Kelvin
q is the electron charge q=1.602e^-19 coulomb

This yields approximately a Vt of 25mV @ 20 or
26mV @ 25
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