07-03-2016, 06:27 AM
Really interesting project and a great write-up 
It just so happens I did a "statistical regulator" project on low-noise HV screen regulators for audio pentodes & tetrodes last year - the project write-up is here (not nearly as detailed/impressive as yours!): http://www.audio-talk.co.uk/phpBB3/viewt...=11&t=6204 (I'm called "Jack" on Audio-Talk
) - it's based on Morgan Jones' write-up in the latest (4th) edition of "Valve Amplifiers".
In my case, the primary objective was very low noise, not absolute precision - also, I was using long strings of low-voltage Zeners so temperature correction was applied using the ZPWL model - see http://www.audio-talk.co.uk/phpBB3/viewt...45#p128632
I know noise is not a design metric in your project, but the LM317 is a noisy old thing so I used a pair of depletion-mode FETs (the ubiquitous DN2450) as the CCS, delivering about 10mA to the Zener string. Also, with the large number of Zeners I was using (up to 80), voltage accuracy for each was not important - it averaged out to a nominal 5.6V, though the temperature & current-related compensation was significant.
As you note, in a statistical regulator the voltage accuracy improves as a function of the square root of the number in the series string - obviously, for the first one in your string (2.5V), there is no improvement - the improvements will be for the higher voltages where more devices come into play.
As a DMM reference, into MΩ input impedances, this would be fine, but being the forum we are here, maybe worth emphasising "definitely not for AVOs"
Possibly consider a precision voltage-follower to lower the output impedance so the loading becomes insignificant?
It just so happens I did a "statistical regulator" project on low-noise HV screen regulators for audio pentodes & tetrodes last year - the project write-up is here (not nearly as detailed/impressive as yours!): http://www.audio-talk.co.uk/phpBB3/viewt...=11&t=6204 (I'm called "Jack" on Audio-Talk
) - it's based on Morgan Jones' write-up in the latest (4th) edition of "Valve Amplifiers".In my case, the primary objective was very low noise, not absolute precision - also, I was using long strings of low-voltage Zeners so temperature correction was applied using the ZPWL model - see http://www.audio-talk.co.uk/phpBB3/viewt...45#p128632
I know noise is not a design metric in your project, but the LM317 is a noisy old thing so I used a pair of depletion-mode FETs (the ubiquitous DN2450) as the CCS, delivering about 10mA to the Zener string. Also, with the large number of Zeners I was using (up to 80), voltage accuracy for each was not important - it averaged out to a nominal 5.6V, though the temperature & current-related compensation was significant.
As you note, in a statistical regulator the voltage accuracy improves as a function of the square root of the number in the series string - obviously, for the first one in your string (2.5V), there is no improvement - the improvements will be for the higher voltages where more devices come into play.
As a DMM reference, into MΩ input impedances, this would be fine, but being the forum we are here, maybe worth emphasising "definitely not for AVOs"
Possibly consider a precision voltage-follower to lower the output impedance so the loading becomes insignificant?
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