Keeping Shunt current output consistent - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 28th April 2012, 01:11 PM   #1
skibum is offline skibum  France
diyAudio Member
 
Join Date: Dec 2009
Default Keeping Shunt current output consistent

Hello,

Newbie here!

I am making several DC power supplies. They are shunt power supplies.

The problem I am having is that the shunt current of each PS varies. I think that must be due to the gain of the PNP transistor (used for the shunt current) is varying from one device to the next.

I can put a pot on each one and adjust the gain individually but I think it would be better to devise some circuit that ensures the amount of current coming through the PNP transistor is the same even when the gain from transistor to transistor varies.

This must be a common issue?

Are there any circuits out there?

Thanks,

skibum
  Reply With Quote
Old 28th April 2012, 02:17 PM   #2
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
What is the variation you see? Is it important to be super accurate?
Remember that a shunt has the task to keep the voltage the same, so as the load current changes or the input voltage changes, the shunt current will adapt itself to keep the output voltage constant.
The only requirement is that the idle shunt current is high enough not to run out of current for max load current situations.
Do you have a circuit you can post? It is difficult to advise without knowing the circuit.

jan didden
__________________
I won't make the tactical error to try to dislodge with rational arguments a conviction that is beyond reason - Daniel Dennett
Check out Linear Audio Vol 7!
  Reply With Quote
Old 28th April 2012, 02:31 PM   #3
diyAudio Member
 
Join Date: Nov 2008
Location: Oakmont PA
Jan,

Nice to see you here. I suspect you have the correct issue, almost. I would ask about the feedback method.

BTY did you notice you flags are 90 degrees out of phase?

ES
  Reply With Quote
Old 28th April 2012, 03:00 PM   #4
Elvee is offline Elvee  Belgium
diyAudio Member
 
Elvee's Avatar
 
Join Date: Sep 2006
Quote:
Originally Posted by skibum View Post
The problem I am having is that the shunt current of each PS varies. I think that must be due to the gain of the PNP transistor (used for the shunt current) is varying from one device to the next.

I can put a pot on each one and adjust the gain individually but I think it would be better to devise some circuit that ensures the amount of current coming through the PNP transistor is the same even when the gain from transistor to transistor varies.
Schematic(s)?
__________________
. .Circlophone your life !!!! . .
♫♪ My little cheap Circlophone© ♫♪
  Reply With Quote
Old 28th April 2012, 03:28 PM   #5
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Quote:
Originally Posted by simon7000 View Post
Jan,

Nice to see you here. I suspect you have the correct issue, almost. I would ask about the feedback method.

BTY did you notice you flags are 90 degrees out of phase?

ES
You mean my country flag? Seems OK to me - we can use it up until it is very short. Unlike the French
Yes the shunt needs a feedback control for settability but since there's no schematic, I didn't want to speculate.

jan
__________________
I won't make the tactical error to try to dislodge with rational arguments a conviction that is beyond reason - Daniel Dennett
Check out Linear Audio Vol 7!
  Reply With Quote
Old 29th April 2012, 09:58 AM   #6
skibum is offline skibum  France
diyAudio Member
 
Join Date: Dec 2009
Quote:
Originally Posted by janneman View Post
What is the variation you see? Is it important to be super accurate?
Remember that a shunt has the task to keep the voltage the same, so as the load current changes or the input voltage changes, the shunt current will adapt itself to keep the output voltage constant.
The only requirement is that the idle shunt current is high enough not to run out of current for max load current situations.
Do you have a circuit you can post? It is difficult to advise without knowing the circuit.

jan didden
Hello Jan,

Yes, I would like to figure out how to get a super accurate shunt current. I would also like this to be consistent from one PS to the next. I made a mistake and the variation in the transistor to transistor gain does not seem to be the biggest factor in shunt current variation.

I am using a DC power design similar to this:

http://www.twistedpearaudio.com/docs..._0_1_schem.pdf

I would like to remove VR1 and just use a fixed resistor R1. The shunt current variation seems to be mostly coming from the variation in the JFET current source.

I replaced the JFET current source with a current limiting diode but that didn't really help. I think the part to part current variation in the current limiting diode was also to large.

What I was thinking is to somehow take the voltage across R3 (this voltage is caused by the shunt current) and use that as feedback somehow to a current source.

I found this constant current source circuit:

http://www.electronicdesign.com/file...66/fig_01a.gif

Could I use the voltage across R3 as the "Vin" to the LMP7731 opamp?
Is anything like this possible or am I making things much more difficult than I should be? : )
  Reply With Quote
Old 29th April 2012, 10:20 AM   #7
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Do you understand how a CCS plus Shunt voltage regulator works?
__________________
regards Andrew T.
  Reply With Quote
Old 29th April 2012, 10:38 AM   #8
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: Great City of Turnhout, Belgium
Blog Entries: 7
Well the problems with these type of circuits is that they try to generate an accurate reference with D1 and D2 which have undefined voltage levels and also vary from one to the other unit.
So even when you adjust the current through them there still will be unit-to-unit variations.
That's one of the things with diy audio. There are very good engineers that developed reference diodes and shunt reference regs specifically for this purpose, but hey, someone in diy audio uses LEDs so they must be the best and everybody uses them, even if they are not the best choice.

Then if you want to explain how this really works, someone comes up and will say, but it sounds the best and that's the end of the discussion.
So, I believe when you continue to use LEDs you will continue to see this variations.
The give-away is the trimmer - a proper designed shunt doesn't need any trimmers!

Use something like an LM4040-5V as a 5V ref instead of the LEDs and you're done.

jan
__________________
I won't make the tactical error to try to dislodge with rational arguments a conviction that is beyond reason - Daniel Dennett
Check out Linear Audio Vol 7!

Last edited by jan.didden; 29th April 2012 at 10:40 AM.
  Reply With Quote
Old 29th April 2012, 11:01 AM   #9
skibum is offline skibum  France
diyAudio Member
 
Join Date: Dec 2009
Quote:
Originally Posted by AndrewT View Post
Do you understand how a CCS plus Shunt voltage regulator works?

Honestly, no. But I am trying to learn!
  Reply With Quote
Old 29th April 2012, 11:08 AM   #10
skibum is offline skibum  France
diyAudio Member
 
Join Date: Dec 2009
Quote:
Originally Posted by janneman View Post
Well the problems with these type of circuits is that they try to generate an accurate reference with D1 and D2 which have undefined voltage levels and also vary from one to the other unit.
So even when you adjust the current through them there still will be unit-to-unit variations.
That's one of the things with diy audio. There are very good engineers that developed reference diodes and shunt reference regs specifically for this purpose, but hey, someone in diy audio uses LEDs so they must be the best and everybody uses them, even if they are not the best choice.

Then if you want to explain how this really works, someone comes up and will say, but it sounds the best and that's the end of the discussion.
So, I believe when you continue to use LEDs you will continue to see this variations.
The give-away is the trimmer - a proper designed shunt doesn't need any trimmers!

Use something like an LM4040-5V as a 5V ref instead of the LEDs and you're done.

jan
In my curcuit I have replaced the two LEDs with a single 1N4448 diode. I did need that much voltage.

Should I replace just the 1N4448 with the LM4040?

OR

Replace the 1N4448, the JFET and R5 with the LM4040?

Thank you!
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Hall-effect current sensor for output stage over-current protection panson_hk Solid State 4 6th September 2011 03:07 PM
Shunt Current Feedback Stee Solid State 24 28th August 2011 08:14 PM
Crazy Current Mode Shunt Regulator wrenchone Analogue Source 5 30th November 2010 06:18 PM
shunt PSU and output device Stefanoo Pass Labs 3 26th November 2008 10:42 PM
A Shunt-Reg with current source for DAC/Phono needed Blitz Solid State 2 11th February 2003 08:34 AM


New To Site? Need Help?

All times are GMT. The time now is 05:04 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2