If you need assistance, please send an email to forum at 4hv dot org. To ensure your email is not marked as spam, please include the phrase "4hv help" in the subject line. You can also find assistance via IRC, at irc.shadowworld.net, room #hvcomm.
Support 4hv.org!
Donate:
4hv.org is hosted on a dedicated server. Unfortunately, this server costs and we rely on the help of site members to keep 4hv.org running. Please consider donating. We will place your name on the thanks list and you'll be helping to keep 4hv.org alive and free for everyone. Members whose names appear in red bold have donated recently. Green bold denotes those who have recently donated to keep the server carbon neutral.
Special Thanks To:
Aaron Holmes
Aaron Wheeler
Adam Horden
Alan Scrimgeour
Andre
Andrew Haynes
Anonymous000
asabase
Austin Weil
barney
Barry
Bert Hickman
Bill Kukowski
Blitzorn
Brandon Paradelas
Bruce Bowling
BubeeMike
Byong Park
Cesiumsponge
Chris F.
Chris Hooper
Corey Worthington
Derek Woodroffe
Dalus
Dan Strother
Daniel Davis
Daniel Uhrenholt
datasheetarchive
Dave Billington
Dave Marshall
David F.
Dennis Rogers
drelectrix
Dr. John Gudenas
Dr. Spark
E.TexasTesla
eastvoltresearch
Eirik Taylor
Erik Dyakov
Erlend^SE
Finn Hammer
Firebug24k
GalliumMan
Gary Peterson
George Slade
GhostNull
Gordon Mcknight
Graham Armitage
Grant
GreySoul
Henry H
IamSmooth
In memory of Leo Powning
Jacob Cash
James Howells
James Pawson
Jeff Greenfield
Jeff Thomas
Jesse Frost
Jim Mitchell
jlr134
Joe Mastroianni
John Forcina
John Oberg
John Willcutt
Jon Newcomb
klugesmith
Leslie Wright
Lutz Hoffman
Mads Barnkob
Martin King
Mats Karlsson
Matt Gibson
Matthew Guidry
mbd
Michael D'Angelo
Mikkel
mileswaldron
mister_rf
Neil Foster
Nick de Smith
Nick Soroka
nicklenorp
Nik
Norman Stanley
Patrick Coleman
Paul Brodie
Paul Jordan
Paul Montgomery
Ped
Peter Krogen
Peter Terren
PhilGood
Richard Feldman
Robert Bush
Royce Bailey
Scott Fusare
Scott Newman
smiffy
Stella
Steven Busic
Steve Conner
Steve Jones
Steve Ward
Sulaiman
Thomas Coyle
Thomas A. Wallace
Thomas W
Timo
Torch
Ulf Jonsson
vasil
Vaxian
vladi mazzilli
wastehl
Weston
William Kim
William N.
William Stehl
Wesley Venis
The aforementioned have contributed financially to the continuing triumph of 4hv.org. They are deserving of my most heartfelt thanks.
Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
As Watson once said, "I have no doubt that I am very stupid." I appreciate the insights that Proud Mary, Marko, Steve Conner, and Steve Maurer have brought to my little project. It's been a long time coming to fruition, and thanks to you, it has a good chance of getting done. I've learned a heck of a lot along the way.
This is what I've got to be going on for now:
(1) If oscillation is in the triode, then first try grid stopper resistance. Then anode, grid, and cathode chokes (in that order?) as well as bypass caps in anode chain (20kV, 100nF!)
(2) If oscillation is in the feedback loop vis-a-vis the op-amp and MOSFET, then try an RC filter across the output to the negative input.
Re: (2), I've done some RC calculations and think that R=100k and C=.1uF or so should give -20dB attenuation at 100Hz. Sound good? Am I right in thinking that this arrangement (kinda RCR) acts like an RC filter in both directions?
I'd like to try (2) first since its fairly easy to solder a couple little parts together and jam their little legs in those DIP sockets. We'll see.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
sngecko wrote ...
Something like this?
Yes!
Marko's comments may well be valid too, but oscillations in the FET/opamp part of the circuit are a separate issue.
FWIW I've built similar current regulators, albeit minus the high voltage cascode, and they worked without compensation. As far as the op-amp's feedback is concerned, the MOSFET is functioning as a source follower, so the loop gain can never be more than 1.
Registered Member #133
Joined: Fri Feb 10 2006, 10:27PM
Location: Pensacola, Florida
Posts: 47
Steve and Marko both have good ideas. Definitely try installing the grid resistor first. If oscillation persists, then pursue the op-amp section also. The last op-amp stage may be modified to form a low pass filter if needed.
Clarification concerning the 24 V SMPS common mode filter: If a SMPS happens to run erratically under certain dynamic loading conditions (due to high frequency noise presented to the output SMPS), then a common mode filter placed in line with the output leads of the SMPS (not the AC line cord input to the SMPS) will sometimes help. Note that the load on the power supply plays a part in the loop response of the SMPS.
(On a side note: A good way to verify SMPS stability is to test the output with a fast rise time pulsed load at several frequencies. I usually check my switch mode supplies at pulsed frequencies of 100 Hz, 1 kHz, and 10 kHz.)
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
sngecko wrote ...
As Watson once said, "I have no doubt that I am very stupid." I appreciate the insights that Proud Mary, Marko, Steve Conner, and Steve Maurer have brought to my little project. It's been a long time coming to fruition, and thanks to you, it has a good chance of getting done. I've learned a heck of a lot along the way.
This is what I've got to be going on for now:
(1) If oscillation is in the triode, then first try grid stopper resistance. Then anode, grid, and cathode chokes (in that order?) as well as bypass caps in anode chain (20kV, 100nF!)
(2) If oscillation is in the feedback loop vis-a-vis the op-amp and MOSFET, then try an RC filter across the output to the negative input.
Re: (2), I've done some RC calculations and think that R=100k and C=.1uF or so should give -20dB attenuation at 100Hz. Sound good? Am I right in thinking that this arrangement (kinda RCR) acts like an RC filter in both directions?
I'd like to try (2) first since its fairly easy to solder a couple little parts together and jam their little legs in those DIP sockets. We'll see.
Hello -
nah that doesn't look right - you need some resistance in your feedback path (wire going from your sense resistor to opamp negative input) to form a meaningful divider. And for start I'd just replace your CRC-whatever with a single 1uF cap to make a simple I regulator. This should stop the oscillations and you can always upgrade it alter if you need better impulse response (don't know if you want to audiomodulate the lasers or whatever?)
Also, only now I've realized that the filter cap on the triode grid doesn't go to it's cathode, but all way to the ground. If you think the tube might be unstable, maybe it would be wiser to connect the cap straight to cathode (again it doesn't matter for static operation). This should nullify any chances of tube squeg or whatever better than any grid resistors, as I think.
Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
Thanks Marko and Steve---
That contraption I put together there didn't make any sense to me either. Sorry, Marko. At least a 1uF cap along that path makes sense to me. I understand that the feedback signal will be slightly out of phase, so the cap sort of smooths the gap a bit, right? Since my input will be manual, fed from a pot off a linear 15v supply, there shouldn't be any weird HF inputs to interact with that cap.
I'll get to work at once on the grid stopper and see what happens, then try out the opamp cap.
Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
THIS JUST IN: I attached a 1.5k grid stopper and fired up the laser. Unfortunately the symptoms persisted, but I at least learned that only the main 24V case fan is slowing in the presence of this oscillation. The other two 24V boost fans (one for each tube channel) seems to operate at or near normal parameters during the noise.
AGAIN: I just confirmed that with a .22uF cap across the output and negative input pin... no change. Still, an audible squeal increasing in frequency with current through the circuit. So, the last culprit is, in fact, the triode, right?
I guess the 1.5k stopper resistor didn't do the job. Also, I need to look closer at the input to that silly fan. I just can't see where it's coupled to the RF noise -- drive circuitry as someone suggested, I guess.
What would be the effect of tying the grid to the cathode with a .5uf capacitor, as suggested above?
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Not sere why, but you just seem to insist on leaving out that crucial resistor that would give sense to the whole thing. You need something to form a voltage divider on your feedback signal!
It's the RC time constant that slows down the response, and if you make it into like seconds (1 meg , 1uF) I have strong feelings that oscillations should stop on that part.
That's of course the most basic solution of all and you have some big homework to do on regulators if you wish to optimize your impulse response speed, overshoot and so on.
I also have no idea if such a simple solution is even proper for a negative resistance load such as a laser tube - you could have a well working current regulator which still creates a relaxation oscillator with the unintuitive aid of some capacitor. (needles to say, don't use a "filter capacitor" across your tube!)
To test the regulator, always use dummy resistance first (or just keep it shorted) until you've gotten it to work properly.
Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
You know? That's a really good point that I seem to have overlooked... test the regulation portion with a dummy load, then work back.
I just started a new thread that addresses what you put up there, essentially an inverting integrator, right? Well, the way my circuit is set up, I have a non-inverting, unity, closed loop feedback chain. I am learning more and more how this is an inherently metastable arrangement that often leads to RF oscillations of all sorts, especially with the TL080 series op-amps. They apparently have a relatively high output impedance that just keeps the oscillations coming. I've considered the following solutions:
(1) Twist the gate drive and source wires together all the way down to the off-board regulator elements (2) Replace the TL084s with AD713s, with lower output impedance (3) I like this one for some reason: Put a 30-something Ohm resistor in the feedback loop with several turns of magnet wire around it (parallel hookup) to block HF component, but pass the DC component (4) Build up a non-inverting integrator!
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
sngecko wrote ...
You know? That's a really good point that I seem to have overlooked... test the regulation portion with a dummy load, then work back.
I just started a new thread that addresses what you put up there, essentially an inverting integrator, right? Well, the way my circuit is set up, I have a non-inverting, unity, closed loop feedback chain. I am learning more and more how this is an inherently metastable arrangement that often leads to RF oscillations of all sorts, especially with the TL080 series op-amps. They apparently have a relatively high output impedance that just keeps the oscillations coming. I've considered the following solutions:
(1) Twist the gate drive and source wires together all the way down to the off-board regulator elements (2) Replace the TL084s with AD713s, with lower output impedance (3) I like this one for some reason: Put a 30-something Ohm resistor in the feedback loop with several turns of magnet wire around it (parallel hookup) to block HF component, but pass the DC component (4) Build up a non-inverting integrator!
No... just add a 1 meg resistor into your feedback path.
This site is powered by e107, which is released under the GNU GPL License. All work on this site, except where otherwise noted, is licensed under a Creative Commons Attribution-ShareAlike 2.5 License. By submitting any information to this site, you agree that anything submitted will be so licensed. Please read our Disclaimer and Policies page for information on your rights and responsibilities regarding this site.
Oscillations Redux
