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 #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
sngecko wrote ...
I am presuming at the outset that a relatively large positive voltage on the grid is indeed hitting the bias supply. This might explain the slowing of the fans and destruction of the +/- 15V power supply.
Only one side of the neon was glowing, indicating a DC signal, right? Does a strong nearby oscillation cause this behavior in neons?
I don't know how to detect oscillations (I don't have an O-scope yet), but I can take any kind of voltage/current measurements. It's a little tough without a dummy load that's light-able, but I can (and have) put the power supply directly on the plate.
In this case, the presumed oscillatory behavior does not occur.
Good observation about those neon electrodes!
Now this is what I think is going on: If the external connection to the grid were interrupted, the grid would very quickly take on a positive charge as if it was a potential divider between anode and cathode.
Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
I will certainly take your precautionary steps regarding the diode and resistor. I will perform some tests as soon as I rebuild the regulator.
I find it most odd that the problem only occurs as soon as the laser tube is lit but not when there is that full voltage (at least 13kV) at the plate directly.
When I turn up the Variac to start the tube, perhaps the sudden demand for electrons at the plate and the very low current requirements (say 6mA) causes an oscillation between the cathode and drain...
I did record >100mA between drain and cathode when the tube was lit. How was this possible? Supporting this observation, I discovered today that the central tube of the HeNe tube was crackled, as with high localized heating. I guess I just blew, literally, US$100.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
sngecko wrote ...
I will certainly take your precautionary steps regarding the diode and resistor. I will perform some tests as soon as I rebuild the regulator.
I find it most odd that the problem only occurs as soon as the laser tube is lit but not when there is that full voltage (at least 13kV) at the plate directly.
When I turn up the Variac to start the tube, perhaps the sudden demand for electrons at the plate and the very low current requirements (say 6mA) causes an oscillation between the cathode and drain...
I did record >100mA between drain and cathode when the tube was lit. How was this possible? Supporting this observation, I discovered today that the central tube of the HeNe tube was crackled, as with high localized heating. I guess I just blew, literally, US$100.
I think it would be best to bypass the resistor to Earth with a capacitor in parallel. This will add a time constant element to the grid's ability to respond, so you should determine the value empirically. Try 100nF at a suitable voltage rating to start with, and adjust according to results.
Now as for the second issue: until the instant when the laser gas ionises and conducts, there will be negligible voltage drop across each of the components in series with it, including the triode. Once conduction occurs, voltages appropriate to the current flowing will appear across each element of the string.
Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
I see your points.
Regarding the second issue though, I've put 20kV at the plate (without a laser tube upstream) and regulated the current up to >30mA like a charm. I even did it after the first laser tube incident and it continued to perform as designed. Do you know if the laser tube itself introduces oscillations?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The laser tube has time-dependent negative resistance like any other gas discharge tube. That makes it harder for your regulator to drive stably than any resistive load, or even direct connection of the plate to the power supply, which is zero load resistance but still not negative.
John Doran may have used a ballast resistor in series with his tube, or maybe a CO2 discharge has less negative resistance.
Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
One purpose of the regulator is to do away with the ballast resistor in the first place. That tube was listed as starting at 10kVmax and operating at 1.3kV. Proportionally that is a great deal more time-dependent negative resistance than most larger bore CO2 lasers. I predict that the laser head I'm also building will start around 20kV and operate around 8-10kV.
I would like to retrofit this regulator for stable operation of even HeNe tubes, if possible. I suppose that I will have to start with the evil oscillations, per Proud Mary's advice. Is there a reason why an oscillation might cause much higher current to develop at the triode cathode, or do you suppose that the oscillation resulted in a pulsing short to ground (very possible considering the close proximity of grounded elements around the FET)?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
If the whining note persists, the only explanation can be oscillation, as Steve proposed above.
Setting aside any oscillatory behaviour the laser tube might exhibit as a Pearson–Anson or negative resistance device (about which I know very little) general remedies for instability and unwanted oscillation in a directly-heated power triode circuit could include:
Sticking some chokes in the filament lines, with decoupling capacitors to Earth.
Placing anti-parasitic suppressors (enamelled copper wire wound round carbon resistors) in the grid and anode lines, connecting them hard up to the socket tags using the shortest possible wiring. The grid and anode suppressors should have different values to attenuate tuned-anode-tuned-grid oscillatory tendencies.
Improve HT supply decoupling so that the HT isn't bouncing up and down in a way that could infiltrate other circuit elements by unintentional coupling - the close proximity of critical wiring etc.
In decoupling and bypassing applications use two or three capacitors of different values in parallel to get a wide frequency response without resonant kinks and quirks e.g. 100pF, 10nF, and 100nF.
Separate critical circuit elements into screened compartments, with power supplied by feedthrough capacitors.
There may well be no need to do all of these things. Some chokes in the filament lines - bypassed to Earth by good quality capacitors - are what I'd try first, but bear in mind that I know a lot more about making a good Sunday roast than I do about gas lasers.
Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
I've replaced the power supply and discovered that there is a short in or connected to the regulator control board. I'm not sure when this developed, but it certainly would stress the power supply.
Registered Member #3447
Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97
U1 and U2, both TL084-style op-amp packages, were damaged somehow at some stage of testing. I've replaced both of them and so far everything seems stable. I have not tested the AD633s, but I think that they were mostly isolated from U1 and U2.
Both U1 and U2 perform similar functions and must have been damaged at the "bang" previously mentioned.
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.
High Voltage Current Regulated Discharges
