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 #1389
Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
I was searching around on Ebay for good HV stuff, and came across a guy who sells HV polystyrene and polypropylene capacitors. The deal of $50 for 12 30kVDC 2nF caps seems pretty good, but I don't know how good polystyrene is for CWs and Marx Generators (which is where I would use them). The only dielectric I know is 'good' is PET, boPET, or PETP.
So, would this be a good investment, or would it be better to use seriesed CDE 942s or homemade overhead caps instead?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
I too looked at the huge polystyrene capacitors. Look at the terminations .. not a pulse cap I think. Note that the stated applications do not include any pulse duty.
Polystyrene has a very low melting point and traditionally the 'suflex' brand were prone to contamination via the wound ends.
Polystyrene is a good low loss dielectric at rf (not the best) and is easily metalized for manufacturing but one of it's key benefits is a negative temperature coefficient of capacitance which almost exactly cancels the positive temperature coefficient of inductance of an air-wound coil, great for rf tuned-circuits but not particularly useful for a TC.
So overall I think they're beautiful capacitors but not ideal for TC work. (I think these caps may have to be potted or under oil to achieve the voltage rating)
P.S. I have had for years 11x 18nF 2.5kV Suflex brand polystyrene caps for general experimenting and I've not once tried them in TC service ...... too nice to kill!
P.P.S. In order of dielectric loss at TC frequencies Vacuum, air, ptfe, etfe, pp, pe, ps............. MKT then unsuitables such as paper/oil, Y- or Z- ceramics etc.
Registered Member #1389
Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
Thanks Sulaiman. I hadn't intended to use them in TC duty, I was planning to use them in Marx or CW multipliers. Since you said they probably are not good in pulse duty, I guess I have to take Marx generators off the list, which leaves the CW multiplier. Knowing that they are less lossy than PET or PP is a bonus too!
Here are the Caps. What do you think, good for CW service with 15-20kV input?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
Look a few inches down in the listing, it clearly states " should be used in insulating oil without aromatic series or be used in silicone rubber." If that's ok for your project then good.
My list was in order of dielectric "goodness", best at the left .. sorry.
These capacitors look so well made (completely transparent at the ends) they're worth buying as a 'visible' capacitor. There must be a new manufacturer of polystyrene film, so these capacitors may be better than my old experiences.
Registered Member #1389
Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
Putting it under oil shouldn't be a problem. I don't know much about aromatic series' (never was particularlly interested in O-Chem), but do mineral or vegitable (probably sunflower) oils have them? Those would probably be the easiest things to get my hands on for this.
Registered Member #1630
Joined: Sat Aug 09 2008, 11:36AM
Location: Seoul Korea
Posts: 115
PS dielectric isn't very good. I would be tempted to use Polyethylene for marx generators as it has much higher current capability. 940C 942C from CDE are perfect for building marx or Walter Crofton impulse generators or those sorts of things.
Registered Member #1389
Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
Thanks Seoul. I am planning a large-ish 60-100kV CW multiplier using the caps I ultimately decide to use. I was asking about these caps, because even half rated, their voltage stand-off (under oil anyway) is much cheaper per kV than CDE caps, and I wasnt terribly concerned about the capacitance. I've used 22nF film caps in a small multiplier at high frequency and got a very reasonable result. Heck, I've used 56 pF caps, and they still worked well, though I believe the driving frequency was around 40 or 50kHz.
Also, Seoul, unless you have very high current diodes that can handle the abuse (which I think you would be hard pressed to find), a Cockcroft Walton multiplier is typically used as a high ripple, low current high voltage source, not an impulse generator. Typical current is generally less than a few milliamps, though Steve's seemed to have more than that (probably around 20mA), as the arc was a yellow power arc instead of a purple low current arc.
Registered Member #1497
Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801
From what I can figure, 940c are available from Digikey, but 942c's are the most sought after with the highest dV/dT are not available from digikey (I think the last time I checked they were from Mouser). 942c's are available on ebay however I'm not sure how reputable some people are.
Registered Member #1630
Joined: Sat Aug 09 2008, 11:36AM
Location: Seoul Korea
Posts: 115
Firefox wrote ...
Thanks Seoul. I am planning a large-ish 60-100kV CW multiplier using the caps I ultimately decide to use. I was asking about these caps, because even half rated, their voltage stand-off (under oil anyway) is much cheaper per kV than CDE caps, and I wasnt terribly concerned about the capacitance. I've used 22nF film caps in a small multiplier at high frequency and got a very reasonable result. Heck, I've used 56 pF caps, and they still worked well, though I believe the driving frequency was around 40 or 50kHz.
Also, Seoul, unless you have very high current diodes that can handle the abuse (which I think you would be hard pressed to find), a Cockcroft Walton multiplier is typically used as a high ripple, low current high voltage source, not an impulse generator. Typical current is generally less than a few milliamps, though Steve's seemed to have more than that (probably around 20mA), as the arc was a yellow power arc instead of a purple low current arc.
Diodes aren't hard to find here in Seoul ( have have seen loads of them), I would suggest MOT oven diodes. They will probably work fine. They are rated for 2-3Kv 450mA ... I have never built a large one before but if my calcs are correct this should be perfectly fine. They are exactly cheap.
Registered Member #1389
Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
Diodes aren't a problem, in fact, I just ordered 40 8kV 10mA diodes for this CW. I'm just saying that you need fairly good ones to use a CW as an impulse generator. Putting a large limitting resistor (100x 10k resistors in series, for example) on the output solves that problem entirely by limiting the output current to those levels.
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.
Capacitor Dielectrics and Applications
