high voltage dummy loads

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Capacitive loads for my pulse generator, just copperclad FR4. Should be good for a few KV. I had to Dremel the little one on the back side to get it down to 5 pF.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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John Larkin
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You left out the dimensions.

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Reply to
Winfield Hill

Trade secrets.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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Reply to
John Larkin

He doesn't know how to do the calculations.

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has got a chapter on Impedance Standards, starting at page 50 with the Thompson-Lampard calculable cross-capatitor and proceeding to slightly more practical secondary standards over the next 19 pages.

Better to go to the horse's mouth than the horse's ass.

Amazon seems to be claiming that the book is back in print, which is good news.

Maybe John could buy one. It doesn't say nice things about him - written too early (in 1984) - or about rich Republicans - not the target audience, so he may not be interested.

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Bill Sloman, Sydney
Reply to
bill.sloman

Sometime if I need a high voltage capacitor of somewhat larger capacitance I'm gonna try an "array" of pickle jars filled with aqueous ammonia and wrapped in aluminum foil, immersed in a mineral oil bath.

Reply to
bitrex

I used copperclad FR4 capacitors in an induction heater, but they got very hot and I had to change them to something less lossy.

Reply to
Chris Jones

A PTFE (teflon) substrate might be less lossy.

Rogers offer a wide choice of substrates.

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Bill Sloman, Sydney
Reply to
bill.sloman

LOL! Seriously, you should go into production with those, John. The market for multi KV 5pf dummy loads is seriously under-served. :-)

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Reply to
Cursitor Doom

John Larkin wrote in news: snipped-for-privacy@4ax.com:

High voltage coax. Very specific per foot capacitance... at HV levels. Very easy to construct a precise HV capacitive load with it.

Reply to
DecadentLinuxUserNumeroUno

Depends on the risetime. Say 30pf is 1 ft. A 100ps risetime pulse would ring like crazy. During the initial portion, the coax would not look capacitive. It would look like the characteristic impedance of the coax. I think JL wants a lumped capacitance.

Reply to
Steve Wilson

Steve Wilson wrote: ..

If you can supply kV with 00ps risetime you are a genius :-)

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Uwe Bonnes                bon@elektron.ikp.physik.tu-darmstadt.de 

Institut fuer Kernphysik  Schlossgartenstrasse 9  64289 Darmstadt 
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Reply to
Uwe Bonnes

It certainly is less lossy.

There are ex-USSR capacitors on ebay claiming to have PTFE dielectric, which might be cheaper and thinner than the Rogers stuff, though some pictures of disassembled ones on the internet suggest that the capacitors as sold may not have extended foil nor connections with high enough current rating. Disassembling them for the PTFE film might be reasonable value though, if you don't have another source of it.

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Reply to
Chris Jones

IIRC, 100ps is just possible in the lowish kV, with a liquid filled spark gap in a coaxial trigger setup.

I'm not sure what Steve is thinking about lines as capacitors, though -- as soon as you've got a time spec, if you want the line to look like a capacitor, the line length must be significantly shorter than that. At

100ps, your "line" has to be smaller than a 1206 chip cap. Otherwise, a line is a line, and... what's wrong with that? Transmission lines are only slightly more work than capacitors.

Tim

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Reply to
Tim Williams

Not my idea. snipped-for-privacy@decadence.org proposed:

"High voltage coax. Very specific per foot capacitance... at HV levels. Very easy to construct a precise HV capacitive load with it."

I was the one who pointed out the risetime problem.

Reply to
Steve Wilson

=1

s.

ing

e.

ants

Nobody is insisting that you use just one chunk of coax. A nice radial arra ngement of shorter bits of coax would present a lower characteristic impeda nce, and shorter propagation times. Using different lengths for the various chunks of coax could smear out the voltage-doubling reflections from the u nterminated open ends.

Plenty of room for ingenuity, of the kind John Larkin seems to think he off ers.

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Bill Sloman, Sydney
Reply to
bill.sloman

It can be done with avalanche transistors or Grehkov's second (avalanche breakdown) diode effect.

Apparently microwave weapons use some sort of closing switch, maybe diamond films, to pump terawatts into antenna arrays.

My pulses are fast enough that I need a lumped capacitor as the dummy load. Coax looks like 50 ohms at first.

I could order a bunch of HV caps from Digikey, but the FR4 was just there.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

Steve Wilson wrote in news:XnsA8B33082B814Eidtokenpost@69.16.179.22:

raw=

I see what you are saying. The end of the segment gets insulated. That is what makes a small HV capacitor out of it. So you are saying the the shape of the "plates" makes a difference and that his FR4 spaced copper plates have no such characteristics?

Reply to
DecadentLinuxUserNumeroUno

It's the length of the coax, the prop delay, and the risetime of the signal.

Copper plates definitely have a resonance. They are used as microwave antennas for GPS and smartphones. But I think that's a bit above where JL is using them.

Reply to
Steve Wilson

Ah, re-reading it now, I see what you meant. Cheers,

Tim

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Seven Transistor Labs, LLC 
Electrical Engineering Consultation and Contract Design 
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Reply to
Tim Williams

You just need more, shorter, pieces of coax. ;-)

Reply to
krw

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