Win's next 10kV project, a 1us ramp

[Winfield Hill]

Ken Smith wrote...

Winfield Hill wrote:
[....]

I can't quite picture what you're saying, can you expand? What
do you mean, splitting the shield? Are you saying the coax will
have 10kV across its dielectric for the last primary section?

I've never done this at 10KV but this is what I think he means to do:

============= =============== ===============
Center =============================================================
============= =============== ===============
A B A B A B

You get a 3:1 turns ratio if you put this on a core and hook the "A"s
together and the "B"s together.

Yes, but with 10 sections and 10kV, it's a sure recipe for coax-
breakdown trouble, especially with the high voltage gradients at
the ends of each section for the sections near the 10kV end. It
should provide low leakage inductance, likely better than I need.

 

[Ken Smith]

Ken Smith wrote... [....]

============= =============== ===============
Center =============================================================
============= =============== ===============
A B A B A B

You get a 3:1 turns ratio if you put this on a core and hook the "A"s
together and the "B"s together.

Yes, but with 10 sections and 10kV, it's a sure recipe for coax-
breakdown trouble, especially with the high voltage gradients at
the ends of each section for the sections near the 10kV end. It
should provide low leakage inductance, likely better than I need.

Its not a "sure recipe" for breakdown but certainly a good start on
getting breakdown. COAX that is able to handle 10KV is going to be bulky,
perhaps too bulky to handle.


You may be able to get good enough leakage with ordinary methods. One
thing worth looking at is enclosing the transformer in copper. If you
keep the lines of force from getting away, you generally reduce the
leakage inductance.

 

[Sven Wilhelmsson]

Ken Smith wrote... [....]

============= =============== ===============
Center =============================================================
============= =============== ===============
A B A B A B

You get a 3:1 turns ratio if you put this on a core and hook the "A"s
together and the "B"s together.

Yes, but with 10 sections and 10kV, it's a sure recipe for coax-
breakdown trouble, especially with the high voltage gradients at
the ends of each section for the sections near the 10kV end. It
should provide low leakage inductance, likely better than I need.

Its not a "sure recipe" for breakdown but certainly a good start on
getting breakdown. COAX that is able to handle 10KV is going to be bulky,
perhaps too bulky to handle.


You may be able to get good enough leakage with ordinary methods. One
thing worth looking at is enclosing the transformer in copper. If you
keep the lines of force from getting away, you generally reduce the
leakage inductance.

There seems to be a fundamental problem here: What we gained in lower
leakage inductance, must be payed for with extra capacitance.
So the answer would be to have a floating driver on each section of primary,
be it coax or conventional. Complicated but possible.
/Sven Wilhelmsson

 

[John Larkin]

Ken Smith wrote... [....]
============= =============== ===============
Center =============================================================
============= =============== ===============
A B A B A B

You get a 3:1 turns ratio if you put this on a core and hook the "A"s
together and the "B"s together.

Yes, but with 10 sections and 10kV, it's a sure recipe for coax-
breakdown trouble, especially with the high voltage gradients at
the ends of each section for the sections near the 10kV end. It
should provide low leakage inductance, likely better than I need.

Its not a "sure recipe" for breakdown but certainly a good start on
getting breakdown. COAX that is able to handle 10KV is going to be bulky,
perhaps too bulky to handle.


You may be able to get good enough leakage with ordinary methods. One
thing worth looking at is enclosing the transformer in copper. If you
keep the lines of force from getting away, you generally reduce the
leakage inductance.

There seems to be a fundamental problem here: What we gained in lower
leakage inductance, must be payed for with extra capacitance.
So the answer would be to have a floating driver on each section of primary,
be it coax or conventional. Complicated but possible.
/Sven Wilhelmsson

Then you may as well put the floating drivers in series and dump the
transformer.

Actually, I think a 10:1 or 12:1 transmission-line transformer, made
from RG-58 maybe, would work fine.

John

 

[Rich Grise]

Over the years I've completed a number of projects involving
roughly 10kV voltages: fixed sources, fast-shutoff, reversible
programmable DC sources, precision-ramped AC sources at 300kHz
to 1MHz with 10kV maximum amplitude, etc.

Now I'm challenged with a creating a moderately-precise (5%),
programmable, fairly-fast 10kV ramp, of 1 to 50us in duration.
It's floating on 3kV. The 1us-long, 3kV to 13kV ramp spec is
tough, but may have a relaxed 5%-precision spec, provided it's
repeatable.

And you, of all people, are asking for advice from this pack of
weinerheads? ;-P

Good Luck!
Rich

 

[Winfield Hill]

Rich Grise wrote...

And you, of all people, are asking for advice from this pack
of weinerheads? ;-P

What, you didn't enjoy the two, three days of discussion?

 

[martin griffith]

Rich Grise wrote...

What, you didn't enjoy the two, three days of discussion?

Yep, good fun, almost understood some of it. This Vbe stuff, has
anyone documented it, or can I download the internet onto a floppy or
two?


martin

 

[Michael A. Terrell]

Yep, good fun, almost understood some of it. This Vbe stuff, has
anyone documented it, or can I download the internet onto a floppy or
two?

martin

No, it takes three DVDs. ;-)

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Terry Given]

Joerg wrote...



Wow, up to 6-inch diameter by 3/4-inch thick. Nice.

I saw a powdered iron pot core a few years back that was about 12" OD,
perhaps 10" high, and far too heavy to hold in one hand. Alas I forget
who made them.

Cheers
Terry

 

[Mark Fergerson]

martin griffith wrote:

No, it takes three DVDs. ;-)

Hmmm, subtract the porn, the copyrighted stuff that's illegal anyhow,
the political BS, manuals for stuff nobody owns anymore, and you're
getting close...


Mark L. Fergerson

 

[Michael A. Terrell]

Hmmm, subtract the porn, the copyrighted stuff that's illegal anyhow,
the political BS, manuals for stuff nobody owns anymore, and you're
getting close...

Mark L. Fergerson

Well, if you include that stuff, you'll need a few more DVDs. ;-)


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Joseph2k]

John Larkin wrote...

Guess it's going to be a stack of FETs...

Please check me on this but there is supposed to be something like a field
emission switch, speed like a fast SCR, power handling like a thyratron.
It would switch kilovolts in 10 to 20 nanoseconds and handle 10's to 100's
of Amperes. decent durability too.

 

[Winfield Hill]

Joseph2k wrote...

Please check me on this but there is supposed to be something like a field
emission switch, speed like a fast SCR, power handling like a thyratron.
It would switch kilovolts in 10 to 20 nanoseconds and handle 10's to 100's
of Amperes. decent durability too.

You're probably thinking of something made by PerkinElmer,
http://optoelectronics.perkinelmer.com/catalog/Category.aspx?CategoryName=High+Energy+Switches

 

[Joseph2k]

Joseph2k wrote...

You're probably thinking of something made by PerkinElmer,
http://optoelectronics.perkinelmer.com/catalog/Category.aspx?CategoryName=High+Energy+Switches

I looked through the catalog but those things do not behave like what i read
about. One specific advantage was faster switching that the spark gap
things.

 

[Joseph2k]

I still sort of like the idea of a linear Marx generator. Have, say, a
dozen mosfet ramp generators, all sitting on ground. Charge their
supply caps to 800 volts apiece, then trigger them to start ramping
together, with steering diodes to "erect" the string.

Transformers might be interesting, except for the 50:1 time spread.

John

The more i read and think about this the more it sounds like a sweep circuit
with a high voltage flyback transformer for the output ramp.

 

[Joseph2k]

It's been a lo-o-o-ong time since I've lurked around a TV repair shop.
Wonder how fast HV regulator tubes like 6BD4/6BK4 can be pushed?

...Jim Thompson

i have been digging around and they can handle about 20 to 25 mA continuous
and about 200 mA peak 1% duty cycle and gave up on them as a poor target.
But that set me off onto power tetrodes from Eimac. Easy to get serious
overkill but they are available and you could probably drive them grounded
grid from a single 1kV 3A FET. Assuming relatively clean triangular pulses
the power dissipation in the FET won't be too bad. And a 4CX35000 can
certainly do its part; 20 kV anode / plate, 6A, PD 35000W (used in VHF
transmitters to 110 MHz and 195 kW). Last problem is that output mode is
still current rather than voltage, and i don't know how important getting a
voltage ramp is. I don't recall what the intended repetition rate is but
IIRC it as about 1 PPS? This setup could go 1000 times faster (save test
time?)

 

[Winfield Hill]

Ancient_Hacker wrote...

Look up any of the radar modulator tubes. The old WW2 715-ABC can
handle like 3 AMPS at 20KV. At a short duty cycle of course. They're
relatively cheap. Still being made under some other number IIRC.

I haven't found any of them, but 5D21 tubes are available from
WW-II days. Thanks for the suggestion, they look promising.

 

[Winfield Hill]

Joseph2k wrote...

i have been digging around and they can handle about 20 to 25 mA continuous
and about 200 mA peak 1% duty cycle and gave up on them as a poor target.

Looking at the curves, I'm amazed they can go that high, is that with
a positive grid and a high plate voltage?

But that set me off onto power tetrodes from Eimac. Easy to get serious
overkill but they are available and you could probably drive them grounded
grid from a single 1kV 3A FET. Assuming relatively clean triangular pulses
the power dissipation in the FET won't be too bad. And a 4CX35000 can
certainly do its part; 20 kV anode / plate, 6A, PD 35000W (used in VHF
transmitters to 110 MHz and 195 kW).

Last problem is that output mode is still current rather than voltage,
and i don't know how important getting a voltage ramp is.

We need a -3kV to -13kV voltage ramp, but a good way to get that is
a constant sink current into the node capacitance. For example, if
the capacitance is 100pF we need 1A for 1us. I checked the 4CX35000.
Whoa! It's huge, 50# and 17" high by 10" dia, and needs 300W in the
filament, sheesh! http://www.geocities.com/aaron_white/mwtube.html

I don't recall what the intended repetition rate is but IIRC it as
about 1 PPS? This setup could go 1000 times faster (save test time?)

We're still evaluating the cycle time.

 

[Joseph2k]

Joseph2k wrote...

Looking at the curves, I'm amazed they can go that high, is that with
a positive grid and a high plate voltage?



We need a -3kV to -13kV voltage ramp, but a good way to get that is
a constant sink current into the node capacitance. For example, if
the capacitance is 100pF we need 1A for 1us. I checked the 4CX35000.
Whoa! It's huge, 50# and 17" high by 10" dia, and needs 300W in the
filament, sheesh! http://www.geocities.com/aaron_white/mwtube.html


We're still evaluating the cycle time.

I did not say it was small, i said it would be a "one part" solution to the
kV problem with a 1A current.

 

[Winfield Hill]

Joseph2k wrote...
Yes, harder... I can easily handle a design for 50:1 spread with
low-droop requirements, for an "ordinary" transformer, but for one
insulated for 10kV, I'm not so confidant.

The more i read and think about this the more it sounds like a sweep
circuit with a high voltage flyback transformer for the output ramp.

Maybe, maybe not. What you get that's linear on a typical HOT is
the charging-current ramp, and not necessarily the transformer's
output-voltage, which doesn't have an ramp as a goal. Still, a
discharging flyback transformer could exhibit a constant output
current, making a linear ramp into a capacitor... So, I'm still
thinking about it.