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CW multiplier current limiting?

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Daniel Uhrenholt

Wed Mar 23 2011, 05:32PM

Registered Member #125 Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155

Hi All,

I'm collecting parts for an 600kV CW multiplier at the moment. I have 50 40Kv doorknob capacitors, and 120 20kV 10mA rectifiers arriving within 14 days.

But my big question is how to limit the current the best way so I don't blow the rectifiers?
I have done some LTspice simulations on this, but now it is an question about money...

If I put a 200k resistor in series with the diode, the current don't go over 100mA. But I can't find any resistors rated at 40kV to a price that don't get the hair in my neck to vibrate.

Any suggestions on this?

I will start an thread on this when the parts arrive, this thing is going to be a monster:-)

Cheers, Daniel

Steve Conner

Wed Mar 23 2011, 05:41PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The best way I know of is to use a huge high voltage resistor in series with the output. A helix of plastic tubing filled with copper sulphate solution, with a copper bung in each end, is the classic cheap HV resistor.

Some commercial CWs use one resistor in series with each diode instead. The highest voltage resistors I know of that are cheap and easy to get are the VR37 series.

Diodes will stand brief pulses considerably over their rated current, maybe 50 or 100 times. But a CW is a nasty environment, if the output is suddenly shorted by an arc, the entire stack resonates with its stray inductance and the diodes may end up absorbing most of the energy that was in the capacitors.

Proud Mary

Wed Mar 23 2011, 06:07PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Hi Daniel!

For use with applications needing up to about 2mA, I use a 2.2MÎ© 40kV 10W resistor in series with the CW output, but go down to 1MÎ© 40kV 10W if the voltage drop across the first resistor becomes too large.

This gives me a good level of protection against impulse damage - unwanted sparkdowns and flashovers - as Steve describes.

I also use anti-surge resistors between C&W stages, and between the transformer and the first stage, to mellow the steep cliff time constant.

jpsmith123

Wed Mar 23 2011, 07:03PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

I've been agonizing over this same problem for a while now.

One of my first questions would be: How much load current do you expect to draw from it?

With diodes rated at only 10 mA, depending on the number of stages and the load current, I can see the diodes having a problem, without even drawing an arc.

In any case, according to the manufacturer, those diodes are meant to be operated in oil or in an encapsulant like epoxy. This is necessary because they'll flash over in air at a point below their rated voltage. The oil or epoxy also helps remove heat, which will make them somewhat less susceptible to failure from over-current.

Personally I don't like the idea of having to worry about electrically/thermally fragile diodes blowing out in the event of an arc or overload of some kind (especially if they have to be in oil or encapsulated in epoxy or RTV), so I ended up just spending some extra money and buying the 10kv/1A diodes from OT, but I'm going with a full wave quadrupler, so I don't need too many pieces.

If I was going to build something really big, I think I would resign myself to stringing some low voltage diodes together, something like what Uzzors did. (BTW he draws some nice hot arcs from his and uses no limiting resistors, AFAIK).

IIRC, you can buy Vishay 2 kv, 0.5 A diodes for $0.10. It would no doubt be a big PITA, but that may be the way to go.

Daniel Uhrenholt

Wed Mar 23 2011, 07:48PM

Registered Member #125 Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155

Steve,

I know that I need a output resistor, and for that I will make an 30mega ohm one to connect to the toroid. I'm aiming to get 500uA-1mA out of this machine:-)

I think I will sleep on the informations you guys gave me, and run some numbers when I get to work tomorrow. I will update this thread with my design goals tomorrow afternoon, and my calculations on this.

Cheers, Daniel

Arcstarter

Wed Mar 23 2011, 08:50PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Another homemade high value resistor i have seen is black vacuum cleaner hoses. They contain carbon so have some conductivity, but i have never used such a thing. It would be much easier and cleaner than a copper sulphate solution, but i cannot seem to find anything on google showing anything about their performance. But, at only 1ma, i think a length of some small tubing would work for such a thing.

Just another crazy idea.

klugesmith

Wed Mar 23 2011, 11:05PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Steve McConner wrote ...
The best way I know of is to use a huge high voltage resistor in series with the output. A helix of plastic tubing filled with copper sulphate solution, with a copper bung in each end, is the classic cheap HV resistor.

[edit] my comment below pertains to instrumentation resistors, not simple current limiting (as this thread is titled!) Sorry.

Yes, but the big & little ends of the voltage divider should be made of the same kind of tubing,
because the electrolyte conductivity is -very- termperature dependent & can warm quickly.
I can plug this thing into the wall & watch the current increase, second by second, as the bottle warms up.

James

Thu Mar 24 2011, 01:02AM

Registered Member #3610 Joined: Thu Jan 13 2011, 03:29AM
Location: Seattle, WA
Posts: 506

Arcstarter wrote ...

Another homemade high value resistor i have seen is black vacuum cleaner hoses. They contain carbon so have some conductivity, but i have never used such a thing. It would be much easier and cleaner than a copper sulphate solution, but i cannot seem to find anything on google showing anything about their performance. But, at only 1ma, i think a length of some small tubing would work for such a thing.

Just another crazy idea.

Vacuum cleaner hose? I've heard of using automotive vacuum hose, it's small diameter black rubber tubing, but I've never heard of using vacuum cleaner hose. There's no particular standard in that stuff.

Proud Mary

Thu Mar 24 2011, 09:25AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I've bought HV resistors from Alex a number of times with good results, and Patrick did too recently.

Daniel Uhrenholt

Tue Mar 29 2011, 01:37PM

Registered Member #125 Joined: Fri Feb 10 2006, 01:52PM
Location: Aalborg, Denmark
Posts: 155

Hi All,

I know that Iâ€™m a bit late with this, but I have been busy with other things.

I have done a lot of simulations on this, and I think that I have a design that will work now. Each diode will have a limiting resistor of 300k, and the first caps will be limited with two 200k resistors. The big limiting resistor will be made out of 40 1 Mega Ohm VR68 resistors, as they are good for 10kv each, so they will live under over voltage, but I think it will work just fine. This will keep things cold and under control, and hopefully not blow the thing to pieces.

This is the schematic of the thing

This is the output voltage and current of two of the diodes

1301404017 125 FT111381 Rectifier Current And Output Voltage

short circuit current across the diodes when the output is shorted to ground

1301405355 125 FT111381 Short Circuit Current Diodes

Shortcircuit current of the CW

1301405488 125 FT111381 Short Circuit Current

I still have a great deal of things to design and simulate before I will begin the build. A 20kHz 20kV center tapped supply to drive this beast from.
I will keep you all updated.

Cheers, Daniel

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