Author Topic: My 14-stage Cockroft-Walton voltage multiplier  (Read 663 times)

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My 14-stage Cockroft-Walton voltage multiplier
« on: April 13, 2021, 09:35:51 PM »
After 6 months of work I have finally finished my Cockroft-Walton voltage multiplier. It is a half-wave design with 14-stages. The specifications are:

* 14-stages, half-wave, oil immersion
* 28 TDK doorknob capacitors, each with 1.7 nf rated for 30 KV
* Each diode string consists of 3 diodes in series, with each diode being rated for 20 KV and 30 ma
* HV power supply consists of 2 AC flybacks in parallel driven by a ZVS circuit
* HV power supply voltage is 15 KV peak AC with 30 volt, 10 amp LV DC power supply
* HV power supply charging frequency is 28 Khz
* LV DC power supply is a SMPS capable of 30 volts at 10 amps
* Designed output voltage for 750 microamp load is 387 KV to 415 KV
* Output streamer is approximately 32 cm in length with toroids
* Load resistance consists of 24 Ohmite resistors in series-parallel with each resistor rated for 2.5M ohms, 50 KV, 12.5 watts
* Composite load resistance spec is 6.8M ohms, 400 KV, 300 watts

There is a front panel DC voltmeter to ensure that the input voltage is between 15 and 30 volts before switching on the multiplier. This is to prevent the ZVS driver transistors from being damaged due to insufficient input voltage. There is a remote on-off switch so that you don't have to be too close when you switch it on.

The entire multiplier stack is housed in a 4" ID clear PVC tube filled with mineral oil. The load resistance and discharge tower operate in air. Everything was designed to be easily serviceable. I have already had to remove the multiplier stack from the tube for modification. Even with oil it wasn't that bad.

The peak AC input voltage to the multiplier stack was measured with a custom-made HV full-wave bridge rectifier that I made just for this purpose. A 2 nf smoothing capacitor is connected to the output of the rectifier. I then use a commercial HV DC probe with built-in voltmeter to measure the DC output voltage of the rectifier. This then is the peak voltage of the AC waveform. With this rectifier, which is removed after the measurement, I determined that a 30 volt DC input yields a 15 KV peak AC output from the flyback transformers.

I had to modify the ZVS driver slightly by adding an additional 0.66 uf of capacitance to the existing capacitance. Two extra 0.33 uf MKP capacitors identical to the six capacitors on the circuit board were used to do this. This was done without having to modify the circuit card by merely connecting the the two extra capacitors across the output terminals. The extra capacitors can be seen in the attached pictures as a small outboard auxiliary circuit card. Adding this extra capacitance increased the streamer length by amount 9 cm so it was a substantial improvement. This addition set the charging frequency at 28 Khz. This is similar to tuning a Tesla coil.

I must say that its sort of creepy to operate if you are within 4 feet of it. The HV DC charges everything that is close by. It makes all of the hair on your body try to stand up and it feels like your skin is crawling. Its like a Van De Graaff generator on steroids. I have to constantly discharge my body to avoid a big charge build-up. Maybe I need to start wearing a ground strap.

Another interesting observation involves optics. The capacitors and diodes are mounted on an acrylic carrier. Apparently, mineral oil and acrylic have an almost identical index of refraction. This causes the acrylic carrier to be almost invisible when immersed in oil. It appears as if the capacitors and diodes are suspended in the oil without support which looks interesting. The tubular shape of the clear PVC tube also acts as a sort of lens which magnifies and distorts the appearance of the capacitors and diodes.

Here are 2 links to Youtube videos from my Youtube channel. The first video is an overview. The second video shows it in operation.

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« Last Edit: April 28, 2021, 10:54:34 PM by MRMILSTAR »
Steve White
Cedar Rapids, Iowa
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Re: My 14-stage Cockroft-Walton voltage multiplier
« Reply #1 on: April 13, 2021, 09:47:39 PM »
Nice craftsmanship!  Looks like you are set for a museum display.
David Knierim

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Re: My 14-stage Cockroft-Walton voltage multiplier
« Reply #2 on: April 13, 2021, 10:17:24 PM »
Nice craftsmanship!  Looks like you are set for a museum display.

Thanks Dave! Half the fun of building these projects for me is making them to a high standard of craftsmanship so it takes me a long time to complete these things.
« Last Edit: April 13, 2021, 10:47:34 PM by MRMILSTAR »
Steve White
Cedar Rapids, Iowa
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Re: My 14-stage Cockroft-Walton voltage multiplier
« Reply #3 on: April 14, 2021, 10:03:39 AM »
Hi Steve,

Great work you have done. I really like the construction techniques you have done here 👍.
And I can understand the amount of time this takes. Very cool 😎.
I watched these 2 videos of this great fella. Looked to see what else you possibly had also, and watched all the videos of other great stuff also, that were recently uploaded too.
👍 Up again...

Kinda of related, to good construction techniques, fairly recently, I have finally decided and have been planning and starting on finalizing my 304er VTTC. Only been, oh I think about 3-4 years now of "heavy" changing around and experimenting with it... decided it is time...I still remember quite a while ago, you mentioning about possibly doing a final setup of it.
Going to take a bit here, just about have everything I need.
Will get around to doing some posts on it.

Again... great job here... 🤓

Chris Reeland
Ladd Illinois USA
Chris Reeland
Ladd Illinois USA
Former Printer

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Re: My 14-stage Cockroft-Walton voltage multiplier
« Reply #4 on: April 14, 2021, 10:31:23 AM »

I must say that its sort of creepy to operate if you are within 4 feet of it. The HV DC charges everything that is close by. It makes all of the hair on your body try to stand up and it feels like your skin is crawling. Its like a Van De Graaf generator on steroids. I have to constantly discharge my body to avoid a big charge build-up. Maybe I need to start wearing a ground strap.


Hi Steve again,

I think I can relate to this experience some... being a former printer on old large newspaper printing presses...on dry air days during the winter time...the press in certain areas would charge up. You could feel this in your whole body in this charge field. Quite the strange feeling. Some areas of the press would also discharge arcs at times 3-4 foot long! Quite scary up close! Glad I never got hit!
We also at times..."goofed off" a little with these charge fields...we would "touch" a co-worker. Did not really touch for real...a arc would jump that was typically 3-4 inches long...once near enough. This hurt like hell for the both of us. So in actuality...we did not goof too much...since the mischievous person got it too...

Chris Reeland
Ladd Illinois USA
Chris Reeland
Ladd Illinois USA
Former Printer

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Re: My 14-stage Cockroft-Walton voltage multiplier
« Reply #5 on: April 14, 2021, 03:51:36 PM »
Chris, thanks for the kind words! Here are a few detailed in-progress construction pictures of the voltage multiplier which I didn't originally post because I thought they were too detailed. These pictures show how the multiplier stack makes electrical contact with the HV AC power supply. I wrestled with this problem for a long time. The main problem was that the multiplier stack was to be immersed in oil. So how to make a good electrical contact without oil leaking out and also allowing the multiplier stack to be easily removed for service? I hope that these pictures clarify that.

There are two copper contact plates at the bottom of the tube which are fixed and attached to a GP03 carrier (red material). Each contact is connected to a through-hole screw terminal like you see on commercial power supplies. Each through-hole terminal is screwed into a tapped threaded hole in the bottom of the PVC base. The bottom of the multiplier stack has two spring-loaded electrodes. When the stack is lowered into position the spring electrodes make contact with the copper contact plates mounted on the base. There is a horizontal slot that I machined into the GP03 carrier between the two copper contact plates. There is a matching acrylic bar at the bottom of the multiplier stack. The slot-and-bar arrangement acts as a lock-and-key which allows positive alignment to be made between the spring-loaded electrodes and the fixed copper plates. The slot-and-bar arrangement also allows me to do this blindly by feel since I can't see to the bottom of the multiplier stack tube when I'm lowering the stack into the tube. The slot and bar also lock into place and prevents the electrodes from rotating out of alignment. The acrylic bar on the bottom of the multiplier stack also limits the compression depth of the springs so that they are not crushed by the weight of the multiplier stack.

I poured slow-cure (24 hours) 2-part epoxy resin, which flows easily, into the bottom of the base mount up to the level of the bottom of the machined slot between the fixed copper plates. The resin flowed all around the threads and holes of the through-hole screw terminals thus sealing these holes against any oil leakage to the outside. I haven't observed any sign of oil leakage. As an unplanned test I've had to pull the multiplier stack out of the oil-filled tube once for modifications and indeed this system worked well. Service was easy and I was able to lower the stack blindly back into the tube by feel into proper alignment.

There's also some custom PVC supports shown here. I hope that this description made sense.
« Last Edit: May 01, 2021, 04:44:52 PM by MRMILSTAR »
Steve White
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Re: My 14-stage Cockroft-Walton voltage multiplier
« Reply #6 on: April 14, 2021, 07:34:33 PM »
Quote
We also at times..."goofed off" a little with these charge fields...we would "touch" a co-worker. Did not really touch for real...a arc would jump that was typically 3-4 inches long...once near enough. This hurt like hell for the both of us. So in actuality...we did not goof too much...since the mischievous person got it too...

At least for smaller static sparks, if you reach out towards someone with a metal object (piece of foil, key, ...) in your hand, then you will feel little of the shock.  The metal object spreads out the current across a much larger area of skin.  I make use of this regularly when stepping out of my car on dry days.  A firm grip on the inside door handle (metal) while stepping out avoids me feeling any of the shock.
David Knierim

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Re: My 14-stage Cockroft-Walton voltage multiplier
« Reply #7 on: April 14, 2021, 07:42:50 PM »
Hi Steve,

Thanks for some more details and pictures of the electrical connections. Like this a lot.

I have just downloaded all 21 pictures, so I can study them further, closely examing all I can see. I enjoy studying all kinds of details of great construction.

Chris
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Re: My 14-stage Cockroft-Walton voltage multiplier
« Reply #8 on: April 14, 2021, 07:54:38 PM »
Hi David,

I will have to keep that in mind if ever in this situation ever again 😁.

But unfortunately 🤕, I am no longer a Printer anymore...not by choice...they are all gone, the presses, in a very large area...just big city presses left. All smaller newspapers have given up on presses. It is farmed out. Even some fairly large newspapers and newspaper groups, have given up on their own presses. Changing world...

So after 25-30 years...I had to change careers.
I will always have some ink in my blood though...

Chris

Chris Reeland
Ladd Illinois USA
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Re: My 14-stage Cockroft-Walton voltage multiplier
« Reply #9 on: April 14, 2021, 09:34:11 PM »
Hi Steve,

Thanks for some more details and pictures of the electrical connections. Like this a lot.

I have just downloaded all 21 pictures, so I can study them further, closely examing all I can see. I enjoy studying all kinds of details of great construction.

Chris

I attached 2 more pictures at the end of the original post if you want them.
Steve White
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Re: My 14-stage Cockroft-Walton voltage multiplier
« Reply #10 on: April 14, 2021, 10:56:15 PM »
Here's the full-wave bridge rectifier that I built to measure the output of the flyback transformers. Each leg consist of two fast 20 KV diodes in series. The smoothing capacitor is 2 nf rated for 30 KV. The voltmeter is a commercial HV DC unit rated for 40 KV. The measurement is the peak AC voltage output of the flybacks minus a small amount of voltage droop.
« Last Edit: April 15, 2021, 05:53:29 PM by MRMILSTAR »
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Re: My 14-stage Cockroft-Walton voltage multiplier
« Reply #11 on: April 30, 2021, 04:55:18 PM »
A beautiful project! Looks very professional and neatly finished. Thank you very much for sharing!

Best regards
kilovolt  ;)
All information on my part without guarantee! I reject any liability for personal and/or material damage. Everyone is responsible for his own safety.

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Re: My 14-stage Cockroft-Walton voltage multiplier
« Reply #12 on: April 30, 2021, 11:36:20 PM »
Thanks kilovolt!
Steve White
Cedar Rapids, Iowa
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Re: My 14-stage Cockroft-Walton voltage multiplier
« Reply #13 on: Today at 08:50:26 PM »
Very impressive build , looks really good ^^ .

I like how the multiplier joins as the base with the flat plates to provide the connection .

When you calculate the output resistance needed , is this simply the expected output voltage divided by the target current you need ?

Cheers

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Re: My 14-stage Cockroft-Walton voltage multiplier
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