Suggestions for CW HV feed-through

[MRMILSTAR]

My next project is going to be a large (300 - 400 KV) Cockroft-Walton generator. I want to house it in a 24" x 4" PVC tube. The CW generator will be operated vertically. The entire unit will be immersed in oil. My conundrum is serviceability. I am not going to assume that I will never need to repair this, therefore I will need to be able to remove the electronics from the 4" tube. I am envisioning all of the components mounted on a plastic strip or smaller PVC tube that fits inside the 4" main tube. That would enable me to withdraw the entire assembly from the 4" tube for repair if needed.

My biggest unknown right now is how to get the two HV power input leads through the bottom of the tube without leaking oil and to be able to remove the electronics assembly. I obviously won't be able to reach down into the bottom of the tube to disconnect the two HV power input leads. The best idea that I have come up with is to be able to slide the outer 4" tube off of the base. The base would probably be some kind of PVC end-cap. To make that joint oil-tight I thought about machining a groove into the 4" tube so that an O-ring could be installed between the tube and the end-cap.

Any ideas?

[shrad]

I know some shops do custom visit chambers in PVC, by joining pieces together in one piece assemblies... maybe they could assemble smaller tubes at the bottom of a bigger one to make for convenient feeds?

[klugesmith]

The separable joint with O-ring sounds like a good idea.  Mind the compatibility of your oil and O-ring material.
It might be easier to make a bottom end _plug_ with o-ring on its outside, or cap with o-ring groove on inside.
Here's a great handbook with groove design information and much more.
https://www.parker.com/literature/Praedifa/Catalogs/Catalog_O-Ring-Handbook_PTD5705-EN.pdf

For non-separable electrical feedthroughs, how about metal through-bolts sealed with glue?  Or little spark plugs?

[Twospoons]

I posted this on 4hv too:
PCB edge connector, with some of the central pins removed, or just a couple of banana sockets sealed into the bottom. Either would allow you to simply unplug your multiplier.

[MRMILSTAR]

I posted this on 4hv too:
PCB edge connector, with some of the central pins removed, or just a couple of banana sockets sealed into the bottom. Either would allow you to simply unplug your multiplier.

Do you think that the banana plug would make a good electrical connection being immersed in oil?

I am also thinking about using clear PVC tubing for aesthetic reasons. This would also allow me to see if the banana plug male and female ends were in alignment.

[davekni]

Banana plugs/jacks sounds like a good solution as long as you have the alignment issue solved.  The stamped metal "banana-peel" part typically produces sharp edges of contact, which likely have enough pressure to break through the oil film.  You could bend the banana-peel tabs a bit to create even more pronounced edge contact rather than contact along the surface of the stamped metal.  I'd suggest having the male plug parts on the multiplier in case you need to tweak the tab bending and/or find a part with stronger (higher insertion force) tabs.

BTW, have you ran any simulations on the multiplier including parasitic capacitance between the two capacitor columns?  I was surprised how much voltage loss came from capacitive coupling between columns.  (If you are making a high-power unit with relatively large column capacitors, stray capacitance may not be significant.  I was purposely using as small capacitors as possible for safety - using it as a Van de Graaff alternative.  Also added a string of 30 x 10meg 10kV resistors in series with the output.)

[MRMILSTAR]

Banana plugs/jacks sounds like a good solution as long as you have the alignment issue solved.  The stamped metal "banana-peel" part typically produces sharp edges of contact, which likely have enough pressure to break through the oil film.  You could bend the banana-peel tabs a bit to create even more pronounced edge contact rather than contact along the surface of the stamped metal.  I'd suggest having the male plug parts on the multiplier in case you need to tweak the tab bending and/or find a part with stronger (higher insertion force) tabs.

BTW, have you ran any simulations on the multiplier including parasitic capacitance between the two capacitor columns?  I was surprised how much voltage loss came from capacitive coupling between columns.  (If you are making a high-power unit with relatively large column capacitors, stray capacitance may not be significant.  I was purposely using as small capacitors as possible for safety - using it as a Van de Graaff alternative.  Also added a string of 30 x 10meg 10kV resistors in series with the output.)

I haven't done any simulations yet. I am still in the conceptual and learning phase right now. I do have 28 TDK doorknob capacitors with 1.7 nF capacitance that I bought some time ago for a future project such as this. They are rated for 30 KV so they are physically rather large, probably about 2" in diameter by 0.5" thick. If I make a half-wave multiplier I could have up to 14 stages. I am planning on using an AC flyback transformer driven by a ZVS driver.

Given that the parts count of a full-wave circuit is higher than a half-wave circuit, is there any significant advantage of using a full-wave circuit over that of a half-wave circuit? I don't really care about voltage ripple since I am only using this to make long sparks.

[davekni]

Steve,

I'd thought that "full-wave" applied to only voltage-doubling.   Is there a circuit for higher multiplication factors that's called "full-wave"?

At 1.7nF and 14 stages, parasitic capacitance will have some effect, but probably not excessive.  (I have 16 stages starting with only ~550pF.)

Another thought for your interconnect at the bottom:  How about using a pair of battery-contact springs on the bottom of the multiplier and two pieces of nickle or nickel-plated sheet metal at the bottom of the housing?  Alignment could have very loose tolerances that way.  I expect the weight of your multiplier is well more than twice the contact force of a typical AA battery holder.

[klugesmith]

Dave's suggestion for horizontal contact surfaces (vertical contact force) inspires thought about radial geometry.
If there are only two connections to be made, have one on axis and the other at some fixed radius and any angle (one part being a flat ring or short cylindrical sleeve).
Additional contacts are straightforward (roundforward?) in concept.

Nice thing about [blind] mating between multiplier rack and the bottom of oil pot:  you can pull out the multiplier without even draining the oil.

Steve's clear tubing idea would still be clearly better for show and tell purposes, or in case of internal D or C failure.  Maybe you can include little neon lamps, with leads serving just as antennas, whose glows would show that each stage is working.  If you can afford the parasitic power loss.

There are clear plastic tube materials other than PVC, for example acrylic and polycarbonate, worth checking prices on. Tap Plastics comes to mind.
Diameters and wall thicknesses are different from PVC pipe.  Where do you all like to go for clear PVC by the foot (or meter)?

[edit] Re. the topic title, electrical feedthroughs without much fluid pressure:
A set I made 13 years ago was still holding water last time I checked.  Couldn't find it tonight, but found it in a picture from 2006. 

The plastic water bottle holds copper sulfate solution and two flat copper plates, connected to outside with soldered-on copper screws, through holes sealed with clear RTV silicone caulk.   Designed for about 100 ohms at room temperature, to discharge multi-kJ high voltage caps quickly and quietly.  Tested to work well for that.  In a trial a year or two ago, I connected it to 120 volts AC and monitored the current, which rises as the electrolyte warms up. 
Don't know if that 2x4 is still around, with the most wane I have ever seen in a piece of dimensional lumber.  What are the words in other languages for that kind of defect in a saw mill product (butcher shop for trees)?

[MRMILSTAR]

Steve,

I'd thought that "full-wave" applied to only voltage-doubling.   Is there a circuit for higher multiplication factors that's called "full-wave"?

At 1.7nF and 14 stages, parasitic capacitance will have some effect, but probably not excessive.  (I have 16 stages starting with only ~550pF.)

Another thought for your interconnect at the bottom:  How about using a pair of battery-contact springs on the bottom of the multiplier and two pieces of nickle or nickel-plated sheet metal at the bottom of the housing?  Alignment could have very loose tolerances that way.  I expect the weight of your multiplier is well more than twice the contact force of a typical AA battery holder.

Dave,

Attached is a comparison between the circuits for a half-wave and a full-wave CW multiplier. I won't be using the full-wave circuit because of the increased parts count. The full-wave circuit also requires a center-tapped transformer. I like the battery spring concept idea.

[MRMILSTAR]

I also like the circular contact idea.

I have seen clear PVC tubing for sale on E-Bay. I actually have some acrylic tubing. The problem is that my research seems to indicate that acrylic tubing is susceptible to attack by petroleum-based oils while PVC is not.  If anyone knows of some type of oil that will not degrade acrylic over time, I would like to hear about it. I am not really interested in vegetable oils though because they can turn rancid over time.

[davekni]

Interesting full-wave schematic.  Requires two AC inputs, presumably 180 degrees out of phase.  When reduced to a voltage-doubler, that does not match the normal circuit called a "full wave voltage doubler", which uses a single AC input:


Of course, what circuit is called what doesn't matter.  The voltage multiplier you are building is the one that makes sense for your project (and for mine and likely most others).

[MRMILSTAR]

I really like this person's 20-stage CW generator. It has the type of construction that I am considering.

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[klugesmith]

I actually have some acrylic tubing. The problem is that my research seems to indicate that acrylic tubing is susceptible to attack by petroleum-based oils while PVC is not.  If anyone knows of some type of oil that will not degrade acrylic over time, I would like to hear about it.

How soon can you cut a piece from your acrylic tubing, and one from PVC pipe, and start an immersion test under plain mineral oil?  Maybe even under elastic stress. For example, a ring shaped sample could be made oval with a stick inside that's too long. Test could be made in a cookie can if there's no sufficiently wide-mouth jar on hand.

One could spend days looking up compatibility charts on the Internet, mostly with short list of elastomers or plastics and long list of chemicals.
There are some with short list of chemicals, like dielectric oil families. 

https://www.midel.com/blog/midel-7131-materials-compatibility/
MIDEL 7131, an ester based insulating oil, is claimed to play well with PMMA (acrylic) and polycarbonate; no mention of PVC there.
At the risk of getting off topic, there's plenty of paper and wood used under oil in great big transformers.