Super High Voltage Capacitor

[Tsindur]

I've got this high frequency x-ray transformer with a full bridge rectifier at the output which can do 150kvp 400ma for like 5 seconds. I am planning on the removing the diode strings and put two pairs in series for a high voltage Tripler. The issue is capacitors. How could I buy a 2x 300+kv capacitor for a reasonable price or at least one that is a cylindrical shape and not like a huge cube since that is all stacking mylar will get me. I do know it is possible since I have seen ones online for sale, however they only sell to businesses.

[Mads Barnkob]

I would solve that with many smaller capacitor in series, just like we do for Tesla coils.

I am not sure if there is a better way for you to do this, my experience with voltage multipliers is too weak.

I made a multi mini capacitor calculator that could help you: https://kaizerpowerelectronics.dk/calculators/mmc-calculator/

[Tsindur]

I would solve that with many smaller capacitor in series, just like we do for Tesla coils.

I am not sure if there is a better way for you to do this, my experience with voltage multipliers is too weak.

I made a multi mini capacitor calculator that could help you: https://kaizerpowerelectronics.dk/calculators/mmc-calculator/

I could also try to make one myself however I'm not sure how I could make one that is relatively lean and last a decent amount of time. The issue with the smaller capacitors in series is that the capacitance would me atrociously small once I achieve the 300kv needed and would probably cost more than just buying a professional 300kv cap from online.

[Twospoons]

So how much capacitance do you actually need? A few 100pF at more than 300kV is going to be big no matter how you do it.

[Tsindur]

So how much capacitance do you actually need? A few 100pF at more than 300kV is going to be big no matter how you do it.

I would need like about 350+pf I am fine with it being large just cylindrical. If I try making my own capacitor with mylar rated for 300kv it wouldn't even be able to roll up.

[Twospoons]

If you do 'roll-your-own' (in both senses of the phrase) then using multiple thin dielectric sheets is better than using a single thick sheet. The rationale for this is that all plastic sheet will have defects, but with multiple thin sheets these defects are unlikely to line up, and so the multi-sheet stack has better performance.

Then again, you'd need ~0.06" (1.5mm)  total thickness in Mylar, so that will be quite a few layers.

Alternate would be something like 100mm foil discs with 3mm PMMA,PET or PC sheet - that would net you ~80pf, so thats only 5 metal layers to hit ~300pf. You could hot-press the stack to seal the edges and get rid of air.
A 200mm dia foil would get you 300pf in one layer. Then you can take your lead out from the middle of the disc, on each side.
I think I've seen PET sheet thats thinner than 3mm, so that would also be a option.
If it must be cylindrical you could use PMMA tube, foil on the outside, and either metal tube on the inside, or possibly salt water?

[Tsindur]

If you do 'roll-your-own' (in both senses of the phrase) then using multiple thin dielectric sheets is better than using a single thick sheet. The rationale for this is that all plastic sheet will have defects, but with multiple thin sheets these defects are unlikely to line up, and so the multi-sheet stack has better performance.

Then again, you'd need ~0.06" (1.5mm)  total thickness in Mylar, so that will be quite a few layers.

Alternate would be something like 100mm foil discs with 3mm PMMA,PET or PC sheet - that would net you ~80pf, so thats only 5 metal layers to hit ~300pf. You could hot-press the stack to seal the edges and get rid of air.
A 200mm dia foil would get you 300pf in one layer. Then you can take your lead out from the middle of the disc, on each side.
I think I've seen PET sheet thats thinner than 3mm, so that would also be a option.
If it must be cylindrical you could use PMMA tube, foil on the outside, and either metal tube on the inside, or possibly salt water?

I considered the idea of using the tube method with foil around a very thick multi plater pvc or abs pipe with more pips inside that and inside a copper rod, and I might try that since it is quite easy. Another Idea that I have had was just to buy 30pcs of 30kv 1nf small ceramic caps from alibaba for 60 cents each to make a 300kv 300pf bank, right now I am not too sure. Thanks for the suggestion!

[Twospoons]

Doing a string of 10 x 30kV caps isn't a great way to build a 300kV cap, unless you add balancing resistors. Typical cap tolerance is 10%, which means the voltage across any individual cap in the string is going to vary. So for 300kV overall, you probably want 12 or 13 x 30kV caps minimum (without balancing resistors).
You will also find that 1nF 30kV ceramic caps, while measuring 1nF at low volts, are a lot less than 1nF at 30kV.

[Tsindur]

Doing a string of 10 x 30kV caps isn't a great way to build a 300kV cap, unless you add balancing resistors. Typical cap tolerance is 10%, which means the voltage across any individual cap in the string is going to vary. So for 300kV overall, you probably want 12 or 13 x 30kV caps minimum (without balancing resistors).
You will also find that 1nF 30kV ceramic caps, while measuring 1nF at low volts, are a lot less than 1nF at 30kV.

I ended up buying 50pcs of 30kv 4.7nf ceramic capacitors! For a quintupler, I will use 12 in series.