Dielectric oil in "air gap" variable capacitor?

Thread Starter


Joined Jan 18, 2021
Greetings. I'm experimenting with Cockroft-Walton voltage multipliers. It occurs to me that the ability to vary the capacitors during operation might be a quick way to optimize capacitance for the other variables. My fixed capacitors are in the range of 2 uF.

With that in mind, I'm wondering if I could use several (two for each stage), identical variable air gap capacitors submerged in oil to increase their capacitance.

Any thoughts about this? Thanx,


Joined Aug 7, 2020
You'll probably not find an air-dielectic capacitor over 1nF. Most of them already will be variable.
I'd recommend simulating it on SPICE. There are some real experts on SPICE on this forum who could help you simulate all sorts of weird load conditions.


Joined Jun 4, 2014
This site gives the dielectric constant of various substances and that number is the ratio of increase of your capacitor.
It lists oil (unspecified type) as 2, olive oil as 3.1, and isopropyl alcohol as 20.1.
So a 1nF air spaced variable would be 20nF if immersed in IPA.


Joined Aug 7, 2020
Isopropanol probably isn't the best choice in practice. "Cockcroft-Walton multiplier" implies a high voltage - I don't know what its dielectric strength is, but its flashpoint of 11°C might be a little worrisome! It also tends to absorb water, and is itself slightly conductive.
Dielectric constant of water is 80, and pure water doesn't conduct electricity. So you could get 80nF, so long as metal ions from the plates don't start to dissolve.