[Amps] Teflon power capacitor

[David G4FTC]

> However, one thing to be aware of, is when stacking films or sheets of 
> dielectric between plates, you may have tiny air gaps in between the 
> materials, even slight. Think of the capacitor then as a series of small 
> capacitors with a total voltage applied at the ends. The voltage 
> division across the capacitors will be in Cair+Cptfe+Cair. Since the 
> PTFE is thick and has Er=2.1 and the air gaps are thin and have Er=1, 
> you have the potential to develop some high voltages in those tiny air 
> spaces. What happens is that it can make corona in there, and eventually 
> eat the dielectric. Going to thinner Teflon (as voltage rating is high 
> per mil of thickness as Gerald said) would exagerate the problem as the 
> air gap would start approaching the Teflon thickness. If a single air 
> space is 0.01 inches wide and the Teflon is the same (with no air gap on 
> its other side, pressed tight against conductor), then the total voltage 
> would be divided so that ~33% is across the Teflon and ~66% across the 
> single air gap. This is because the Cptfe is 2.1 time Cair, or the 
> reactance is about half. The air space may have microsparking over time, 
> that can cause noise, ozone, and damage the dielectric and the conductor 
> surface.
> 
> A way around this is to have the electrodes tightly bonded to the 
> dielectric, the way commercial capacitors are done. Unfortunately this 
> doesn't work in variable meat-slicer style capacitors. As long as your 
> air space on either side of the Teflon sheet is large, then the 
> ionization should be minimal in the air space. Just something you should 
> watch out for in homemade RF capacitors.


One thing which I have seen done commerically to try and avoid the creation of air bubbles or pockets is to coat the metallic surfaces with a thin coating of silicone grease which then sandwiches the PTFE film. 

 

But I don't know if this was to specifically prevent corona or was just considered "good engineering practise" - the voltage in question was only in the low kV region which I wouldn't have thought to have been high enough for corona.

 

 

David G4FTC

 

 

 

 
 		 	   		  
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