 Over voltage testing of a salt water cap
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Russ Edmonds
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Registered Member #46264
Joined: Sun May 11 2014, 05:27PM
Location: Tucson, Arizona USA
Posts: 61
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Have not tried a two liter bottle but have blown through a PETE plastic bottle with about the same thickness of PETE at 65 KV. The tennis ball container I'm now using seems a little thicker than a two liter bottle and the bottle used previously. I've determined the polarity of outer surface charge to be positive by placing an FET electrometer near the discharged cap. This makes sense because the HV being applied to the inner salt water plate of the capacitor is negative so inducing a positive charge on the outer surface. The outer foil is grounded.
Before I try Newton's idea of adding skirts to increase the voltage limit before flash over occurs, I want to try to coat most of the outer surface of the capacitor with paraffin wax.
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paris
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Registered Member #3042
Joined: Wed Jul 28 2010, 12:36AM
Location:
Posts: 121
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does it matter about the area of the 2 surfaces = the square area of the tin foil vs the surface area of the salt water against the plastic ???
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Russ Edmonds
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Registered Member #46264
Joined: Sun May 11 2014, 05:27PM
Location: Tucson, Arizona USA
Posts: 61
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The level of the salt water was just under the edge of the outer foil. So the area of the salt water was about the same as the foil. I think the non-foil positive outer surface charge is being induced by the HV terminal leading into the salt water but this is just a theory.
I covered the outer foil edge with paraffin wax and again the breakdown was through the PETE plastic of the tennis ball container at 70 KV. This outer non-foil surface charge that remains on the cap after the cap is discharged is very annoying. As this charge dissipates the charge balance of the capacitor shifts inducing a voltage on the capacitor and I keep getting shocked. I have to discharge the capacitor four or five times over a period of several minutes before I stop getting shocked. I need a new design that does not produce this residual outer surface charge after the capacitor is discharged. I don't think this is a dielectric memory effect but I could be wrong.
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