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Saltwater Caps

Quoting adams-at-intranet.on.ca (harry adams):

> Hi Guys,

> A friend and myself are planning a small, low cost TC project. 

Sounds like a good hobby project Harry.

> When specifying caps for the lab I usually take the maximum 
> peak to peak voltage and multiply that by 2.5 to get the DC 
> working voltage for the cap I need. 

A sound approach.

> If I am using saltwater caps where the glass wall thickness is
> 0.125 inches and the puncture voltage is about 200 volts per 
> mil, then the 125 x 200 = 25kV.  Derating by 2.5 gives a 
> working voltage of about 10 kV for the peak to peak voltage or
> about 7 kV RMS.

> Is this the correct approach?

This would be exactly correct when dealing with plastic film or
barium titanate dielectrics. In salt water capacitors the
dielectric strength is actually a bit better than your
calculated. I account for this difference as due to the better
thermal stability of the glass and the very high heat capacity of
the salt water. 

> Also, is the output voltage on an oil burner ignition 
> transformer the RMS or peak to peak value?

Transformer outputs are nearly always measured RMS.

BTW, someone wrote to me the other day and stated they had a
22 ma 10kvDC oil burner ignition transformer. Anyone ever heard
of them rectifing these units? The ones I am familiar with are
always AC.

Richard Quick


... If all else fails... Throw another megavolt across it!
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