RE: budget caps (I did a quick test today)

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Original poster: "Lau, Gary" <Gary.Lau@xxxxxx>

Hi Mike:

The problem that you will face with an HDPE bottle cap is not dielectric
puncture voltage.  The trouble is the corona that inevitably will form
at the top edge where the foil stops, is not something that a plastic
dielectric can withstand.  A glass bottle is much more tolerant of the
heat and UV that such corona inflicts, but any plastic will quickly go
pffft!  The reason that plastic is successfully used in MMC's is that
the voltage across any one pair of plates is limited to <1kV, to prevent
the formation of corona.

Most MMC caps are actually two caps in series, to further reduce the
voltage per stage.  The point at which corona forms is called the corona
inception voltage, and this voltage is not really dependant on the
dielectric thickness.  I think the corona inception voltage is something
like 500-600V, but like all things, we tend to push that somewhat.

Regards, Gary Lau
MA, USA

> Original poster: Mike <megavolts61@xxxxxxxxx>
> Hi all,
>      I did a lil experiment today.   I know a lot of beginners use
> beer bottle caps and a couple weeks ago I was talking about using
> polyethylene containers instead.   I took an empty poly container
> that previously held  Lysol antibacterial wipes.  It just seemed like
> a good one for this test.  Not terribly thick walled, but a nice
cylinder.
>      I measured the dimensins after wrapping it fairly neatly with
> foil...I extended the foil underneath and flattened it as best I
> could.  I then roll an inch wide copper strip (30g) around a plastid
> rod to give it basically a pipe shape... and then affixed it using
> the lid of  container and connected my DMM to it in cap mode.   I
> then began filling it in increments of 40 mL (using a measure
> cup...had 40 mL  graduations...so I used it  haha).  I plotted the
> capacitance vs volume and it came out very linear,  probably over .99
> correlation.   The max came out to 1.175jF, so it would not take too
> terribly many of these to make a nice salt water cap bank.   And
> doing reverse calculation(being I was pretty confident about the
> measured capacitance),  I determined the thickness of the dielectric
> to be about 35 mils..  Now depending on who you ask,  that could make
> some good high voltage standoff.  Conservatively it would take
> 15kVDC, but I saw another reference that said HDPE has a dielectric
> strength of 5000V/mil...I think that could be a typo....but the range
> from the first table said it could go up to 1200v/mil  which means
> this could go as high as 40kVDC.
>      I will destructively test this tomorrow, if possible.  I got
> interrupted today, so I only got the measurments done.   One purpose
> of this 'experiment' is to come up with a cheap alternative to MMC
> arrays.  I don't know what they run, but this cap only cost probably
> 50 cents to make and could aid in tuning simply by adjusting the
> water level in the caps.
> Mike