On 4/11/2011 7:26 PM, Don Klipstein wrote:
, Jeff Thies wrote:

SNIP to getting a nonpolarized 1uF cap suitable for 120 VAC

If not then go buy a 200V non polarized 1uF cap. It is no easy chore.

http://search.digikey.com/scripts/Dk...dll?Cat=131081

200V DC rating is not enough, despite 120 VAC having a peak voltage
of only 169 volts.

AC does make things worse, by rapidly repeated severe voltage swings
combined with some development of heat.

The capacitor needs to have an actual AC rating, and one that includes
your AC voltage with a comfortable safety margin. If it has a DC rating,
chances are that will be around 400 volts DC. And not every cap rated
400 VDC is dsafe for use with 120 VAC, even if non-polarized.

Preferably, it should be "UL recognized" or the like, for some assurance
that it is reasonably reliable against failure, or at least an unsafe
failure.

Back in the early 1980's, in an experimental sodium lamp ballast, I have
blown an 800VDC cap and two 600 VDC ones with 240-260 VAC 60 Hz with less
than 10 operating hours of this combined among the three of them. One of
those capacitor blowups was a spectacular one that left a major oil stain
on the ceiling above. I learned the hard way that actual AC ratings are
required here.

Yes, many moons ago I was playing with adding a mylar cap in series with
the primary of a neon sign transformer. It worked great for making the
neon dimmer which was my attempt, but I recall measuring a much higher
voltage across the cap due to the reactence and impedence of them in
series. I'm guessing but I think it was up in the 400vac or so range
maybe higher. Not good for the transformer winding either. I didn't
blow them but stopped using them after reading the voltages.