Its a tank circuit, an economical way to get high voltage at low current.
The concept is used in pi network filters of B+ supplies, a capacitor tunes
the filter choke.
73
Hank WD5JFR
"Mike Knudsen" wrote in message
...
This may be OT but could apply to some high-voltage transformers that
supply
hihg-powered TX finals.

Does anyone know why the 4 KV secondary of a medium-sized transformer used
in
an electrostatic precipitator air cleaner should have a 0.01 uF cap across
its
secondary? My suppy failed when this cap opened up. Without it, only a
small
fraction of the voltage was developed across the two diodes and 0.08
filter
caps in a voltage-doubler circuit that produces about 8 KV at up to 5 mA.

When I replaced it with half the original value, a 0.047 6KV cap from an
old
color TV parts bin, I got enough DC output to draw a 1/4" arc. But when I
paralleld a second such cap, to pretty much equal the original value, I
got a
3/8" or more arc, and the transformer made buzzing noises like it was
really
doing some work.

I permenently soldered in the two replacement caps and put the air cleaner
back
together. It works good as new again. Oh yes, I added a fuse to the
primary
side, just in case my new caps short out. Odd there wasn't one already.

Anyway, I at first thought that shunt cap was just to protect against line
spikes, but apparently it tunes a resonance in the secondary winding, or
something. Has anyone else encountered such a cap? Obviously a dangerous
place for a cap to be, so I can understand this isn't used often. 73,
Mike K.

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