Resonant HV Secondary Windings?
 

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.

Oscar loves trash, but hates Spam! Delete him to reply to me.

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.

Oscar loves trash, but hates Spam! Delete him to reply to me.

Can you tell me how is the transformer built (e.g. secondary over =
primary, or physically separated windings)? Do you see any air gap in =
the lamination?

I am trying to determine whether the secondary is lightly coupled to the =
primary. Taking into account the scope of the equipment, this could have =
been done to limit maximum power transfer onto the secondary.

Tony, I0JX=20

"Mike Knudsen" ha scritto nel messaggio =
...
This may be OT but could apply to some high-voltage transformers that =
supply
hihg-powered TX finals.
=20
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.
=20
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.
=20
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.
=20
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.
=20
Oscar loves trash, but hates Spam! Delete him to reply to me.

In article , "Antonio Vernucci"
writes:

Can you tell me how is the transformer built (e.g. secondary over =
primary, or physically separated windings)? Do you see any air gap in =
the lamination?

Thanks for picking up on this thread. The two windings are physically
separated, but I didn't see any air gap. The laminated iron core is pretty
heavy. Since the whole PS is rated at 8 KV at 5 mA, it must be good for 40
watts. A lot of modern xformers are wound separately form the primary for
safety isolation.

I am trying to determine whether the secondary is lightly coupled to the =
primary. Taking into account the scope of the equipment, this could have =
been done to limit maximum power transfer onto the secondary.

Yes, I can see that a magnetic gap or other loose coupling would protect the
secondary winding from burning up if a diode or cap shorted. And this would
also make it possible to resonate the secondary. You may well be onto
something here.

BTW, I replaced the original .011 uF cap with what amounts to .0094 uF. I
think I remember the decimal place right.
Tnx es 73, Mike K.

Oscar loves trash, but hates Spam! Delete him to reply to me.