Hey all:

I tore down a transformer I got from ebay it was a Zenith TV power
transformer from days gone by. Re wound it with 17AWG wire on the
secondary and primary. It powers up my 4-1000A filament nicely with
7.25VAC from 120VAC mains. I used a penta-filar winding on the
secondary. Shimmed the bobbin in the core window with some 1/8 thick
virgin teflon. Epoxied the windings to the bobbin. This was a 240VA
transformer and now is a 83watt buzzer / 157watt filament transformer.

So what is the cheapest I can gits away with sealing these laminations
from buzzing, I used up all my epoxy sealing the windings.

73

n8zu

raypsi wrote:
Hey all:

I tore down a transformer I got from ebay it was a Zenith TV power
transformer from days gone by. Re wound it with 17AWG wire on the
secondary and primary. It powers up my 4-1000A filament nicely with
7.25VAC from 120VAC mains. I used a penta-filar winding on the
secondary. Shimmed the bobbin in the core window with some 1/8 thick
virgin teflon. Epoxied the windings to the bobbin. This was a 240VA
transformer and now is a 83watt buzzer / 157watt filament transformer.

So what is the cheapest I can gits away with sealing these laminations
from buzzing, I used up all my epoxy sealing the windings.

73

n8zu
I've sealed rewound filter chokes by dipping them in a bucket of
polyurethane varnish.

I'll second that.

Varnish is the best thing. Warm the varnish and the
transformer first before you dunk it. That will reduce
the viscosity and improve penetration. A little thinner
in the pot may help too. Vacuum/pressure impregnation is
better than dipping... but dipping can work well on a buzz.

There is even special dipping resin to reduce noise that
leaves the part looking like it's been dipped in a candy
coating. It's a soft, rubbery, epoxy coating.

Oh, and the teflon isn't so great as a shim. You might try
nomex paper, G-10 PC board scraps, fish or kraft paper even
typing paper, calendered cardboard (like shoe box cardbord)
or wood. Teflon will withstand the heat but it tends to
cold flow which makes it dimensionally unstable. The amount
of mechanical force between the core halves can be surprising.
Though in your transformer teflon may be be OK. In a choke
it can be a source of bewilderment as the gap decreases. Yes,
I've had this happen.

Any material that will soften when it's hot, like perspex/plexiglas
isn't a good choice either. I found this in an inductor for a
10KW boost converter that an otherwise reliable supplier built
for me.

For varnish, I like Dolphs (Von-Roll.) Failing that I use
marine spar varnish. Some varnishes are corrosive to copper
and over time will cause windings of fine guage to open if
any copper is exposed.




ken scharf wrote in
:

I've sealed rewound filter chokes by dipping them in a bucket of
polyurethane varnish.

"ken scharf" wrote in message
. ..
raypsi wrote:

n8zu
I've sealed rewound filter chokes by dipping them in a bucket of
polyurethane varnish.

This would work even better if you could pull a slight vacuum
on the bucket to get the air bubbles out. I believe that's how
the manufacturer's did it. There was a thread on this over on
AMFONE.NET a few month's back.

pete k1zjh

As was suggested by Grumpy... missed it first time.

Pedro

"Tio Pedro" wrote in message
...

"ken scharf" wrote in message
. ..
raypsi wrote:

n8zu
I've sealed rewound filter chokes by dipping them in a bucket of
polyurethane varnish.

This would work even better if you could pull a slight vacuum
on the bucket to get the air bubbles out. I believe that's how
the manufacturer's did it. There was a thread on this over on
AMFONE.NET a few month's back.

pete k1zjh

A good trick for the lams is clear nail polish as you reassemble them.
Just a quick wipe, as it'll spread as you squeeze in the last few lams
under a "Quick-Grip" clamp.

Cheers!

Uh! Sorry Ray, it was near 2AM and I misread this. Of course
there's no gap in the core it's a linear AC transformer. No
DC in the windings and so no gap.

Usually I'd do everything I can to reduce any gap, inclduing
interleaving the laminations, to reduce the magnetizing current.

I just wasn't thinking, that happens sometimes.


raypsi wrote in news:ea79280c-6148-42a5-be23-
:

Shimmed the bobbin in the core window with some 1/8 thick
virgin teflon. Epoxied the windings to the bobbin. This was a 240VA
transformer and now is a 83watt buzzer / 157watt filament transformer.

Grumpy The Mule wrote:
Uh! Sorry Ray, it was near 2AM and I misread this. Of course
there's no gap in the core it's a linear AC transformer. No
DC in the windings and so no gap.

Usually I'd do everything I can to reduce any gap, inclduing
interleaving the laminations, to reduce the magnetizing current.

I just wasn't thinking, that happens sometimes.


raypsi wrote in news:ea79280c-6148-42a5-be23-
:

Shimmed the bobbin in the core window with some 1/8 thick
virgin teflon. Epoxied the windings to the bobbin. This was a 240VA
transformer and now is a 83watt buzzer / 157watt filament transformer.

Actually "transformers" that carry DC (IE: filter chokes) have a gap
formed by putting ALL the "E"s in one direction and ALL the "I"'s in the
other.

For transformers that carry SOME dc (audio output transformers) group
the "E"'s and "I"s in bundles and assemble with a few groups of bundles
going in opposing directions.

For transformers that carry ONLY dc (power, filament) alternate each
lamination so no two in a row go the same way.

ken scharf wrote:
Grumpy The Mule wrote:
Uh! Sorry Ray, it was near 2AM and I misread this. Of course
there's no gap in the core it's a linear AC transformer. No
DC in the windings and so no gap.

Usually I'd do everything I can to reduce any gap, inclduing
interleaving the laminations, to reduce the magnetizing current.

I just wasn't thinking, that happens sometimes.


raypsi wrote in news:ea79280c-6148-42a5-be23-
:

Shimmed the bobbin in the core window with some 1/8 thick
virgin teflon. Epoxied the windings to the bobbin. This was a 240VA
transformer and now is a 83watt buzzer / 157watt filament transformer.

Actually "transformers" that carry DC (IE: filter chokes) have a gap
formed by putting ALL the "E"s in one direction and ALL the "I"'s in the
other.

For transformers that carry SOME dc (audio output transformers) group
the "E"'s and "I"s in bundles and assemble with a few groups of bundles
going in opposing directions.

For transformers that carry ONLY dc (power, filament) alternate each
lamination so no two in a row go the same way.
I meant for transformers that carry ONLY AC! oops!

Yep, It's all about core saturation. A large utility
transformer can be saturated by just a few amperes
of DC. Which is one reason improvements in load current
THD are important. Zero sequence currents and all
that jazz.


ken scharf wrote in
:

ken scharf wrote:
For transformers that carry ONLY dc (power, filament) alternate each
lamination so no two in a row go the same way.
I meant for transformers that carry ONLY AC! oops!