Has anyone re-wound a power transformer with good results?
I have salvaged some big transformers from microwave ovens, and cut the
secondary winding out of it. Then re-wound it with 14 guage till getting
enough turns to produce about 20 volts. Problem is the thing gets hot even
without a load on it.
Is there something I am overlooking? Thanks.

Hi James,

Yes, there are shunts, which need to be removed. There is/was an article
about how to rewind microwave oven transformers. It was in an ARRL QEX or
similar magazine.

You can find a copy of that article in the Files Section of the yahoo
rfamplifiers group.

http://groups.yahoo.com/group/rfamplifiers/

Pretty much shows you what needs to be done.

Hope that helps...
cheers,
skipp

: James Thompson wrote:
: Has anyone re-wound a power transformer with good results?
: I have salvaged some big transformers from microwave ovens, and cut the
: secondary winding out of it. Then re-wound it with 14 guage till getting
: enough turns to produce about 20 volts. Problem is the thing gets hot even
: without a load on it.
: Is there something I am overlooking? Thanks.

James Thompson wrote:
Has anyone re-wound a power transformer with good results?
I have salvaged some big transformers from microwave ovens, and cut the
secondary winding out of it. Then re-wound it with 14 guage till getting
enough turns to produce about 20 volts. Problem is the thing gets hot even
without a load on it.
Is there something I am overlooking? Thanks.

I found the same thing when I rewound one a few years back.
Apparently the primary is saturated so it will draw 4 or 5 amps with
the secondary loaded or unloaded. This provides some voltage
regulation.
I should have taken a variac and ran a V/I curve from 0 volts to the
point saturation occurs and the current starts to rise. That would
prove that the transformer operates in saturation.
My microwave is rated for continous operation at full load, so
apparently the transformer doesn't get hot enough to let the smoke out,
but it does get hot.

Gary N4AST

James Thompson kirjoitti:
Has anyone re-wound a power transformer with good results?
I have salvaged some big transformers from microwave ovens, and cut the
secondary winding out of it. Then re-wound it with 14 guage till getting
enough turns to produce about 20 volts. Problem is the thing gets hot even
without a load on it.
Is there something I am overlooking? Thanks.


Hi !

That transformer is working in saturation.
Maybe you can see a welded line in the iron going over the plates.
Maybe not. It depends on design

Saturation guarantees good enough regulation for magnetron HT.
Also so called line conditioners and AC stabilizers are working with
saturated core.

Saturated cores are getting warm even withouth any load.
Added load doesn't warm up them remarcable any more.

Maybe saturation is an advantage for you because of regulation phenomena.
Test regulation with variac in input but use some load in secondary !

Line conditioner behaves so that in certain lowered input voltage the
saturation stops and output trends to drop.
To guarantee stabilization and conditioning they need some load all the
time (5 % of max allowed power is normally OK)
Line conditioners are normally working in resonance principle.
Therefore they have a nice flywheel efect. Gaps or glitches in line
sinus form are removed

73 ! Risto, OH2BT