Hallo!

I am seeking some info about using transformers from microwave ovens in HF
PA power supplies. As one side of the secondary winding is connected to the
transformer core, I am a bit confused on how to do the voltage doubling.

Any info on the subject is appreciated.

73 de Sven LA6KJ

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Hi Sven,
QEX Jan/Feb 1998 has an article using four microwave oven transformers
to build a HV power supply. Glancing thru I see that a magnetic shunt is
removed from the transformers as these will cause poor regulation.
I'll read it today a see if I can add more info. I recommend you locate the
article.
Mike


"Sven Arne Astrup" wrote in message
...
Hallo!

I am seeking some info about using transformers from microwave ovens in HF
PA power supplies. As one side of the secondary winding is connected to
the
transformer core, I am a bit confused on how to do the voltage doubling.

Any info on the subject is appreciated.

73 de Sven LA6KJ

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Hi Sven,
The author built a 500V screen supply and a 2700V plate supply.
The magnetic shunts must be driven out using a hammer and a
chisel of about the same size as the shunt.
The author had MOT's with the secondary unconnected to the case.
He did design so if he had to replace a MOT it would work with a
case connected secondary. He mounted the transformers so they were
insulated from everything except the case of the others. The cases were
the center tap.
With the large output capacitor, he designed in a slow start circuit to
prevent damage from large inrush currents trying to charge the cap.

I located an article below that has more info.
I searched " Build a high voltage power supply at low cost QEX"
The author is Randy Henderson WI5W


Michael A. Terrell wrote:


The welded cores can cause problems. This has been discussed on the
various sci.electronics.* groups a number of times

The key phrase to search for is "magnetic shunt". Typically the magnetic
shunt consists of two extra blocks of laminations between the primary
and secondary bobbins. This has been added to give the transformer a
current-limiting characteristic, which is necessary to power a magnetron
- but is exactly the opposite of what you need for normal vacuum tubes.

In many oven transformers, the magnetic shunt blocks can be driven out
with a large punch, to leave a more normal-looking transformer.

The not-so-good news is that you're still left with a transformer that
has a very high secondary voltage and rather high secondary resistance,
which will give poor voltage regulation in typical amateur use. Also one
end of the secondary winding is usually grounded to the core. Even if
you can get to the grounded end to disconnect it, the insulation close
to the core is not designed for typical full-wave bridge applications.

Some people have had limited success with a pair of identical
transformers connected as a "centre-tapped" winding... but remember that
in two separate transformers the core magnetization by the DC components
will not cancel.

It all seems like a lot of effort for rather poor results.

The worst problem is idle current, 1.2-1.5A on 240VAC, so you should
use the extra space and add 100turns for the primary, and reduce the
idle current to 0.3A. This will also reduce the output voltage below
2kV DC. Many cores are standard E120 types and it isn't worth the
trouble of trying to use the original rubbish

I've planned to bring some information on
http://home.online.no/~la8ak/d4.htm , but haven't translated it into
English yet

73
LA8AK
---
J. M. Noeding, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/91n.htm



"Sven Arne Astrup" wrote in message
...
Hallo!

I am seeking some info about using transformers from microwave ovens in HF
PA power supplies. As one side of the secondary winding is connected to
the
transformer core, I am a bit confused on how to do the voltage doubling.

Any info on the subject is appreciated.

73 de Sven LA6KJ

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scanned by NAV 2005.

Hi Sven,

There's a public group on Yahoo all about building rf amplifiers. You can
find it at:

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

In the files section of the group... I've placed a copy of the Microwave
Oven QEX article. There's also some pictures in the photos section of the
same group. The group is open to everyone...

enjoy
skipp

: Sven Arne Astrup wrote:
: Hallo!

: I am seeking some info about using transformers from microwave ovens in HF
: PA power supplies. As one side of the secondary winding is connected to the
: transformer core, I am a bit confused on how to do the voltage doubling.

: Any info on the subject is appreciated.

: 73 de Sven LA6KJ

: --


: All incoming and outgoing E-mail
: scanned by NAV 2005.

Hallo!

Thanks to everyone who responded to my posting. I finally found some
information on the subject, appreciate it very much.

73 de Sven LA6KJ


"Skipp says hello" skrev i melding
...
Hi Sven,

There's a public group on Yahoo all about building rf amplifiers. You can
find it at:

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

In the files section of the group... I've placed a copy of the Microwave
Oven QEX article. There's also some pictures in the photos section of the
same group. The group is open to everyone...

enjoy
skipp

: Sven Arne Astrup wrote:
: Hallo!

: I am seeking some info about using transformers from microwave ovens in
HF
: PA power supplies. As one side of the secondary winding is connected to
the
: transformer core, I am a bit confused on how to do the voltage doubling.

: Any info on the subject is appreciated.

: 73 de Sven LA6KJ

: --


: All incoming and outgoing E-mail
: scanned by NAV 2005.