Andy writes:

I have been thinking about building a high voltage power supply
for a big amp, and am toying with the idea of using a couple of
microwave oven transformers, which I have on hand.

I plan to use two, with the primaries connected in parallel but
opposing, so that each one will supply voltage, half-wave, to a
diode ring. By doing this, I can have each core grounded like it
is used in the ovens....

So, has anyone else done this and run into any problems that
I may not have forseen ?

Thanks,
Andy

AndyS wrote:
Andy writes:

I have been thinking about building a high voltage power supply
for a big amp, and am toying with the idea of using a couple of
microwave oven transformers, which I have on hand.

I plan to use two, with the primaries connected in parallel but
opposing, so that each one will supply voltage, half-wave, to a
diode ring. By doing this, I can have each core grounded like it
is used in the ovens....

So, has anyone else done this and run into any problems that
I may not have forseen ?

Thanks,
Andy

Drew VK3XU had an article in AR magazine recently about this - he used
2 of them, the primaries in series, the secondaries in //. He removed
the magnetic shunts. With effectively half supply on each one, they
didnt go into saturation but maintained full output voltag. He lifted
the ground wire of the secondaries to frame - this was supposed to be a
big no no, but his experiments showed there was no breakdown to earth,
he then used a bridge rectifier on the resultant // secondaries...got
good results,......

Andrew VK3BFA.

AndyS wrote:
Andy writes:

I have been thinking about building a high voltage power supply
for a big amp, and am toying with the idea of using a couple of
microwave oven transformers, which I have on hand.

I plan to use two, with the primaries connected in parallel but
opposing, so that each one will supply voltage, half-wave, to a
diode ring. By doing this, I can have each core grounded like it
is used in the ovens....

So, has anyone else done this and run into any problems that
I may not have forseen ?

Using 1/2 wave rectification on each one will result in big DC currents
in the coils. These DC currents will saturate your cores like nothing else.

If you wanted to do this and be safe you'd want to rewind the primaries
after insulating the core with some appropriate high-dielectric strength
material like Kapton.

If you're in the US you may want to consider using the primaries in
series, and running off of 220V.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

There's a yahoo rfamplifiers group on the web, we've talked about this
subject a few times and a copy of the text is available in our files
section.

Join us if you'd like to talk about hv supplies and rf amplifiers:

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

cheers,
skipp


: AndyS wrote:
: Andy writes:

: I have been thinking about building a high voltage power supply
: for a big amp, and am toying with the idea of using a couple of
: microwave oven transformers, which I have on hand.

: I plan to use two, with the primaries connected in parallel but
: opposing, so that each one will supply voltage, half-wave, to a
: diode ring. By doing this, I can have each core grounded like it
: is used in the ovens....

: So, has anyone else done this and run into any problems that
: I may not have forseen ?

: Thanks,
: Andy

Go to http://groups.google.com and do an advanced search for "microwave
oven transformer" in rec.radio.amateur.*. You'll get a couple of pages
of hits, representing at least a couple of hundred postings on the
topic, many of them from this newsgroup.

Roy Lewallen, W7EL

AndyS wrote:
Andy writes:

I have been thinking about building a high voltage power supply
for a big amp, and am toying with the idea of using a couple of
microwave oven transformers, which I have on hand.

I plan to use two, with the primaries connected in parallel but
opposing, so that each one will supply voltage, half-wave, to a
diode ring. By doing this, I can have each core grounded like it
is used in the ovens....

So, has anyone else done this and run into any problems that
I may not have forseen ?

Thanks,
Andy

Tim Wescott wrote:

Using 1/2 wave rectification on each one will result in big DC currents
in the coils. These DC currents will saturate your cores like nothing
else.



Hey, Tim -

IIRC, there is only one diode attached to the transformer in my microwave.
That would mean it is half-wave rectified as it comes from the factory. Yes?

Cheers,
John

John - KD5YI wrote:

Tim Wescott wrote:

Using 1/2 wave rectification on each one will result in big DC
currents in the coils. These DC currents will saturate your cores
like nothing else.




Hey, Tim -

IIRC, there is only one diode attached to the transformer in my
microwave. That would mean it is half-wave rectified as it comes from
the factory. Yes?

Cheers,
John

Yes, come to think of it -- yes. Perhaps the core is sized to take it.

I guess the next question is how much harder can you push the system if
you _aren't_ putting DC on the core?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

Tim Wescott wrote:

John - KD5YI wrote:

Tim Wescott wrote:

Using 1/2 wave rectification on each one will result in big DC
currents in the coils. These DC currents will saturate your cores
like nothing else.




Hey, Tim -

IIRC, there is only one diode attached to the transformer in my
microwave. That would mean it is half-wave rectified as it comes from
the factory. Yes?

Cheers,
John

Yes, come to think of it -- yes. Perhaps the core is sized to take it.

I guess the next question is how much harder can you push the system if
you _aren't_ putting DC on the core?


Do they not put a capacitor in series with the transformer secondary, and
then connect this combination to a diode in parallel with the magnetron?
In this way the capacitor would prevent DC in the transformer.

Chris

Andrew VK3BFA wrote:
John - KD5YI wrote:

Tim Wescott wrote:


Using 1/2 wave rectification on each one will result in big DC currents
in the coils. These DC currents will saturate your cores like nothing
else.



Hey, Tim -

IIRC, there is only one diode attached to the transformer in my microwave.
That would mean it is half-wave rectified as it comes from the factory. Yes?

Cheers,
John


No. Its actually a full wave voltage doubler - the tube itself is used
as a diode.


Ah! Right you are, Andrew. I had forgotten about that. That makes Tim
Wescott correct, too, about DC in the winding.

Good catch.

John

Andrew VK3BFA wrote:

John - KD5YI wrote:

Tim Wescott wrote:


Using 1/2 wave rectification on each one will result in big DC currents
in the coils. These DC currents will saturate your cores like nothing
else.



Hey, Tim -

IIRC, there is only one diode attached to the transformer in my microwave.
That would mean it is half-wave rectified as it comes from the factory. Yes?

Cheers,
John


No. Its actually a full wave voltage doubler - the tube itself is used
as a diode.
And there not, strictly speaking, "proper" power transformers - a
saturable reactor would be a better description. They use this to
current limit the things cheaply - notice the big "Boing" when they
switch, noticeable on a low duty defrost cycle.

I read an article years ago on using them for powering linear amps --
the first thing you were supposed to do was knock the shunt out of the
core. Apparently this is usually quite easy unless it's spot welded, in
which case it's quite hard.

73 de VK3BFA.

PS - as a self employed electronics technician, working alone, I refuse
to work on the things in my business - too many people been lethally
zapped by them, and no one nearby to give CPR......

This could be wise -- once you broach the case on one of those things
you're into voltages that will deal instant death. But that's the case
for just about any tube linear amplifier.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html