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

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

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

On Fri, 21 Jul 2006, 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


I'm in the process of building a pair of 813s (GG) with microwave oven
transformers. I have two of them, slightly different size and about 4%
difference in secondary voltage for 115 primary. Both ovens had nameplate
current specs of 15 amps, max, but the transformers look like intermittent
duty. From the schematics on both ovens, the transformers had one side of
HV sec grounded, so I'm going to use each transformer for half of each
cycle into its own diode string. Then, a bunch of electrolytics in series
with voltage divider/bleeders on each cap. I've actually had the half wave
version going, putting out 1500 vdc onto the plates of the 813s, and the
transformer I used did not hum at all (I had a variac on the primary and
cranked up from zero and back down to zero). I don't know about the
"magnetic shunt" that some of the other posters are talking about.

The schematics both showed the magnetron as in a circuit where it looked
like the tube was conducting only on one half of each cycle (no diode
rectifier, no filter caps). The filament circuit (filament winding was
separate, and apparently only a few volts and maybe tenish or so amps
judigin by the wire gauge). Stampings on one of the transformers makes it
look like it has max 2.6 kv output, so I'll need to keep primary voltage
lower to keep rectified DC output around 2400 v for the 813s.