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Microwave Oven Transformers Specifications.



 
 
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  #1  
Old April 5th 05, 12:32 AM
Dan/W4NTI
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Posts: n/a
Default Microwave Oven Transformers Specifications.

Does anyone know what kind of AC voltage a Microwave oven transformer would
put out? Are they center taped or just a single coil?

I need a replacement for my homebrew 3-500z single tube amp. I only used it
for low power AM and I don't need "maximum voltage".

Dan/W4NTI


  #2  
Old April 5th 05, 01:44 AM
Michael A. Terrell
Guest
 
Posts: n/a
Default

Dan/W4NTI wrote:

Does anyone know what kind of AC voltage a Microwave oven transformer would
put out? Are they center taped or just a single coil?

I need a replacement for my homebrew 3-500z single tube amp. I only used it
for low power AM and I don't need "maximum voltage".

Dan/W4NTI



They are a single winding on the secondary and not designed for
continuous use. You would have to switch the primary to prevent
overheating when you aren't transmitting.

--
Former professional electron wrangler.

Michael A. Terrell
Central Florida
  #3  
Old April 5th 05, 02:21 AM
John Smith
Guest
 
Posts: n/a
Default

I have gleaned 1.5KW commercial microwaves for their transformers (they had
timers which maxed out at 1 hour!), removed the secondary and replaced it
with heavy copper wire to make 13.8 V high amp supplies for mobile linears
converted to shack use.
The ones I have come across--where I bothered to measure the voltage of the
secondary, were 3000+ V, as I remember 3300-3500 V.
Two matched transformers should be able to serve as a center tapped
transformer without much re-engineering.
In amplifier use, a transformers 50% duty cycle specs should be
sufficient--I wouldn't be surprised if they could serve well in amps (and it
never hurts to make sure the xfmr benefits from a fans output.)
The only red flag which pops up in my mind is if there would be sufficient
isolation of the primary from the secondary at low freq. rf., if not,
something there could probably even be worked out (and, my worry may be
false to begin with.)
In extreme cases, a heat sink could be fitted to the exposed section of the
top and sides of the core (heat conductive epoxy would work.)
And, hey, if I guy gets long winded and smokes a xfmr, a trip to the dump
for a replacement is cheap!
Take care, those transformers can pack a lethal wallop!

Regards,
John
--
I would like to point out, I do appreciate the "Been there--done that!"
posts. Indeed, now your observations, comments and discourse should be
filled with wisdom--I am listening!!!
"Dan/W4NTI" wrote in message
news
Does anyone know what kind of AC voltage a Microwave oven transformer
would put out? Are they center taped or just a single coil?

I need a replacement for my homebrew 3-500z single tube amp. I only used
it for low power AM and I don't need "maximum voltage".

Dan/W4NTI




  #4  
Old April 5th 05, 03:08 AM
Michael A. Terrell
Guest
 
Posts: n/a
Default

John Smith wrote:

I have gleaned 1.5KW commercial microwaves for their transformers (they had
timers which maxed out at 1 hour!), removed the secondary and replaced it
with heavy copper wire to make 13.8 V high amp supplies for mobile linears
converted to shack use.
The ones I have come across--where I bothered to measure the voltage of the
secondary, were 3000+ V, as I remember 3300-3500 V.
Two matched transformers should be able to serve as a center tapped
transformer without much re-engineering.
In amplifier use, a transformers 50% duty cycle specs should be
sufficient--I wouldn't be surprised if they could serve well in amps (and it
never hurts to make sure the xfmr benefits from a fans output.)
The only red flag which pops up in my mind is if there would be sufficient
isolation of the primary from the secondary at low freq. rf., if not,
something there could probably even be worked out (and, my worry may be
false to begin with.)
In extreme cases, a heat sink could be fitted to the exposed section of the
top and sides of the core (heat conductive epoxy would work.)
And, hey, if I guy gets long winded and smokes a xfmr, a trip to the dump
for a replacement is cheap!
Take care, those transformers can pack a lethal wallop!

Regards,
John



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

--
Former professional electron wrangler.

Michael A. Terrell
Central Florida
  #5  
Old April 5th 05, 06:03 AM
John Smith
Guest
 
Posts: n/a
Default

I have missed that. What are the problems?
Even the power supplies I have made which power 1500+ liners have held up
fine. I do notice they seem to run a bit warmer than their expensive
counterparts.
I am convinced without arguement that their efficiency is lower than a more
conventional xfmr--one reason--microwaves are usually only in operation for
short periods of time.
However, all I can see which might be happening is my electric bill is a few
cents higher...

Regards,
John

--
I would like to point out, I do appreciate the "Been there--done that!"
posts. Indeed, now your observations, comments and discourse should be
filled with wisdom--I am listening!!!
"Michael A. Terrell" wrote in message
...
John Smith wrote:

I have gleaned 1.5KW commercial microwaves for their transformers (they
had
timers which maxed out at 1 hour!), removed the secondary and replaced it
with heavy copper wire to make 13.8 V high amp supplies for mobile
linears
converted to shack use.
The ones I have come across--where I bothered to measure the voltage of
the
secondary, were 3000+ V, as I remember 3300-3500 V.
Two matched transformers should be able to serve as a center tapped
transformer without much re-engineering.
In amplifier use, a transformers 50% duty cycle specs should be
sufficient--I wouldn't be surprised if they could serve well in amps (and
it
never hurts to make sure the xfmr benefits from a fans output.)
The only red flag which pops up in my mind is if there would be
sufficient
isolation of the primary from the secondary at low freq. rf., if not,
something there could probably even be worked out (and, my worry may be
false to begin with.)
In extreme cases, a heat sink could be fitted to the exposed section of
the
top and sides of the core (heat conductive epoxy would work.)
And, hey, if I guy gets long winded and smokes a xfmr, a trip to the dump
for a replacement is cheap!
Take care, those transformers can pack a lethal wallop!

Regards,
John



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

--
Former professional electron wrangler.

Michael A. Terrell
Central Florida



  #6  
Old April 5th 05, 07:08 AM
Ted Bruce
Guest
 
Posts: n/a
Default

On Mon, 4 Apr 2005 22:03:21 -0700, "John Smith"
wrote:

I have missed that. What are the problems?
Even the power supplies I have made which power 1500+ liners have held up
fine. I do notice they seem to run a bit warmer than their expensive
counterparts.
I am convinced without arguement that their efficiency is lower than a more
conventional xfmr--one reason--microwaves are usually only in operation for
short periods of time.
However, all I can see which might be happening is my electric bill is a few
cents higher...

Regards,
John

I couldn't get the laminations apart because of the welds. I would go
down to the garage and chisel away at them every couple of day, but I
finally gave up. My intended use was a hi current 13.8 V supply

Te KX4OM
  #7  
Old April 5th 05, 08:23 AM
Ian White G3SEK
Guest
 
Posts: n/a
Default

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.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
  #8  
Old April 5th 05, 06:04 PM
John Smith
Guest
 
Posts: n/a
Default

Yes, I see.
I never attempted to get the laminations apart. Using a hacksaw blade, I
cut the secondary turns and drove them out of the core with a punch.
Next, I used teflon tape around the core for insulation, and wound the heavy
thermaleze insulated wire though the windows in the core to complete my low
voltage winding.

Regards,
John
--
I would like to point out, I do appreciate the "Been there--done that!"
posts. Indeed, now your observations, comments and discourse should be
filled with wisdom--I am listening!!!
"Ted Bruce" wrote in message
...
On Mon, 4 Apr 2005 22:03:21 -0700, "John Smith"
wrote:

I have missed that. What are the problems?
Even the power supplies I have made which power 1500+ liners have held up
fine. I do notice they seem to run a bit warmer than their expensive
counterparts.
I am convinced without arguement that their efficiency is lower than a
more
conventional xfmr--one reason--microwaves are usually only in operation
for
short periods of time.
However, all I can see which might be happening is my electric bill is a
few
cents higher...

Regards,
John

I couldn't get the laminations apart because of the welds. I would go
down to the garage and chisel away at them every couple of day, but I
finally gave up. My intended use was a hi current 13.8 V supply

Te KX4OM



  #9  
Old April 5th 05, 08:03 PM
Reg Edwards
Guest
 
Posts: n/a
Default

Ian, welcome back!

I trust your move has been successful.
----
Reg.


  #10  
Old April 5th 05, 10:51 PM
Ian White G3SEK
Guest
 
Posts: n/a
Default

Reg Edwards wrote:
Ian, welcome back!

I trust your move has been successful.


Well, not quite, not yet. Still in the process of selling up down south,
and doing what work we can on the new place in Scotland.

My silence of the past several weeks has concealed a mad scramble to
finish all the outstanding jobs, before being sent to W6 for another
three weeks solid work.

Thank goodness for jet-lag, and an excuse to take it easy for a few
days.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
 




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