James Sweet writes:



I've never seen one with a ferro-resonant transformer. They use a
standard laminated core power transformer with a pair of magnetic shunts
to regulate the current.

[Johnny Carson voice] "I did not know that...."

All I've ever had to a u-w power supply was to replace the rectifier
stack; or junk the oven because it was clearly smoked...

Someone one mentioned they were F-R, and a casual look seemed to confirm
that, so I never questioned it. A F-R is also current limited; short the
output and it delivers rated current, period..



--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

David Lesher writes:

James Sweet writes:



I've never seen one with a ferro-resonant transformer. They use a
standard laminated core power transformer with a pair of magnetic shunts
to regulate the current.

[Johnny Carson voice] "I did not know that...."

All I've ever had to a u-w power supply was to replace the rectifier
stack; or junk the oven because it was clearly smoked...

Someone one mentioned they were F-R, and a casual look seemed to confirm
that, so I never questioned it. A F-R is also current limited; short the
output and it delivers rated current, period..

Never seen an FR uwave. Why would they use that when the basic
circuit is adequate and reliable (more or less!)?

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Sam Goldwasser wrote:

Never seen an FR uwave. Why would they use that when the basic
circuit is adequate and reliable (more or less!)?

Mostly less! :-) My Panasonic inverter unit just released the magic
smoke. First the magenetron died, and then after I replaced that, an
IGBT in the switcher shorted and did a fair bit of collateral damage.

I finally did find a service manual...in spanish (which I read
poorly,) but it did at least tell me what all the small resistors and
diodes were supposed to be (before they melted.)

So, $100 in parts later, I now have a working microwave again. And, if
I get tired of it, I can sell it on Craigslist for at least $35! :-/

On Fri, 30 May 2008 01:50:29 GMT, (Fiat Sparks) wrote:

Sam Goldwasser wrote:

Never seen an FR uwave. Why would they use that when the basic
circuit is adequate and reliable (more or less!)?

Mostly less! :-) My Panasonic inverter unit just released the magic
smoke. First the magenetron died, and then after I replaced that, an
IGBT in the switcher shorted and did a fair bit of collateral damage.

I finally did find a service manual...in spanish (which I read
poorly,) but it did at least tell me what all the small resistors and
diodes were supposed to be (before they melted.)

So, $100 in parts later, I now have a working microwave again. And, if
I get tired of it, I can sell it on Craigslist for at least $35! :-/


Don't try to operate that oven from a cheap generator with a less than perfect
sine output. That's another excuse for the blue smoke to leak out. BTDT.

In my case I wasn't about to spend that kind of money to repair an oven that
barely cost that much, especially since I used it in my restaurant always on
high. Therefore I yanked out all those fancy electronics and installed the
transformer/diode/cap assembly from another old oven. I drilled a hole
through that nice touch pad and installed an Intermatic spring-wound timer
from Home Depot.

Viola, good as new and bullet-proof against nasty power.

John

--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com -- best little blog on the net!
Tellico Plains, Occupied TN
The secret to creativity is knowing how to hide your sources -Albert Einstein

(Fiat Sparks) writes:

Sam Goldwasser wrote:

Never seen an FR uwave. Why would they use that when the basic
circuit is adequate and reliable (more or less!)?

Mostly less! :-) My Panasonic inverter unit just released the magic

Sorry, I was referring mostly to the normal transformer/capacitor/diode
circuit.

smoke. First the magenetron died, and then after I replaced that, an
IGBT in the switcher shorted and did a fair bit of collateral damage.

I finally did find a service manual...in spanish (which I read
poorly,) but it did at least tell me what all the small resistors and
diodes were supposed to be (before they melted.)

So, $100 in parts later, I now have a working microwave again. And, if
I get tired of it, I can sell it on Craigslist for at least $35! :-/



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Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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Important: Anything sent to the email address in the message header above is
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(Fiat Sparks) writes:

Mostly less! :-) My Panasonic inverter unit just released the magic
smoke. First the magenetron died, and then after I replaced that, an
IGBT in the switcher shorted and did a fair bit of collateral damage.

Ouch. We have one that's only a few months old; I hope it does better
than that. I love it because all of the power settings from "3" to "10"
actually vary the magnetron RF output while keeping it on continuously.
At "3", I can cook a single egg in the oven without it exploding.

Conventional microwaves set to "3" run the magnetron about 4.5 seconds
at full power followed by about 10.5 seconds off, and 4.5 seconds of
full power into a single egg generates enough steam for an explosion.

(On the Panasonic, settings of "1" and "2" actually run the magnetron at
30% power, using 1/3 and 2/3 on time).

So, $100 in parts later, I now have a working microwave again. And, if
I get tired of it, I can sell it on Craigslist for at least $35! :-/

For that much, I'd be tempted to scrap it and buy a new oven.

Dave

(Dave Martindale) wrote in news:g1pa8d$ljv$2
@swain.cs.ubc.ca:

Conventional microwaves set to "3" run the magnetron about 4.5 seconds
at full power followed by about 10.5 seconds off, and 4.5 seconds of
full power into a single egg generates enough steam for an explosion.



Saw something in a catalogue: a plastic 'Egg' shaped chamber that holds 4
eggs and some water. I am guessing that it is a 'plastic pressure cooker'.
Anyway, they claim you can hard boil eggs in it in a microwave oven.



--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

remove ch100-5 to avoid spam trap

(Dave Martindale) wrote:

Ouch. We have one that's only a few months old; I hope it does better
than that. I love it because all of the power settings from "3" to "10"
actually vary the magnetron RF output while keeping it on continuously.
At "3", I can cook a single egg in the oven without it exploding.

Mine lasted about eight years with daily use for a family of 5. So,
hopefully you've got a ways to go. I agree, it does cook nicely, which
is why I even bothered to repair it.

So, $100 in parts later, I now have a working microwave again. And, if
I get tired of it, I can sell it on Craigslist for at least $35! :-/

For that much, I'd be tempted to scrap it and buy a new oven.

You know, that's what the wife said! :-)

Had I known it was going to go out again after the magenetron died, I
probably would have. But, my pride was at stake!

On Thu, 29 May 2008 22:19:09 +0000 (UTC), David Lesher
wrote:

All I've ever had to a u-w power supply was to replace the rectifier
stack; or junk the oven because it was clearly smoked...

Someone one mentioned they were F-R, and a casual look seemed to confirm
that, so I never questioned it. A F-R is also current limited; short the
output and it delivers rated current, period..

The transformer goes by several names, depending on where you are. Variable
reluctance, leakage flux, stray flux, etc. It is exactly the same
construction and operating principle as a neon transformer, some kinds of HID
light ballasts and some series streetlight constant current transformers.

The core is an almost standard "E" core (or "H" core if you prefer) with one
exception. The center leg has an air gap. The windings are on the end legs
of the "E" instead of the center leg.

There are two magnetic paths around the core for the field set up by the
primary to travel. Around the periphery and across the secondary and around
the center leg and across the air gap. Field that travels along the center
leg does not cross the secondary and induces no voltage.

With no load applied, the bulk of the field travels the peripheral, very much
lower reluctance solid iron path, inducing full secondary voltage proportional
to the turns ratio. As current flows in the secondary, counter-MMF raises the
reluctance of the peripheral path so that some of the flux travels through the
center leg. With less flux traveling around the periphery and cutting across
the secondary, the secondary voltage drops as the current remains about the
same. At the limit, if the secondary is shorted, the peripheral path has so
much reluctance that most of the flux travels the center leg and across the
air gap. The same current as before flows through the secondary but at zero
volts.

When the dimensions of the core and gap are set up correctly, the transformer
behaves as an almost perfect constant current device. That is, the secondary
voltage varies as necessary to keep the same current flowing through a varying
load. Just what the doctor ordered to keep the magnetron happy.

The secondary current can be increased by opening up the air gap. This raises
the reluctance of that path and forces more field through the secondary leg.
Closing the gap has the opposite effect.

The center leg is often called the magnetic shunt and frequently it is a
separate piece of laminated iron stuck between the coils and TIG welded in
place. It is a common trick for Tesla Coilers to open up a neon transformer
and either knock out the shunt entirely or grind it down to open the air gap.
This modification causes the transformer to output much more current than it
is designed for - for a little while, at least :-) The same thing works with
microwave oven transformers (MOT).

This design in a microwave oven is a vital part of keeping the magnetron anode
current within spec. The magnetron is electrically a diode. A diode that
isn't emission-limited would draw destructive current if not externally
limited. With this design, the filament can be heated good and hot for long
life and not have the tube run away. The design also is vital for protecting
the magnetron from potentially damaging conditions such as operating the oven
empty, arcing, etc.

It's popular to use several MOTs to build an arc welder. This works quite
well specifically because these transformers are constant-current devices -
exactly the characteristic stick welding needs. If they were conventional
transformers, the first time the rod touched the work and shorted the
secondary, fault current would flow and the breaker would trip or blue smoke
would leak out.

Along similar lines, one can cut off the high voltage secondary and replace it
with a suitable number of turns of heavy wire, connect a bridge rectifier and
have a nice constant current battery charger. Select the turns carefully and
it'll do the bulk/absorption stages of the smart 3 stage charging algorithm.

John
--
John De Armond
See my website for my current email address
http://www.neon-john.com
http://www.johndearmond.com -- best little blog on the net!
Tellico Plains, Occupied TN
In theory, there is no difference between theory and practice. In practice, there is.

Neon John wrote:
see originsl post

It's popular to use several MOTs to build an arc welder. This works quite
well specifically because these transformers are constant-current devices -
exactly the characteristic stick welding needs. If they were conventional
transformers, the first time the rod touched the work and shorted the
secondary, fault current would flow and the breaker would trip or blue smoke
would leak out.

Along similar lines, one can cut off the high voltage secondary and replace it
with a suitable number of turns of heavy wire, connect a bridge rectifier and
have a nice constant current battery charger. Select the turns carefully and
it'll do the bulk/absorption stages of the smart 3 stage charging algorithm.

Another use for defunct MOT's, even those with a bad HV winding, is
using two as a line isolation transformer. Just remove the HV winding,
and connect the two magnetron heater windings together. The normal line
winding on the second transformer then supplies your workbench with
power isolated from the line. (You might need to modify the "shunt" core.)

With a Variac on the input, this is a handy for working on line operated
devices that might have a line-to-chassis defect.

--
Virg Wall

VWWall writes:

Neon John wrote:
see originsl post

It's popular to use several MOTs to build an arc welder. This works quite
well specifically because these transformers are constant-current devices -
exactly the characteristic stick welding needs. If they were conventional
transformers, the first time the rod touched the work and shorted the
secondary, fault current would flow and the breaker would trip or blue smoke
would leak out.
Along similar lines, one can cut off the high voltage secondary and
replace it
with a suitable number of turns of heavy wire, connect a bridge rectifier and
have a nice constant current battery charger. Select the turns carefully and
it'll do the bulk/absorption stages of the smart 3 stage charging
algorithm.

Another use for defunct MOT's, even those with a bad HV winding, is
using two as a line isolation transformer. Just remove the HV
winding, and connect the two magnetron heater windings together. The
normal line winding on the second transformer then supplies your
workbench with power isolated from the line. (You might need to
modify the "shunt" core.)

With a Variac on the input, this is a handy for working on line
operated devices that might have a line-to-chassis defect.

Do you actually use a setup like that? I'd think that for any sort
of current, the filament windings would be melting down even though
they are made of fat wire. For example, at 300 VA of output, you're
looking at ~100 A in the filament winding.

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Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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Important: Anything sent to the email address in the message header above is
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With a Variac on the input, this is a handy for working on line
operated devices that might have a line-to-chassis defect.

Do you actually use a setup like that? I'd think that for any sort
of current, the filament windings would be melting down even though
they are made of fat wire. For example, at 300 VA of output, you're
looking at ~100 A in the filament winding.




Connecting the HV windings together would probably work a lot better.
Would want to knock out the magnetic shunts too, that can be tricky but
I've done it on several without damaging the windings.

James Sweet wrote in news:HT%%j.670$BV.298
@trndny05:




With a Variac on the input, this is a handy for working on line
operated devices that might have a line-to-chassis defect.

Do you actually use a setup like that? I'd think that for any sort
of current, the filament windings would be melting down even though
they are made of fat wire. For example, at 300 VA of output, you're
looking at ~100 A in the filament winding.




Connecting the HV windings together would probably work a lot better.
Would want to knock out the magnetic shunts too, that can be tricky but
I've done it on several without damaging the windings.

I wanted some heavy copper wire to wind a coil with so I tore xxxx started
to tear up an old microwave transformer.
Found out that pretty copper wire was aluminum with a copper colored layer
on top of it.




--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

remove ch100-5 to avoid spam trap

David Lesher writes:

Someone one mentioned they were F-R, and a casual look seemed to confirm
that, so I never questioned it. A F-R is also current limited; short the
output and it delivers rated current, period..

They look similar. Both transformers have magnetic shunts, and there's
a big capacitor near the transformer. But the circuits are different.

The FR transformer puts the capacitor across the secondary winding, or
two secondary windings in series, and the windings plus capacitor
resonate at the designed line frequency. This causes the voltage to
rise above what you'd expect from the turns ratio alone, but the voltage
is limited by the portion of the core that the secondary is wound on
going into saturation. Sometimes the secondary voltage waveform looks
pretty square because of this peak clipping, but sometimes there's a
third winding that (somehow) reduces the second harmonic and gives
something closer to a sine wave.

There's a magnetic shunt between the primary and secondary windings so
that the primary current doesn't go through the roof when the iron in
the secondary saturates. It limits the shorted output current to about
*twice* the rated current, not equal to the rated current.

Still, the FR transformer runs hot with no load, dissipating about 20%
of its full output rating as heat.

In comparison, the capacitor in the microwave is wired as part of a
voltage doubler; it doesn't resonate with the transformer secondary.
The transformer iron is not designed to saturate (though, as an
intermittent-duty transformer that is fan-cooled in use, it is
apparently designed to operate close to saturation to minimize the
amount of iron).

Dave