Maker Pro
Maker Pro

Transformer from a microwave oven

E

Engineer

Jan 1, 1970
0
Hi, all...
This may seem an odd post but I'm an avid re-user and recycler of old
and/or scrapped electronic stuff (especially tube equipment), so I
hope all will be clear...

I have just picked over the carcass of a dead microwave oven (free
junk from a church sale) to get a thermal switch to repair our main
unit, but that's another story...

I pulled out all the usable bits (not many!) but also the power
transformer. It has a 1.75 x 1,25 inch core, weighs 9 1/4 lbs, and is
120 VAC to a scary 1,925 VAC (measured off load.) The core has two
magnetic shunts between the windings to give, I think, a degree of
voltage regulation (I may be able to press or knock them out, but I've
not tried yet.) The 120 volt primary resistance is about 0.3 ohm; the
2 KV secondary is 115 ohms.

Can anyone think of something useful to do with this this transformer.
All I can come up with is:

1. Use two of them back to back as a bench isolation transformer (but
watch the 2 KV floating around!)
2. Use as the OPT for a weird single-ended tube amplifier with 3.8
Kohm plate impedance (rather low) to a 15 ohm speaker (forget about 8
ohm speakers - too low; also need to isolate the secondary from the
frame (easy enough to do).)
3. Use it with a voltage doubler or tripler for an electrostatic
filter (would not fit inside most units so serious safety engineering
required.) Much too big, though.
4. Run it off a suitable battery and interrupter (NOT AC mains!) to
power an electric fence.
5. Use it as a choke in a tube power supply. I'm not sure of the
secondary winding inductance or current rating. Again, isolate the
secondary.
6. Use to replace another failed one <g> Do they ever fail?

That's it. Thanks for all ideas.
Cheers,
Roger
 
R

rex

Jan 1, 1970
0
I pulled out all the usable bits (not many!) but also the power
transformer. It has a 1.75 x 1,25 inch core, weighs 9 1/4 lbs, and is
120 VAC to a scary 1,925 VAC (measured off load.) The core has two
magnetic shunts between the windings to give, I think, a degree of
voltage regulation (I may be able to press or knock them out, but I've
not tried yet.) The 120 volt primary resistance is about 0.3 ohm; the
2 KV secondary is 115 ohms.

Can anyone think of something useful to do with this this transformer.

This discussion comes up fairly often. I suggest start by doing an
advanced google search in groups.google.com and use < transformer
"microwave oven" > limited to only this group. I just did that and came
up with lots of old posts.
 
J

John Popelish

Jan 1, 1970
0
Engineer said:
Hi, all...
This may seem an odd post but I'm an avid re-user and recycler of old
and/or scrapped electronic stuff (especially tube equipment), so I
hope all will be clear...

I have just picked over the carcass of a dead microwave oven (free
junk from a church sale) to get a thermal switch to repair our main
unit, but that's another story...

I pulled out all the usable bits (not many!) but also the power
transformer. It has a 1.75 x 1,25 inch core, weighs 9 1/4 lbs, and is
120 VAC to a scary 1,925 VAC (measured off load.) The core has two
magnetic shunts between the windings to give, I think, a degree of
voltage regulation (I may be able to press or knock them out, but I've
not tried yet.) The 120 volt primary resistance is about 0.3 ohm; the
2 KV secondary is 115 ohms.

Can anyone think of something useful to do with this this transformer.
All I can come up with is:

1. Use two of them back to back as a bench isolation transformer (but
watch the 2 KV floating around!)
2. Use as the OPT for a weird single-ended tube amplifier with 3.8
Kohm plate impedance (rather low) to a 15 ohm speaker (forget about 8
ohm speakers - too low; also need to isolate the secondary from the
frame (easy enough to do).)
3. Use it with a voltage doubler or tripler for an electrostatic
filter (would not fit inside most units so serious safety engineering
required.) Much too big, though.
4. Run it off a suitable battery and interrupter (NOT AC mains!) to
power an electric fence.
5. Use it as a choke in a tube power supply. I'm not sure of the
secondary winding inductance or current rating. Again, isolate the
secondary.
6. Use to replace another failed one <g> Do they ever fail?

That's it. Thanks for all ideas.
Cheers,
Roger

The magnetic shunts are there to limit the line current during a
secondary overload, effectively adding leakage inductance between the
two windings. This makes the voltage regulation worse, instead of
better.

If you take the shunts out, the idling current will rise and the
transformer will no longer be as overload proof. These transformers
are also designed to use just about every bit of flux capability of
the core, so they are quite lossy and so, rated for intermittent
operation and/or high core temperature rise. The secondaries are also
often not very well insulated on one end, since one end is grounded in
most microwave oven designs.

If you remove the shunts and use the space to add some primary turns,
to lower the flux swing, they may settle down and look more like a
normal step up transformer (better regulation, with lower core losses,
magnetizing current and audible hum. This will, of course, lower
the output voltage, too.

Want to build a big tube amplifier?
 
R

Roger Lascelles

Jan 1, 1970
0
Engineer said:
2. Use as the OPT for a weird single-ended tube amplifier with 3.8
Kohm plate impedance (rather low) to a 15 ohm speaker (forget about 8
ohm speakers - too low; also need to isolate the secondary from the
frame (easy enough to do).)

The high voltage secondary must not work floating from the frame - the
insulation from frame end of windings to frame is not rated for this.

Its a nasty transformer, designed for fan cooling and limited duty cycle.

Roger Lascelles
 
E

Engineer

Jan 1, 1970
0
Engineer said:
Hi, all...
This may seem an odd post but I'm an avid re-user and recycler of
old and/or scrapped electronic stuff (especially tube equipment), so
I hope all will be clear...

I have just picked over the carcass of a dead microwave oven (free
junk from a church sale) to get a thermal switch to repair our main
unit, but that's another story...

I pulled out all the usable bits (not many!) but also the power
transformer.

(snip)

Some informative, if depressing, replies. Thanks for all.

Firstly, since posting I knocked out the magnetic shunts and rechecked
the secondary voltage. It's now 2000 VAC (off load) for 120 VAC in, a
ratio of nearly 1:17.

Of course, John (two of you, other one from "tubes" NG) is right about
these shunts, regulation is worse. I was confusing them with that
resonant CVT design that uses a capacitor and magnetic shunts in a
weird way. Mea culpa. <g>

That's a good point about the insulation at the frame end (Roger L.),
it's not good for 2 KV but, in other uses, it may not have to be.
Anyway, I'd decided not to push it very far but thought it would be
quite safe at normal tube B+ (DC.) However, from the tube NG, Phil
said the LF and HF response would be very bad. I could readily agree
the HF would suffer from the crude winding layout but figured that, at
a couple of watts, the LF would be OK - lot of iron! Anyway, impedance
match is impossible and the matter of DC in the core is yet another
problem for SE amps and P/S chokes.

The more I think about it the more it looks like a door stop. Too
bad...
Thanks again,
Roger
 
K

Ken Taylor

Jan 1, 1970
0
Engineer said:
(snip)

Some informative, if depressing, replies. Thanks for all.

Firstly, since posting I knocked out the magnetic shunts and rechecked
the secondary voltage. It's now 2000 VAC (off load) for 120 VAC in, a
ratio of nearly 1:17.

Of course, John (two of you, other one from "tubes" NG) is right about
these shunts, regulation is worse. I was confusing them with that
resonant CVT design that uses a capacitor and magnetic shunts in a
weird way. Mea culpa. <g>

That's a good point about the insulation at the frame end (Roger L.),
it's not good for 2 KV but, in other uses, it may not have to be.
Anyway, I'd decided not to push it very far but thought it would be
quite safe at normal tube B+ (DC.) However, from the tube NG, Phil
said the LF and HF response would be very bad. I could readily agree
the HF would suffer from the crude winding layout but figured that, at
a couple of watts, the LF would be OK - lot of iron! Anyway, impedance
match is impossible and the matter of DC in the core is yet another
problem for SE amps and P/S chokes.

The more I think about it the more it looks like a door stop. Too
bad...
Thanks again,
Roger
I've once changed a transformer from a microwave oven which failed - a ball
of sparks and flame, but I'd consider that pretty rare. This one was being
used by my parents' in their caravan annex by the beach, so was in a pretty
corrosive atmosphere. Not to say they don't fail (Sony had a spare on the
shelf) but it isn't a terribly common occurrence to my knowledge. Anyway,
yours isn't much of a spare now... :)

Ken
 
Engineer said:
Hi, all...
This may seem an odd post but I'm an avid re-user and recycler of old
and/or scrapped electronic stuff (especially tube equipment), so I
hope all will be clear...

I have just picked over the carcass of a dead microwave oven (free
junk from a church sale) to get a thermal switch to repair our main
unit, but that's another story...

I pulled out all the usable bits (not many!) but also the power
transformer. It has a 1.75 x 1,25 inch core, weighs 9 1/4 lbs, and is
120 VAC to a scary 1,925 VAC (measured off load.) The core has two
magnetic shunts between the windings to give, I think, a degree of
voltage regulation (I may be able to press or knock them out, but I've
not tried yet.) The 120 volt primary resistance is about 0.3 ohm; the
2 KV secondary is 115 ohms.

Can anyone think of something useful to do with this this transformer.
All I can come up with is:

1. Use two of them back to back as a bench isolation transformer (but
watch the 2 KV floating around!)
2. Use as the OPT for a weird single-ended tube amplifier with 3.8
Kohm plate impedance (rather low) to a 15 ohm speaker (forget about 8
ohm speakers - too low; also need to isolate the secondary from the
frame (easy enough to do).)
3. Use it with a voltage doubler or tripler for an electrostatic
filter (would not fit inside most units so serious safety engineering
required.) Much too big, though.
4. Run it off a suitable battery and interrupter (NOT AC mains!) to
power an electric fence.
5. Use it as a choke in a tube power supply. I'm not sure of the
secondary winding inductance or current rating. Again, isolate the
secondary.
6. Use to replace another failed one <g> Do they ever fail?

That's it. Thanks for all ideas.
Cheers,
Roger

HI, I've used them for making test gear. Flash tester (insulation
tester), you can use 1 or 2 with a variac to give you a 2 or 4KV
variable supply. Put a suitable resistor in the high voltage lines to
limit the current to less than 1mA. Fast transient burst generator (emc
test) if you make 1 of these you need a variable 2kv dc supply to drive
the pulse generator, the transformer/diode/cap from the oven is
perfect, you still need the variac, if you build them all together you
can use the same 1.
 
C

Chris Jones

Jan 1, 1970
0
HI, I've used them for making test gear. Flash tester (insulation
tester), you can use 1 or 2 with a variac to give you a 2 or 4KV
variable supply. Put a suitable resistor in the high voltage lines to
limit the current to less than 1mA. Fast transient burst generator (emc
test) if you make 1 of these you need a variable 2kv dc supply to drive
the pulse generator, the transformer/diode/cap from the oven is
perfect, you still need the variac, if you build them all together you
can use the same 1.

A resistor in the primary is safer than resistor in the secondary, since
there is then no place where high current, >1kV is available from the
circuit. Probably the magnetising current will mean that the resistor in
the promary can't be as large as you would need to make the output current
safe, so the best approach might be to put a resistor in the primary and
also a string of resistors in the secondary such that several resistors
have to fail before it becomes dangerous. A metal box suitably earthed is
absolutely necessary. Also beware of the voltage rating of the resistor,
and the possibility of arcing between any wiring associated with the
resistors.

If at all possible, I would suggest avoiding using the high voltage from a
MOT and instead use a flyback converter which is incapable of putting out
so much current.

Chris
 
C

Chris Jones

Jan 1, 1970
0
Engineer said:
Hi, all...
This may seem an odd post but I'm an avid re-user and recycler of old
and/or scrapped electronic stuff (especially tube equipment), so I
hope all will be clear...

I have just picked over the carcass of a dead microwave oven (free
junk from a church sale) to get a thermal switch to repair our main
unit, but that's another story...

I pulled out all the usable bits (not many!) but also the power
transformer. It has a 1.75 x 1,25 inch core, weighs 9 1/4 lbs, and is
120 VAC to a scary 1,925 VAC (measured off load.) The core has two
magnetic shunts between the windings to give, I think, a degree of
voltage regulation (I may be able to press or knock them out, but I've
not tried yet.) The 120 volt primary resistance is about 0.3 ohm; the
2 KV secondary is 115 ohms.

Can anyone think of something useful to do with this this transformer.
All I can come up with is:

1. Use two of them back to back as a bench isolation transformer (but
watch the 2 KV floating around!)
2. Use as the OPT for a weird single-ended tube amplifier with 3.8
Kohm plate impedance (rather low) to a 15 ohm speaker (forget about 8
ohm speakers - too low; also need to isolate the secondary from the
frame (easy enough to do).)
3. Use it with a voltage doubler or tripler for an electrostatic
filter (would not fit inside most units so serious safety engineering
required.) Much too big, though.
4. Run it off a suitable battery and interrupter (NOT AC mains!) to
power an electric fence.
5. Use it as a choke in a tube power supply. I'm not sure of the
secondary winding inductance or current rating. Again, isolate the
secondary.
6. Use to replace another failed one <g> Do they ever fail?

That's it. Thanks for all ideas.
Cheers,
Roger

The only reasonably safe suggestion I can think of is to cut away the
secondary and sell it to the scrap dealer as recyclable copper, and then
thread a few turns of welding cable or copper bar through the core, to make
a very high current low voltage transformer. You might be able to spot
weld things with it, or at least melt nails, etc.

For spot welding, I have found that a relatively low current works if the
electrodes are not very tight on the sheet metal that you are welding, but
that you need much more current if the electrodes are clamped down hard
like in a real spot welder. I am suspicious of the welds produced under
light pressure. Recently, I have seen some websites about making spot
welders from about a Farad of capacitors charged to 50V or so, and
discharging this into some probes. These are supposed to work for
attaching tabs to Lithium cells, but I don't know if they are very good or
not.

Chris
 
B

Barry Lennox

Jan 1, 1970
0
Hi, all...
This may seem an odd post but I'm an avid re-user and recycler of old
and/or scrapped electronic stuff (especially tube equipment), so I
hope all will be clear...

I have just picked over the carcass of a dead microwave oven (free
junk from a church sale) to get a thermal switch to repair our main
unit, but that's another story...


There's a number of websites detailing how to make a spot welder.

I built one and it works great. Hacksaw off the secondary, and wind 4
turns as a new secondary, using as many parallel bits of wire than
will fit into the window. I control the primary using a 555 timer
driving a solid state relay. I also switch a large WW resistor into
the primary to give a high-low setting. With the resistor and the
timer variable between 30 and 350 mSec, I spotweld pretty much any
thickness of sheet metal, from foil to about 0.9 mm.

Contact pressure is important and you need to mechanically control
that to get consistent results. However, I use it a lot, and, for
one-off jobs, I can gauge it pretty well by hand.

Barry Lennox.
 
B

Barry Lennox

Jan 1, 1970
0
HI, I've used them for making test gear. Flash tester (insulation
tester), you can use 1 or 2 with a variac to give you a 2 or 4KV
variable supply. Put a suitable resistor in the high voltage lines to
limit the current to less than 1mA. Fast transient burst generator (emc
test) if you make 1 of these you need a variable 2kv dc supply to drive
the pulse generator, the transformer/diode/cap from the oven is
perfect, you still need the variac, if you build them all together you
can use the same 1.

I'm interested in the Fast transient burst generator, can you supply
more details of this?

Thanks,

Barry Lennox
 
E

ehsjr

Jan 1, 1970
0
Engineer said:
Hi, all...
This may seem an odd post but I'm an avid re-user and recycler of old
and/or scrapped electronic stuff (especially tube equipment), so I
hope all will be clear...

I have just picked over the carcass of a dead microwave oven (free
junk from a church sale) to get a thermal switch to repair our main
unit, but that's another story...

I pulled out all the usable bits (not many!) but also the power
transformer. It has a 1.75 x 1,25 inch core, weighs 9 1/4 lbs, and is
120 VAC to a scary 1,925 VAC (measured off load.) The core has two
magnetic shunts between the windings to give, I think, a degree of
voltage regulation (I may be able to press or knock them out, but I've
not tried yet.) The 120 volt primary resistance is about 0.3 ohm; the
2 KV secondary is 115 ohms.

Can anyone think of something useful to do with this this transformer.
All I can come up with is:

1. Use two of them back to back as a bench isolation transformer (but
watch the 2 KV floating around!)
2. Use as the OPT for a weird single-ended tube amplifier with 3.8
Kohm plate impedance (rather low) to a 15 ohm speaker (forget about 8
ohm speakers - too low; also need to isolate the secondary from the
frame (easy enough to do).)
3. Use it with a voltage doubler or tripler for an electrostatic
filter (would not fit inside most units so serious safety engineering
required.) Much too big, though.
4. Run it off a suitable battery and interrupter (NOT AC mains!) to
power an electric fence.
5. Use it as a choke in a tube power supply. I'm not sure of the
secondary winding inductance or current rating. Again, isolate the
secondary.
6. Use to replace another failed one <g> Do they ever fail?

That's it. Thanks for all ideas.
Cheers,
Roger

http://oak.cats.ohiou.edu/~postr/bapix/813amp.htm
http://repairfaq.ece.drexel.edu/sam/laserasc.htm#ascias
http://repairfaq.cis.upenn.edu/sam/gadget.htm#gadmot

Some ideas on those pages.

Ed
 
J

JustMe

Jan 1, 1970
0
Some informative, if depressing, replies. Thanks for all.

Firstly, since posting I knocked out the magnetic shunts and rechecked the
secondary voltage. It's now 2000 VAC (off load) for 120 VAC in, a ratio
of nearly 1:17.

The more I think about it the more it looks like a door stop. Too bad...
Thanks again,
Roger

I actually use a couple of these transformers, winding connections clipped
short, on my general workbench. They provide a nice compact anvil like mass
for hosts of little jobs that I don't want to walk or bounce when using
percussive persuasion. One transformer in a cloth is also just right to hold
in your hand for the occasional "biffing without bouncing" session on a wide
variety of house projects.

Don't drop it off a ladder. It will make a hole in concrete! Don't ask :-(

JustMe
 
E

Engineer

Jan 1, 1970
0
John Popelish said:
(snip)

(snip)


The magnetic shunts are there to limit the line current during a
secondary overload, effectively adding leakage inductance between
the two windings. This makes the voltage regulation worse, instead
of better.
Correct.

If you take the shunts out, the idling current will rise and the
transformer will no longer be as overload proof.

After taking them out I measured the no load magnetizing current.
Very high: 3 amps at 120 VAC on the primary. But I think it is high
with the shunts in anyway (but not measured.)
These transformers are also designed to use just about every bit of
flux capability of the core, so they are quite lossy and so, rated
for intermittent operation and/or high core temperature rise. The
secondaries are also often not very well insulated on one end, since
one end is grounded in most microwave oven designs.
Correct.

If you remove the shunts and use the space to add some primary
turns, to lower the flux swing, they may settle down and look more
like a normal step up transformer (better regulation, with lower
core losses, magnetizing current and audible hum. This will, of
course, lower the output voltage, too.

Actually, I measured it before (1,925 VAC) and after (2,000 VAC) for
120 VAC input. This is what I would expect.
Want to build a big tube amplifier?

Er... no.
 
E

Engineer

Jan 1, 1970
0
Engineer said:
(snip)


I still have to measure the off-load magnetizing current.

Just did it: 3 amps at 120 volts input. Horrible! Did not measure
the phase angle so I don't know the no load losses.

Cheers,
Roger
 
J

Jasen Betts

Jan 1, 1970
0
Hi, all...
This may seem an odd post but I'm an avid re-user and recycler of old
and/or scrapped electronic stuff (especially tube equipment), so I
hope all will be clear...

I have just picked over the carcass of a dead microwave oven (free
junk from a church sale) to get a thermal switch to repair our main
unit, but that's another story...

I pulled out all the usable bits (not many!) but also the power
transformer. It has a 1.75 x 1,25 inch core, weighs 9 1/4 lbs, and is
Can anyone think of something useful to do with this this transformer.
All I can come up with is:

cut the secondary out (I used a chisel) and replace it with a low
voltage winding, you'll be getting close to 1 volt-per-turn ...

The heart of an indestructable bench, or CB radio supply... etc

Bye.
Jasen
 
Fast transient burst generator (emc
I'm interested in the Fast transient burst generator, can you supply
more details of this?

Thanks,

Barry Lennox

What do you want to know about it? Basicaly it provides a pulse (50nS)
up to 1kV to simulate a spark. These are repeated at 5KHz for 15mS
(burst) each burst is repeated every 300mS. The pulses are injected
into the power lead of the equipment under test. See European standard
EN 61000-4-4 .
 
Chris said:
[snip] See European standard EN 61000-4-4 .

Is there a cheap way to see it?
Chris

I wish there was. It's bad enough trying to comply with them and they
change them every so often. There was a time when I thought they would
put me out of business but it seems the lead free directive (june 2006)
will do that.
 
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