Microwave Oven Transformer

[K9ACT]

I was shopping around for a transformer to use in an 866 tester project.  I bought a bunch of them but it's a lot of trouble to plug them into my rig and find out they are no good or worse.

After getting an Ebay jewel that won't do the job, I remembered the microwave oven out on the junk pile and did some reading after pulling it out.

I pulled out the shunts and added about 50 turns to the primary and now have a nice tester that runs an 866 at 2kv with a load of 100ma and cost me nothing.

I added a bridge and some caps and rigged up some resistors to provide a 300 ma load for a very short time and it still held at 2kv.

I look at this thing and at my Dahl transformer that is at least 5 times bigger and heavier (not to mention the price) and wonder where did Peter go wrong?

Aside from the fact that the frame has to be insulated from a metal chassis, is there any reason why this should not be used for my next project?

Instead of testing 866's, I am pondering a 4-250 at about 400W input.

It would be very simple to build the supply on a wooden chassis like the one for my 811 rig.

Am I missing something?

js

[w4bfs]

I have a question, JS ... after taking out the shunts and before adding the additional primary winding, did you measure how much current it took just to magnetize the core with 120v applied ?   I had tried several and most took 3 or so amps .... then how much did it drop after adding the 50 turns (in phase, of course) ? .... this is important because it speaks to eddy current losses in the core ... may not be too important for switched hv applications unless in the 'old buzzard' mode ... would be nice to be able to use these ....73...John

[K9ACT]

I have a question, JS ... after taking out the shunts and before adding the additional primary winding, did you measure how much current it took just to magnetize the core with 120v applied ?

Not sure I understand the question but as I increased the voltage, the current surged at around 100V to over 4 amps by the time I got to 120V.  After rewinding, it is well under 1 amp at line voltage and constant.

Adding the turns was fairly simple but a bit tedious.  I spliced into the existing winding and filled up the space there and then filled up the space where the shunts were.  I put two popsicle sticks under the hv winding to maintain at least that much space between the two.

My elmer (K9WEK) and I made all the measurements and he calculated exactly how many turns were needed and then left.  My wife helped keep the measured  wire sorted out as I wound it on. I ended up with a few feet left over but the smaller core size where the shunts were took more windings so I don't really know how many turns went on but it was something around 50.  His calculations called for 44.  (Engineers work)

js

[Opcom]

Assuming mounting the MOT on an insulator, the only issue is the insulation between the poorly insulated low end of the secondary and the primary winding.

[N8ECR]

Hey there was a guy out in the east back about 5 years ago using them. infazct he was using several of them in his rig, including one use as a choke and it worked fine business at that(with out any real warming of the iron).  As I recall it was a 100th rig, does any recall his call???  KA2*** name was Paul  thats all I can remember.

[w4bfs]

ok JS .. you answered my question fb .... the reduction from 4 A to 1 A is encouraging and I am curious if a further reduction is possible with more turns ... further experimentation would clarify just how MOT's could be made first rate ... Patrick ... if you can find 2 alike MOT's then do full wave c.t. topology on secondary ...problem goes away ... 73 ...John

[Ed/KB1HYS]

Yes, they work FB.  I am using two tied together togive me a 2kv transformer in a plate supply.  They are usually free but require the effort to strip them out of the ovens.  Usually there are some other good parts in there too.  The coils off of the fans are a good source of #24 -26 magent wire for coil winding purposes.

I'm glad you mentioned that you added turns to the primary to change the output voltage and input current as I was trying to think of a way to do that and adding primary turns never occured to me!!  (DUH!)

I also used MOT's to make the low voltage High current filament transformers.  The hardest part was mentaly accepting taking a chisel to the secondary ( I hate wasting all that copper!) but once cleaned out it was an easy task to wind enough turns of heavy gauge (#10) wire through the opening to get 5 or 10 volts.   I figure that 4 amps at 120 volts is 480 watts derate for comfort to 300watts and you can get a lot of current at those voltages. (weld thin metal!!)

[W2PFY]

I think this was the guy who used the microwave transformers. Since one side of the secondary is to ground, he just reversed the input ac line to one transformer giving him a full wave center tap configuration. He used 813 in the mod and 100 TH's in the final. He also used a microwave transformer as a heising reactor. His signal was clean and he sounded good. He had somewhat a high pitced voice so I couldn’t tell how the bottom response would have been. I don’t know what happened to him. He’s not on often.

KA1ANX     
Paul M Mileski
339 Lantern Hill Rd
Mystic CT 06355-9801

[W3RSW]

I have a couple of microwaves that are still operable sitting in the barn. They cosmetically had broken this and that but the xfor's still work.  Think I'll open them up and see what's what.  

P Dahl and others have designed their cores to take continuous duty, heat build up over long periods, etc. I'd think that microwave xfors have a very intermittent duty cylcle, on the order of up to, say, a half hours vs, 24.  

So I'm curious how they hold up over time in ham useage.

The other half of the P Dalh question is that the muwave' xfor's are probably made in China, way cheaper and the wire, core lamination material, etc. is probably bought by the linear kilometer rather than meters.  You name it...  all the components when economy of scale is in the millions rather than the tens.

I think economy of scale is the most germain explanation for where "P. Dahl "went wrong."

"Our mission" should we wish to take it  is to continue to find exotic ham usage for everyday items as we've done since 1915.

[K9ACT]

ok JS .. you answered my question fb .... the reduction from 4 A to 1 A is encouraging and I am curious if a further reduction is possible with more turns

Not necessary.   1A is a bad number.  I just measured it again with no load and it is more like .1A or one digit on the 200A range on my clamp-on.

With 200ma load it is about 2A which is about right if you do the math.


> if you can find 2 alike MOT's then do full wave c.t. topology on secondary ...problem goes away

It would still present a problem because the CT becomes grounded if the trans are bolted to the chassis.  As I see it, this limits the ability meter through the CT and keying.

js

[ve6pg]

...hey jack...
..email jim, ve3brl. he built a 813 rig using microwave transformers...

  ve3brl@yahoo.ca
   
  tim..sk..

[N8ECR]

Yes KA1ANX... Back then I started to play around with some on the bench, was going to run my 250TH rig  on them... I connected them up in a old buzzard fassion using two of them that were very simular, I tied the cores together for the B- and tried them in choke input and, capacitance input. keep in mined I left the shunts in place... they worked pretty good in capacitance, but made noise as if they were breaking down  in choke input(could hear this in a near by receiver).  I was getting any where from 900- 1100 volts DC wired in series,  enough for a 811 rig you know. Good for about 250ma ,( not to bad for some thing out of the junk, or sitting along the road.) The regulation was pretty good to.  With no excessive draw of current when idle.  So give 'em a try ;might be a cheep fix for a HV chrap out too.... N8ECR

[w4bfs]

really good, JS ... .1A is really good for unloaded primary current @ 120V ...thanks for rechecking the numbers .....think I'll go dumpster diving .....donk donk doinnnnng !

[WZ1M]

I believe when you add turns to the primary, this REDUCES the output on the secondary.




I'm glad you mentioned that you added turns to the primary to increase the output voltage and drop the inputj current as I was trying to think of a way to do that and adding primary turns never occured to me!!  (DUH!)

[K9ACT]

I believe when you add turns to the primary, this REDUCES the output on the secondary.

Roger.  I meant to comment on that.  I don't recall what the voltage was before but it must have been more than 2KV because that is what it now is.  I am also mixing AC with DC and half wave and full wave and now am totally confused.

The last thing I measured was at the filament of one 866 with the .85 uF across  it and 100 ma resistor load.

js

[Ed/KB1HYS]

Yep, uhh I knew that... brain fart.

[Opcom]

ok JS .. you answered my question fb .... the reduction from 4 A to 1 A is encouraging and I am curious if a further reduction is possible with more turns ... further experimentation would clarify just how MOT's could be made first rate ... Patrick ... if you can find 2 alike MOT's then do full wave c.t. topology on secondary ...problem goes away ... 73 ...John

It does go away, and with each one passing current only half the time, more current could be pulled. The secondary DC resistance of the one my appretice was messing with yesterday was about 110 Ohms.

I'm looking for the shunts to remove them, are there any pictures of this before and after?

[w4bfs]

shunts are strips pf xfmr laminaton material, generally stacked and shoved between the windings and the core like a spacer.... sorry, no feeelthy pictures to show... 73 ... John

[K9ACT]

I'm looking for the shunts to remove them, are there any pictures of this before and after?

Good question but too late for a pic.  Not what I expected at all.

When looking at the transformer face-on, you should see a stack of laminations about 1" long and 1/2" high on each side near the center top to bottom.  They are wrapped in an insulating paper.

The next problem in my case was getting them out as they were really a tight fit.  I ended up with the frame resting over the jaws of a vice and pounding the shunt with a 1/4" screw that was 3" long.  It took a great deal of very hard ponding with a big hammer just  to convince me that it was moving and it didn't get any easier until it was almost out.

Having said all this, I seem to have forgotten why the shunts must be removed.

Someone might review this for me.

js

[w4bfs]

an interesting question, why do they have to be removed ? ... folklore has it that they had some mysterious regulating function in the MO as the load changed ... could be lore or could be fact ...would be interested in knowing, JS ....good question ....73....John

[Opcom]

huh. I found this:
http://wiki.4hv.org/index.php/Microwave_oven_transformer

so the shunt purpose is explained, along with some other things.