Tesla Forums

Hey all,

Many of us have heaps of microwave oven transformers in hopes of one day actually using them  ...
I'm included in this and have littered one shelf of my shop with a dozen or so for the last few years.

The low voltage and high currents are great for many solid state and quenched gap projects,
and this variation makes them a little more practical for typical spark gap Tesla Coils too.



1.  Take two MOTs that have around the same size core.
2.  Carefully use a grinding wheel and cut the weld that holds the core together.  On most MOTs,
the "E" core contains the two windings and is welded to a flat stack of laminations that closes the "E" (and also
typically has the mounting bracket welded to it as a base).
3.  Once you remove this, you have a base and an "E" with P & S windings.  Repeat this with the second transformer.  Now remove the primary winding from one transformer.  (Keep it for a later project)
4.  Slide the primary winding on the other transformer to the center of the middle leg so that it partially overlaps it.
5.  Drop the other transformer "E" on top, taking care to align the secondary coils in opposition for
the finished stack.  Weld the seams, and weld one of the bases back on.  Add more shunts if desired.

The finished product is a 4kV transformer rated at from 250 - 500mA depending on the size.  With a .05 mfd capacitor, it makes a very hot discharge 2 foot long with a small 4.5" secondary coil.  This current was no problem for a 1/2" diameter 3-series tungsten spark gap.  I am hoping to make a bread-board power supply for the 1.5KW coil on a 12" square piece of wood or marble -  transformer, spark gap, and capacitor bank included.



Here is an interesting discharge from a Pancake Coil after the arc hit a drop of wax insulation - glad the camera caught the instant flash ignition of things!  The coil didn't suffer from that flash, ironically! 



This transformer has been my favourite project this month, next to making homemade fermented pickles.  So simple, and so stupid - but, like pickles, its cheap, almost free, and better than your average mot/cucumber...
and nice to have an MOT setup with only 2 wires in and 2 wires out!

More here:
http://www.electrotherapymuseum.com/2009/09Nov09/index.htm

Jeff

Hi Jeff, That sounds like a really cool way of making a nice 4kv~ high current transformer.  One thing i would like to suggest, Why don't you try to use both primary's in parallel?  From my own experience the primary windings tend to get hot fairly quickly and here you are essentially going to be drawing 2x the power through 1 winding.

Just thought i would throw that out there for you to try perhaps.

John,
I think that's definately an option too!  In the past I've used 2 MOTs with primaries in anti-parallel and secondaries in series, with the cores grounded together.  The only thing (and I think in many cases it is an advantage 100%, not a disadvantage) is that I needed an extra ballast or reactance coil to limit the current.  With one primary, you can run it full blast on a 15A line.  This particular one didn't get hot, but I was only running it a few minutes.  I want to toss it in oil in any case, maybe soft wax or vaseline for easier handling.  I haven't tried to limit the current yet.  Addition of more shunts would also be interesting to see the effects on current.

But I love the fact that with the two primaries before I could use a large Powerstat in series as an adjustable reactance coil.  This would allow using very little current (less than 100W) up to 15A or so, in small increments, which was really nice for studying electrical discharges.  I'm definately going to expand on this idea, I have a pile of them to do something with (hopefully)...I hope there are some close enough matched pairs, which is always a pain with MOTs.

Any and all suggestions are welcome!!  Its definately worth trying, they are fun, cheap, and compact.  I'm going to make a housing to resemble a "Baby" pole pig.  6.625" diameter x 8" tall.  I remember Finn Hammer built this awesome homemade pig he called "Pork Chop".  This is my super-crude poor-mans rendition, fast & cheap... maybe "Microwaved half-eaten Pork Chop"?   

Jeff

Here is an update of progress.  I was originally wanting to mount it in an 8" length of 6" PVC (SCH 40, OD 6.625") and make a baby pole pig in appearance.  Then I decided to mount the 2 Sangamo .1 mfd caps inside, and in the end I want the whole power supply built around a 9" diameter footprint left by the primary coil.  The spark gap still needs mounted to the side, and wiring added.   The finished system will be around 9" diameter and 36" tall.  So far it is producing from a 24" - 30" discharge. 

3-Series 1/2" Tungstne Spark Gap added.  More photos here:
http://www.electrotherapymuseum.com/2009/15Nov09/CompactCoil.htm