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Re: MOT Voltage Doubler



>Original Poster: "Gregory R. Hunter" <ghunter-at-enterprise-dot-net>
>
>Dear List,
>
>I've been studying the half-wave voltage doubler circuit (MOT, cap,
diode)
>found in US-type microwave ovens, and I think I understand how it
works.
>What is the ultimate voltage developed by this thing.  Looks like 2.8 x
>RMS.  Is that it?  If I'm correct, then it doesn't really "double"
anything
>does it?
>
>Greg
>
>
nope. It is a simple half wave rectifier. It is not a voltage doubler.
The topology is a little strange however, in that the diode is grounded
on one end shorting the positive going half cycle. The negative going
half cycle
is conducted to the K (cathode or heater) of the magnatron by the
capacitor.
The capacitor is not used as a filter or as an energy storage device.
Note that the voltage rating of the cap. is usually 2.0 or 2.1 kV AC,
the same as the RMS rating of the higher output transformers.
(There are transformers that are rated 3 kV which put out about 1.5 kV
RMS,
and 4 kV rated transformers which put out a nominal 2.1 kV RMS.)
There is some resonant charging effect going on however which accounts
for the voltage increase. The ultimate voltage developed is not more
than the
warning label, or not more than 2 * Vrms. The actual voltage produced
during operation
 is dependent on the load presented by the material in the oven. Higher
with no load
or a light load as in frying a CD. Lower with a heavier load like
boiling a cup of water.
I have never measured the actual voltages in an operating oven because
if the oven doesn't care,
and it doesn't, why should I? Just as long as it cooks.

A simple halfwave DC power source can be made with a MOT, diode and cap.
by arranging
the components in a more conventional fashion. (Diode in series with
transformer, one
side of cap grounded, output taken at the diode-cap junction) This
arrangement will produce
an unloaded output of approx. 1.7 * Vrms. The output at full load (1/2
transformer current) will
be very near Vrms. (1.7*2.1 kV=3.5 kV) If two MOT's are connected in
series (also must double voltage rating of cap. or place two caps in
series) 7 kV may be had. This would be OK for Tesla coil use.

Using a fullwave bridge one may obtain 1.4*Vrms unloaded, Vrms at full
transformer current.

Microwave oven transformers, while not providing much use in AC operated
Tesla coils,
are very useful as DC power sources. Combined with the right kind of
filter caps. and
voltage doubling and/or series connected transformers, they can be used
to make "stiff"
enough and high enough voltage sources to become the "method of choice"
for the future.
(as NST's get harder and harder to come by, and as they get more cheaply
made, and as they get
replaced more and more by solid state devices)

My method of choice for using MOT's as a Tesla coil power source would
be two MOT's
back to back with a fullwave bridge and a rather large filter cap (at
least ten, preferably one hundred
times the tank cap value). This would produce approx. 5 kV DC and be
very stiff. Then a suitable
inductor to couple the filter cap to the tank cap so that resonant
charging may take place...

I had a bad MOT (sec shorted) so I removed the sec. and the shunts and
wound a new sec.
to produce about 8 kV at 120 mA. External current limiting required.
It works OK but if I had paid myself a reasonable wage for the time it
took to do that, well,
I could have bought a couple of new NST's.  Not worth the trouble.
Especially since about a week after completing the rewound MOT, I was
given a 5 KVA 14.4 kV distribution transformer. Oh, well.

Later,
deano