Your concepts are good, and if you do google searches on "microwave oven
transformers" you will get websites that talk about their experiences.

I did buy a couple of used microwave ovens at local thrift stores for $5
each and played with the big transformers inside.

I was not satisfied with the result. One of the things some of the
websites talk about is that you have to remove the "magnetic choke" that
is part of the core. This needs a good hammer and a "punch" and was much
harder to do for one transformer than another. And, after taking it out,
it still did not perform.

The problem I found in both of my transformers is that at low voltages and
a reasonable load, it was fine, but from about 50 volts and on up to line
voltage into the primary, the primary current went up much faster than it
should have considering the load that I put on the transformer secondary.
I do not know what kind of load magnetrons put on power supply
transformers but i suspect it is not like a continuous load. I tried to do
google searches on magnetron current-voltage relationships but could not
find anything. Certainly those "magnetic chokes" must be doing something.

In the end, I wound up buying my plate transformers at hamfests and also
one, new, made by Hammond in Canada. Peter Dahl is out of business, and
Ameritron (I think) will sell plate transformers and some are reasonably
priced for the power and voltage. Fair Radio Sales (Lima, Ohio) and
Radio-Daze (NY) are two that sell Hammond transformers, including large
plate transformers (many types, specs all over the map, too).




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On Sun, 21 Jun 2009, terry wrote:

Date: Sun, 21 Jun 2009 16:21:03 -0700 (PDT)
From: terry
Newsgroups: rec.radio.amateur.homebrew
Subject: Other uses for Microwave oven transformers?

Trying to clear out some 'junk' but have always tended to keep
scrapped parts from old (UNREPAIRABLE/NOT-WORTH-FIXING) microwaves!

e.g. the transformers, fans, fuse holders, overheat switches. And even
occasionally have used some of them for repairs to other microwaves!

I applied 115.7 volts AC to what had been the HV winding of one of
these scrapped transf. recently and got about 6 volts on what had been
the original primary. That established the ratio between the two
windings as approx 20:1.

Also a small voltage (about 0.185 volts) across what had been the
magnetron (Approx. 3 volt?) heater winding. Which gives ratio of about
1:33 to the original primary winding; also a ratio of about 1:700 from
the original HV winding.

So the idea of using the transformer backwards comes to mind????

Depending on the unit they are typically rated as follows?

Input winding 115 volts at whatever total wattage of the magnetron
circuit is. Assume say 1000 watts requiring about 8.7 amps primary
current! Usually intermittent use.

The original HV secondary operates at around 2200 volts, so 1000 watts
(ignore the magnetron heater current for a moment) would average about
400 to 500 milliamps? And pretty peaky because of the half wave high
voltage doubling circuit???

More moderately; at 115 volts and say 400 m/a into the old HV winding
the transformer might be able to supply around 6 volts at around 5 to
7 amps from what used to be the 115 volt primary?
Or at 115v/250 m/a maybe 3 to 5 amps? For 6.3v heaters?

Alternatively could such an ex-microwave power transformer be used in
the plate circuit (non-push/pull) of an audio amp using say a 6v6 or
6L6 (perhaps triode connected?) with a DC plate current of 45 to 75
milliamps?

Thus in such an arrangement the 20:1 ratio to the original primary
could provide something of a match for an 8 ohm speaker? (20)^2 x 8 =
3200

Problems; the laminations of such transformers do not seem to have air
spaces as in some audio transformers carrying DC? The bottom (neutral)
end of HV winding is sometimes at cabinet 'ground'. Hence a problem;
unless the transformer is isolated and even then the transformer frame
could be 'hot' whenever the unit is plugged in if that winding now
used as the primary! Therefore unsafe.

Have also heard of cutting off the old HV winding etc. and using the
remainder of transformer to make battery charger; down rating a
typical transformer to say 500 watts (continuous) for battery charging
that's sort of around 30 to 40 amps at 12+ volts. Not sure what the
turns per volt would be but should be possible to poke a few turns
around the partially stripped core until one gets around 14 to 15
volts RMS.

This is all supposition at this stage! Any comments criticism????