On Tue, 13 Jul 2004 18:02:35 -0700, Eric R Snow
wrote:

Greetings to all the microwave oven scroungers,
I have a job coming up that requires soldering the corners of brass
frames. These frames are made of 1 1/2 x 1 1/2 x 1/4 brass angle. In
the past I've used a torch to heat the brass. This leads to a little
distortion. This can be troublesome because the brass expands so much
that it moves the joint even though everything is clamped. The parts
must then be straightened when cool. Because of the application and
fit the frames must be straight within .010" in 36". I saw some
resistance soldering units in a catalog and the description of how
they operate says that only a small area is heated and gets to heat
fast. This would be ideal. However, the ones I saw were too small and
the price too large for me. But I've got several microwave
transformers and they seem like they might be perfect. A rough
calculation from the specs and pictures given in the catalog leads me
to believe that they output about 12 volts open circuit. Some have
variable outputs. So I have a few questions:
1) Does 12 volts sound reasonable? Would a different voltage be
better?
2) Is DC better than AC? Does it matter?
3)What would be good ways to limit the current? Would a lamp dimmer on
the input side of the xmfr work? Wouldn't that also lower the voltage?
Would that matter?
4) I have a timer that pulses a relay on and off. I can set the length
of the pulses. Sort of what a lamp dimmer does but much longer pulses
(1 second and up) and the voltage would be the same out as in. But the
brass would average the heating. Would this work almost as well as
lowering the current? Better?
Any other input is much appreciated.
Thank You,
Eric R Snow,
E T Precision Machine


How are you planning to connect the transformer to get 12 volts? Put
power to the high voltage secondary?

If you put power to the normal primary you get 10s of thousands of
volts at relatively low current. Only way to use a microwave
transformer for resistance soldering is to remove the HV secondary and
install a very husky secondary of several windings - to give you a
couple volts at very high current.

AC or DC is not important and a"motor duty" dimmer can be used to
control output. A variac works better.