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Using microwave xmfr for resistance soldering?

Discussion in 'Electronic Basics' started by Eric R Snow, Jul 14, 2004.

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  1. Eric R Snow

    Eric R Snow Guest

    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
    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
  2. Guest

    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.
  3. Rich Grise

    Rich Grise Guest

    All you need is to work on your torch technique. Try preheating.
    There's a NG - where they discuss this
    sort of thing. And don't use OA - use propane/air. (i.e. Bernz-O-
    Matic o.e.)

    I've seen spot welders (well, web pages with them) that used a
    microwave transformer, and they replaced the secondary with
    enough turns of wire to get 4V. That sounds like a good starting
    ballpark. Use #2 or #4 weld cable, and big copper electrodes.

    A simple timed on-off switch should work, like a 555 one-shot,
    tranny, and relay (or SSR). You control the heat by on-time.
    You might even be able to use the SSR out of the microwave
    itself. :)
  4. Eric R Snow

    Eric R Snow Guest

    I will remove the secondary and wind my own. I have done this to make
    a spot welder. It works well.
  5. Wild Bill

    Wild Bill Guest

    I dunno much about the specific outputs/capacity of the resistance soldering
    units, but I'll throw a few observations into the thread.
    The units that I've seen in use in a starter/generator repair shop were
    fairly compact units with a transformer housing of about a 6" cube. The
    plier/tweezer-style handpiece appeared to have carbon jaws that made the
    contact to the joint to be soldered. The heat was rapid, and they used heavy
    gauge solder, 1/8" maybe.

    Some butt welding machines that I used to repair were used to weld ends of
    heavy steel wire together (similar to a bandsaw blade welder). The sizes
    ranged up to about 1/4" diameter.
    The secondary of the transformer was only a couple of turns of flat braided
    cable securely clamped at the ends. The cable was the type that was used as
    engine ground strap in autos decades ago. This stuff would be good for using
    as a secondary winding in a modified transformer, easy to thread thru the
    frame aand flat for fitting into a square shape.
    For insulation, a good product would be fiberglas tape.. thin, high temp
    resistant and an effective barrier/insulator.

  6. Eric, I had a simillar idea a few months back when I needed a spot
    welder. The main thing with using a microwave xformer based welder
    is the power output, the "big" xformers are rated for around 1500 VAs.
    That's O.K. for spot welding thin sheet metal & thats what the plans
    available on the web seem to be aimed at, such as

    1/4" brass is probably more than a single transformer home built will
    handle. As a comparison, commercial units rated for spot welding
    3/16" steel are rated around 2500 VAs w/ a 250 V primary. I ended up
    buying a unit off of Ebay, they seem to go for about $70. Thats
    probably a lot cheaper & certainly quicker than trying to roll my own.

    If you're trying to butt weld the brass as I suspect then I really
    have no idea how much juice you'll need but you're certainly talking
    about a lot more than you'll get from a m/w trans. or the 70 bucks on

  7. Dan Caster

    Dan Caster Guest

    1. I suspect 12 volts is a bit high. 2. AC ought to be as good as DC.
    3. Use fewer turns on the secondary. 4 Set the length of the pulse
    so one pulse is the right amount of heat. 5. Use very heavy wire. 6
    gauge or bigger. Might be easier to wind three or more #10 wires in

  8. resistance solder units that I have output between 1 and 2.5 V
  9. Eric R Snow

    Eric R Snow Guest

    Thank You. That's just what I was looking for.
  10. Guest

    Reading the original post it seems that it's soldering
    that you need rather than flash or resistance butt welding
    which would need a much higher peak power input.

    It takes time for heat to travel and distribute itself
    into a workpiece. The shorter the heating time the smaller
    the volume of the workpiece affected and, for a defined
    temperature rise, the smallest total heat input and smallest
    heat affected volume. This means that the aim should be for
    a high peak power input to permit a very short heating time.

    Some resistance soldering units use either one or a
    pair of carbon electrodes sharpened to a point. These can
    apply intense local heat but it flows non uniformly into the
    brass and can only melt the solder after heating a
    relatively large volume of brass.

    A better approach (and probably the one you're
    already intending to try) is to resistance melt the solder
    directly by clamping the transformer output leads to the
    brass frame just either side of the joint and applying
    pressure and time controlled current pulse for a second or
    If you can succeed in mostly filling with copper the
    vacant space left by the high voltage secondary AND removing
    any magnetic shunt pieces, a single microwave oven
    transformer should be enough. You probably need about three
    volts. These transformers are typically about 1 turn per
    volt so two to four turns is the right range.

    If there's a choice solder should be in the form of
    flat preform lightly fluxed on both sides.

    This is all partly informed guesswork so let us know
    how you get on.
  11. Eric R Snow

    Eric R Snow Guest

    Thanks for the input Jim. You have hit exactly on the head what I'm
    trying to do. Often it is hard, for me, to get an idea across
    consisely. And I end up clarifying over and over. It must be hard to
    be a teacher. Since I have already stripped out the secondaries and
    shunts of two xmfrs I'll be wiring them in parallel. I will then
    machine two copper electrodes to match the shape of the pieces to be
    soldered and clamp the wires to them. I hope that getting the
    electrodes within 1/4" of the joint will get me lots of heat fast. The
    method of joining will be to use small .002" thick shim pieces in the
    joint to provide capillary action. I have done this when using a torch
    and it works very well.
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