Using microwave xmfr for resistance soldering?

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.

All you need is to work on your torch technique. Try preheating. There's a NG - news:sci.engr.joining.welding 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. :-)

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

My technique is not the problem. Even preheating the whole frame does not solve the problem. I'm good at this. Had lots of practice. It's because of the really close straightness tolerance. ERS

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.

WB .................

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

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 Ebay.

Howard.

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 parallel.

Dan

resistance solder units that I have output between 1 and 2.5 V

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

Required voltage depends on whether the heat is produced by the resistance of the workpiece (brass) or by the contact which might be copper, SS, tungsten, or even carbon.

I have a Wassco Glo-Melt resistance soldering unit model 105-B2, 500 watts. It's output is variable in steps from 1.0 to 5.8 volts AC.

A microwave transformer's capacity will depend on the power rating of the microwave from which it came. They vary from 600 watts to 1200 watts. I don't know if that power rating is input to or output from the magnetron. A given sized core can only handle so many VA at given frequency (60 Hz), but I'd think a few experiments with such a trannie and your own lowvoltage secondary would have you knowing a good deal more than the "experts", at least about your particular application!

I'd wind a one-turn secondary and measure that voltage as a reference point for volts per turn. Secondary material is up to the guy winding it, and if you built a spot welder than you know the drill.

I'd then experiment with electrode material and configuration. That may affect your results as much as available power.

No, it can handle a certain number of turns per volt (which of course has a factor of frequency, yadda yadda, but we're at 50/60Hz only so forget about it). Current depends on the wire. That's why you can short a transformer and get the windings smokin'...

Personally I'd probably melt some aluminum wire into bars (I've got 13 pounds as weighed, soft crap to burn.... literally? HMMMM.....) and wrap that around the core, a turn or two. Some duct tape would make good insulation I bet. Most MOT welders have uh...3-5T? Maybe pile up four, run them in series-parallel off 240V and loop some heavy welding cable through the space where the secondaries used to be.

Tim

-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @

Greetings Don, I am using the experience of others to get to the starting point. I will be using three xmfrs in parallel. Because I have them and because I don't think two will be enough. I may end up buying a spotwelder on ebay too. The heat must come from the resistance of the brass. Hopefully, right at the joint will be hottest. The best part about this project is expanding my knowledge. Getting the job done is important but learning how and applying it to other situations is the ultimate goal. Cheers, Eric

I mention this to include it in this thread, for anyone that's not familiar with this type of AC circuit wiring. I'm supposing that you've got enough knowledge and experience in setting up this project to know that the transformer windings need to be wired to accomplish the correct phasing to have the circuit perform properly. This same method is required for xfmrs that have multiple windings.. dual secondaries for example, where the two windings could be connected either in series, parallel, or used separately (depending upon the application).

It'd be a problem if the primaries aren't phased, and worser if the secondaries aren't.

WB ..............

Just thought I would add another note here. Since resistance welders are used very intermittantly, I wouldn't hesitate to try just one and overload it for your welding operation. Similar to the Lincoln buzz box welder, it is rated for a certain duty cycle depending on the output current.

A microwave transformer rated at 1000 watts can probably put out twice that power if used at no more than a 10 to 20% duty cycle. Just a guess, but an educated one. :>)

Earle Rich Mont Vernon, NH

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 so. 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. Jim

Bill, I know the windings need to be phased. I've never done this but I gather all that needs to be done is to measure the voltage out. It should be the same when two are connected in paralell. If less, then one is out of phase. I don't know any other way to tell if my home wound windings are connected correctly. I do know that there are phase dots on some xmfrs that I have on the shelf. Eric

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. Eric

Almost correct, BUT!!

Place the two windings in series, If the voltage across the two free windings is zero, then you can connect them together and the windings will be paralleled. Connecting them in parallel wrong will give you maximum fault current.

If you read twice the voltage of either one seperatly, then they are phased wrong. Reversing the primary phase is usually easier than trying to rewire the secondary.

Unless the two transformers are identical, you will probably have some small voltage difference in the two windings. That will cause some loss due to circulating currents, probably not too important unless its more than 1/2 volt or so. Connecting transformer secondaries in parallel is not common due to this problem. But, for an intermittant service where the primaries are switched, I wouldn't worry about that.

Earle Rich Mont Vernon, NH

Well, golly, do you suppose I've been designing transformers wrong for 35 years?

Given 60 Hz, and given max flux density determined by the core material , volts per turn (or turns per volt if you prefer) are a fn of cross sectional area enclosed by the winding. However, for more VA (at given voltage) you need to use larger wire for those turns. This dictates more "window" area so the overall dimensions (envelope), volume and weight of the core must be larger to make a larger window.