Spot welder

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Over the years I have seen a few ways of doing spot welder for battery packs:

But besides the power source, some sort of control is needed, as you are basically shorting the output of your power source and you do not want that to last long. The simplest thing to do is to use a pushbutton, but that is not very precise nor repeatable, so something like an electronic timer can work best. But if you want to provide more than a single pulse a microcontroller can be the best choice for a precise and repeatable switching of the power.

And switching the power can be another source of trouble, especially if the switch is the high-current side. A spot welder can rush hundreds of Amperes and you need something more than a pushbutton to handle that. Some designs use a bank of MOSFET transistors to do the job (power dissipation is not huge due to their low ON resistance, the use of several of them in parallel and the short duration of the ON pulses). Others use an automotive relay, that while it works, makes me wonder about its reliability and lifetime in this particular use case). 

If the system uses a modified microwave oven transformer (MOT) the switch can work on the primary where currents are lower, but it has to switch AC current. So for that job a Triac is the selected component.  It even can be used to control the power applied by carefully timing the ON moments so only a fraction of a sinusoidal semi-cycle is fed into the transformer.

A long time ago, I removed the transformer of a Panasonic microwave oven that served me well for several decades. I kept it for unspecified future use. A few months ago, a controller board caught my attention. It was to be used with a microwave oven transformer and it was driven by a 32bit microcontroller and it featured a display and encoder wheel to select the trigger options. It did not include a foot pedal, but I bought that one separately. 

Getting the original secondary wiring out of the MOT was not really my thing. I did not have an angle grinder and I just sawed the coils with a hand saw, which was a nice exercise, but there was no way I could get the remaining wiring out as it was embedded in epoxy resin. Luckily, my friend Ricardo stepped in and with a bit of persuasion managed to the sawed coils out of the way. 

I used what I could find in the electric shop near my house for the new secondary wiring, a semi-rigid 16 mm^2 copper wire (I wanted 25 mm^2 but they have none). 

I designed and 3D printed a few parts for the boards to sit on top of the transformer and this is the, so far, final look.






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