A simple spot welder using a car battery charger/starter is detailed here:
http://www.nmia.com/~vrbass/models/solderer.pdf

They call it a resistance solderer.

-Adam

Mike Hord wrote:

>So am I then to understand that spot welding, in its simplest form,
>simply involves two electrodes on a metal object?  Say, a ground
>clamp in one corner of the sheet metal and a "wand" with which
>the desired spot weld points are tapped and PING! the metal is
>miraculously* joined?
>
>That sounds like it would be ridiculously easy to fabricate at home,
>but I suppose Lawrence already said that below...
>
>Mike H.
>
>*for those who don't understand electricity, anyway...
>
>  
>
>>That is really interesting.
>>
>>http://www.hobbyspotwelders.com/HS300A2.php
>>
>>The welder controller, which apparently is a DC spot welder, is a curious object.  In a conventional spot welder, you have a big transformer, and the spot weld is controlled by time.  The longer the time, the hotter the weld, and the more metal it will go through.  In this one, apparently, you control the voltage on a capacitor instead.  Nice 24V isolated input from a cheap wall wart.
>>
>>This would not be so hard to build.  It is basically a power supply with a controlled charging time, and a large, low ESR, high current capability capacitor with a high current switched output.  The $325 clams they charge for it sounds pretty steep compared to what it is.  PIC?
>>
>>It is AMAZING what you can do with a spot welder, a notcher, and a press brake.  Boxes, standoffs, mounting brackets, all this stuff becomes a piece of cake.
>>    
>>
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