I recently saw this CD welding video on YouTube and thought: why use supercaps if you can have more continuous bursts of high power using other electronics? I understand the advantages of supercaps, but there can also be many annoyances like charging time and physical size. But more importantly, you cannot charge and discharge them in a timely manner. It can often take many seconds to charge them. It is not at all continuous.

So why not have, say, a med-high voltage (~30-100V) med-low-capacity (~20-200uF) ultra-low esr (2mOhm or less) cap being charged relatively quickly but then discharged much quicker? You could, with a low enough esr, get 100s of amps and 1000s of watts (but for only a small % of the time). Why not just use some power transistors in parallel for charging/discharging, a Schmidt trigger, and a charging resistor? The Schmidt trigger would not let the caps charge past a certain voltage less than the supply and would not let them discharge past a certain point (to limit inrush current and time wasted charging them to max voltage). You could then just add a hall-effect/logic circuit to quickly interrupt dead shorts and to reset when a push-button is pressed or something. It would only need basic logic ICs, would be very fast, and seems like a better design over all. So why does everyone use high-capacity super-caps? What problems are there with this kind of design? Am I ignoring certain requirements for spot welding?

 

So...

 

The whole concept of capacitor discharge spot welding is for it to happen very quickly and then be done so that the heat does not spread. Super caps may not be the very best choice if those used have a higher ESR (effective internal resistance), but they do offer a smaller package than using regular capacitors.

 

@-live wire-
You might be interested in this thread: http://www.eevblog.com/forum/projects/guesses-on-what-i-am-attempting-here/msg1232857/#msg1232857

The primary author (tatus1969) took development to the point of offering a kit here: https://www.keenlab.de/index.php/portfolio-item/kweld/

As I recall, he investigated LiPo batteries and capacitors as his power sources. The LiPo batteries were subjected to brief but very high discharge rates. He found some brands survived better than others. I would be a little concerned about the longevity of LiPo's in that application. The welder I made some years ago used a conventional transformer and rectifier to charge a capacitor pack.

As for increasing the voltage, I would not recommend that approach primarily for safety. You do not want to inadvertently "defibrillate" yourself. I also found with my welder that higher voltages gave more burn through, but I did not pursue whether that was a function of the voltage per se or total power delivered. I get quite adequate welds of nickle strips with about 15 to 16 volts.

 

I get quite adequate welds of nickle strips with about 15 to 16 volts.

People that have never done any welding have a distorted view on how/what is happening in a weld. They for some reason think it is a 'high' voltage operation. When most welding only uses under 50V at high amps.