Supercaps delivered, first welds

A project log for Coin cell battery powered spot welder

Wait overnight and do your welding job!

jaromir.sukubajaromir.sukuba 12/14/2017 at 09:175 Comments

I received two 50F supercaps and didn't hesitate too long to do some torture to them. To get some more angry pixies, I wired them both in paralell, using 2,5mm thick copper wire.

As I was too lazy to wait for my charger from previous logs, I hooked it to laboratory power supply via crocodile clips, set to 2,6V and 2,5A current limit, waiting a while to charge up. After charging I disconnected the clips and shortly pressed the two copper electrodes against nickel strip placed on discharged CR2032 cell. I think I'm going to have some more depleted cells in near future.

Capacitor discharged part of its charge with tiny spark, I was able to do three welds from single charge. Here is the cell after pull-test.

The bonding seems to be reasonably strong.

So, now I can try to charge it from my charger and try to make some welder prototype. I also thought of controlling the discharge via MOSFETs, to get better weld repeatability.


Christoph wrote 12/14/2017 at 14:12 point

Did you use anything special for soldering the caps? the wire is quite thick, after all.

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jaromir.sukuba wrote 12/14/2017 at 14:23 point

Nothing special, just my 40W DIY soldering iron.

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Christoph wrote 12/14/2017 at 14:12 point

Regarding MOSFETs for controlled discharging/welding: do you have any idea about the currents? I searched for designs on the web and they all seem to just use a bunch of huge transistors without much explanation about why they need to be able to deliver *that* high currents.

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jaromir.sukuba wrote 12/14/2017 at 14:21 point

Currents are in order of 100s amps, limited by voltage of caps, resistance and inductance of leads, resistance of welded material, sun phase and average amount of cheddar consumed in Bhutan per capita. The capacitors I have are not particularly low-ESR type, so here it's perhaps 100-200A peak.

Bunch of transistors is for redundancy. Though better transistors in TO-220 can handle dozens of amps continually and hundreds in peak, it's not trivial to ensure proper dividing of current for parallel transistors (especially in hidh dV/dt circuits), so usually you have to overdo it slightly and guesstimate amount of transistors to have better reliability.

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Ted Yapo wrote 12/14/2017 at 13:09 point

Very nice!

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