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schematic

simulate this circuit – Schematic created using CircuitLab

Like many other DIYers with a one-off need to build a few li-ion battery packs and can't justify buying a spot welder, I am planning to build one myself. I did much reading and dug through my junk pile, and are leaning towards this setup built using what I have on hand. The TLDR question is: Will this work?

  • A power supply, connected to...
  • A bank of electrolytic capacitors, connected to...
  • A parallel gang of (I think I now need 10) MOSFETs salvaged from two dead main boards. I could salvage up to 16 70T03GH (60A continuous) or 14 85T03GH (75A), with the gates connected to...
  • A discrete/555 circuit or an Arduino to produce a consistently timed 10ms pulse to control the switching MOSFETs above. All this finally fires through...
  • A pair of copper electrodes.

I have on hand:

  • An old AT power supply, 8A@12v + 24A@5v
  • Rehoused iMac G4 power supply, 10.83A@12v
  • 20A@12v power supply off the 3D printer
  • A "1000A" booster pack anchored by a 12v/20Ah SLA battery that I changed out a year ago.
  • Two dead motherboards with 16 70T03GH and 14 85T03GH MOSFETs between them
  • 555 and LM324 chips
  • Arduino Micro
  • Assorted discrete parts to build a circuit around 555/324/Arduino
  • An assortment of capacitors from 1000 to 3300 uF, 8 of them rated above 12v.
  • NMD-90 14/2 solid copper wires, easily made into electrodes.
  • 4-gauge wires meant for car stereos
  • Some copper pipes to cut electrodes out of, if needed.

My question is thus:

  • Can it work?
  • Is my stash enough for what I'm about to do, at least quantitatively?
  • Can I do it with just a charged cap bank and no switch?

This question has been significantly revised before posting after I came across a youtube video of someone testing a spot welder at different power settings. He says (within the first minute, fortunately for me) that 90J over 10ms at 13v gives the best weld. Based on his other tests I believe 12v could also work. Now I have to design my build around the 90J figure. Some crude and (sometimes naive) calculations led me to 750A surge at 12v, and the cap bank would need to be 3.6F at 5v, well within supercap territory, and 12v supercaps are too expensive. He also said he used 1/0 welding wires to reduce loss. This is easier for me to get than supercaps from digikey.

Edit: I finally figured out how to add a schematic, so I added one representing my plan. Cx is the capacitor bank, 1F is just a placeholder value that it means to be a big bank.

buurin
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    Simply posting a list of parts is kinda like posting a list of lumber and asking if that can be used to build a house. What's important is HOW you plan to connect those parts and the best way is to provide a schematic. BTW - most of the DIY spot welder plans that I've seen use a scrap microwave oven transformer to generate a low-voltage high-current pulse needed. – jwh20 Nov 20 '22 at 01:53
  • I figured out how to add a schematic, so I did. For transformers, the closest I have is one out of a gutted UPS and its secondary has too many wires than I could figure out what they're for and which one I should feed juice through. – buurin Nov 20 '22 at 03:50
  • Decent spot welders are $50. Do yourself a favor and buy one. – winny Nov 20 '22 at 08:17
  • https://www.instructables.com/Homemade-Spot-Welder/ – jwh20 Nov 20 '22 at 11:02

2 Answers2

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The TLDR question is: Will this work?

There is no return current path back to the capacitors (at least in the schematic anyway). the mosfet is the same potential. You need to put the mosfet on the low side of the circuit. Also make sure that if you do parallel fets, that you put some series resistance to even out the current (either PCB trace or resistor)

enter image description here

But the real problem is power, you aren't going to get any more power/current than you would from just wielding with the supply. So if you think you can complete the wield in that 10ms, then go for it. But my experience with welding is you need a sustained arc that can also heat the material.

Voltage Spike
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Direct connection between the supply and the capacitor bank will not work well because the capacitors will likely trip the short-circuit protection in the power supply. you should put some sort of power resistor between these two parts.

The MOSFET will need a positive pulse to turn on.

other than that it looks like it could work.

Jasen Слава Україні
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