capacitive discharge spot welder

Oct 09, 2005 6 Replies
M

In round numbers... If you have 25V and 25 mOhm of total circuit resistance, you'll be limited to 1000 amps. Think about the benefits of parallel vs serial connection in those terms. There are special caps rated for rapid discharge. Parallel caps divide the current. Normal ones won't last long if you keep shorting them. Welding is a reversible process. It's just as easy to unweld with high current.

Real spot welders use about 600V on the caps and discharge it into a transformer. Unitek 125 WS welder does 7000 amps peak.

.005" sheet brass from the hobby store welds nicely. People will argue against brass for a lot of reasons, but when it's all you've got...

If you want consistency, put a lot of effort into the head. You can't get consistent welds by manually poking probes at it.

mike

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K

Here are some 120,000uF 25Volt caps:

If you put the caps in series you will get one eighth the capacitance or 15000 uF. To get 1 Farad you would have to take eight 120,000 caps and put them in parallel. The voltage rating would stay the same.

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J

Hi,



I am working on making a capacitor discharge spot welder based on this design: "

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" and am seeing what the best bet for the capacitors is. Here are some 120,000uF 25Volt caps: "
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How would 8 of those in series compare to one of the big 1Farad caps that people use for car audio? I can get a ~1F car audio cap for cheaper off ebay (no shipping from the UK to Canada!) so if it can work I would like to use one of those. The spot welder is just for welding on battery tabs.



cheers, Jamie



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I

Check voltage rating on these car caps, vs the coltage of your appplication. Car audio caps are usually rated for 12v, and the webpage mentioned 25V power supply.

i
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J

oops I meant parallel..

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R

Just a generic question here - do you intend for the weld current to go through the body of the battery? Or do you poke both electrodes at the tab and battery cap? I read somewhere (very probably here, albeit some time ago) that at the battery factory, they weld the tabs onto the end caps before they assemble the batteries.

Thanks, Rich

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J

Yes, you poke both electrodes on the tab at the same time, so each time you weld you get two little welds, one from each electrode:

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I bet some people have put one electrode on each end of the battery too though, not sure if that works very well or not, but it may be a good way to start a fire with lithium batteries :)

cheers, Jamie

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