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Timer for spot welder
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james_s:
A friend of mine built a spot welder, he used an AVR microcontroller that counts cycles from the mains frequency, setting the weld time is effectively setting the number of cycles the thing is turned on for, works quite well.
rhb:
An MCU has the advantage of being able to generate more complex pulse patterns.   I came across a long discussion of the subject, but it was several years ago.  IIRC one example was to have a short pulse followed by a delay and then a longer pulse for the weld.  But this is a pretty good start.

www.jfe-steel.co.jp/en/research/report/020/pdf/020-18.pdf

But a 555 based single shot with a pot and a trigger switch will do very well.  In grad school 35 years ago I built an enlarger timer using a 555 and a series of slide switches, each of which was double the time of the previous one.

This looks like a pretty good tutorial on building one.

https://www.electronics-tutorials.ws/waveforms/555_timer.html

I just followed the monostable example in the datasheet.

But you could easily have two pots, one for the length of the preheat pulse and and the second for the weld pulse with a fixed delay between them.  I designed and built a burglar alarm with entry and exit delays and siren timeout entirely with 555s.

With your limited electronics background I suggest just using a single monostable per the tutorial to start with.  That's actually a popular beginners project in a lot of books.  And it is fine for metal of equal thickness.

Measuring temperature is pointless. It's got to get hot enough to melt steel.  I have a small commercial spot welder that just has a toggle switch. It's a Chinese version of a little unit that Hobart sold.   It's really difficult to get consistent results manually.  Especially if you only make 3 or 4 welds  several years apart.
james_s:
Yeah there's no way you'd ever measure the temperature of a spot welder properly, the actual weld is a tiny point and it's sandwiched tightly between the two electrodes and gets hot enough to be molten. The whole point of spot welding is that it applies a tremendous amount of heat to a very tiny area welding it without heating up the surrounding metal.

For battery packs a traditional spot welder is too slow, I use a capacitive discharge type that dumps a capacitor bank of approximately 1F charged to 20V. This results in a very short pulse I measured at roughly 10kA and works with two side by side electrodes.
Doctorandus_P:
Hackaday has a nice overview of spot welder hobby projects in various states of usefullness:
https://hackaday.com/?s=spot+welder

And there seems to be a big distinction between spot welding batteries or steel plates.
Batteries tend to be capacitor dischare or sometimes from high-current battery packs, while for steel plates a slower, but much higher energy solution seems more applicable.

Measuring the spot weld temperature is even more difficult than suggested.
The spotweld itself is on the conact area between the 2 plates, and the outside where the steel plates are in contact with the electrodes is already a much cooler temperature.
You could try to measure mechanical distortion. A part of a good spotweld is to first heat the material enough to be plastic, and then mechanically force the (almost) motlen material into each other to make a good connection. The spotweld electrodes can only move closer to eachother when the base material is hot enough to be deformed. But in praktice it seems enough to simply controll the total energy input with no feedback.

It may be that they use such a mechanical feedback system for example in robotic welders in the car industry to ensure quality control.
abyrvalg:
I would recommend this controller: https://avdweb.nl/popular/spotwelder/spot-welder-controller-with-tft-display
I’m not affiliated with the author, just used it to upgrade a Sunkko 737 battery welder and found it working perfectly. The key features are wide adjustment ranges and correct transformer commutation phase (minimises inrush current, preventing the mains protection tripping). There is a high power version for bigger machines too.
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