Timer for spot welder

[AzzyAsi]

Hello

I'm a mechanical engineer but have bases of electrics and some concepts about electronics but never did any electronic project. I'm more into mechanical and basic electrical stuff (wiring for home, etc). I have soldering skills, multimeter and stuff, but never did more than replacing bad capacitors, transistors, switches, cables, etc.

Anyway, I've built a big transformer to give 1.2V AC at ~2000 Amps. At this moment i only have a powerswitch on the mains AC input and use that manually to set the time (with my wrist watch chronometer and shutting the power off when i think it's ok).
I have a big mechanical relay rated for 50Amps AC with coil voltage of 12V, and have a readymade 12V PSU (a brick type) that can operate that relay. I plan to use this insted of the switch on the mains (220V 50Hz here), that will draw some 10-14Amps with spikes over 18 at start.. this is what a clamp-meter says when i'm using it)

Now I'd like to add a timer circuit to do the following: Have a range from 0.1s to 30s that can be set with a potentiometer (or some switches for rough adjustments), a diplay would be nice but i don't know anything about them (i can calibrate that potentiometer with a chronometer and make a scale for what it does) - it's not about precision since it's eyeballing type of weld but would be nice to remember what settings worked best on certain types of sheetmetal. And a button to start that timer to close the relay to turn on the power to transformer to start the welder and keep the power going for that amount of time. Increments in 0.1seconds would be great, and tipically 10-20s would be enoung but would like to extend that to max 1-2min if within range for experimenting with thicker plates on car bodys

So from what i know i need either an arduino to do this (i'm not into programming arduino but i have base knowledge about programming in general.. will have to learn first), or some 555timer to control that relay (how do i use this to give me the range i want and to do a single count then stop until that button is pressed again?)
Never dealt with either of them.. so what would be an easy approach? like what components to use, what to buy?

[nsrmagazin]

I am not sure about the middle voltage electrical part, but as for the timing circuit a Timer555 as a monostable circuit will do. It will trigger once for 30 sec and then stop. The timer can be control by replacing the time resistors with pots.

[Doctorandus_P]

https://www.aliexpress.com/wholesale?SearchText=welder+control+panel

[AzzyAsi]

Thanks a lot. Was looking for exactly this around me (in Romania) and never found one. I'll research into those to see what fits my needs and place an order on aliexpress.

Temperature measurement? with what? that thing gets amazingly hot and eyeball the HAZ (heat affected zone) of the weld and stop at around twice the diameter for thin panels with a 5mm flat copper tip and a lot of pressure on it (i made a cam clamping device.. if i want to refine it would do it in a way that i can predict the force applied to be consistent). It is an ideea, but not sure how to implement it of it's relevant.

I weld panels for car bodies on the outer skin, and would experiment with more heavy gauge steel like frame steel (1.5-3.5mm thick) to see if i can spotweld (already did some tests and works at around 40-50seconds, goes glowing red triple the diameter of the tip and need more pressure to do something reasonably sound.. atm i'll MIG/TIG those if needed), also some enclosures of folded sheetmetal, and various applications.

I work on my cars as a hobby. I'm not a repair shop.

[Doctorandus_P]

Measuring the temperature of a spotweld is simply plain silly. Probably well meant, but not practical. The normaly way for spotwelders is to have an eye that gets heated and the diameter is defined by your electrodes. Then the peak temperature is reasonably well defined by the energy you put in your spotweld.

There are plenty of DIY spotwelders on Youtube, often made from microwave transformers.
They go from pretty bad, to reasonably well. The main problem with almost all of them is they do not have a proper timer 
You should keep the metal clamped down untill the metal hs cooled enough to take the load.
Most very simple spotwelders simply constantly power the electrodes and will keep puting energy in the spotweld as long as the electrodes touch the spotweld. This is horrible. A decent spotwelder timer is a very valuable addition to such a circuit.

[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.