110vdc capacitive discharge spot welder - possible?

Thread Starter

RogueRose

Joined Oct 10, 2014
375
I've seen some pretty simple spot welders, one uses 4 2200uf 35v caps in parallel hooked to a 30vdc transformer and it is being used to weld battery tabs. I think it is under powered as it is only about 4j of energy.

I was wondering if it would be possible to build one using 120ac hooked to a full wave bridge rectifier, mine gives 110vdc output. I have a lot of larger (300-800uf 450v caps as well as many caps that are 250v and higher at much higher capacitance).

If I run the caps in parallel and use a momentary switch, would I be able to charge the caps with the 110vdc then discharge them with the switch? I know that this is a pretty simple explination and I'd add a diode between the rectifier and the caps (any where else?) but would this concept work and if not, can it be made to work by adding something else?

I need a way to weld onto batteries so it need not be very powerful (though I would like to build one that is capable of sheet metal at some point.

I also have some 250w metal halide transformers that step up 120 to 277vac which may be more useful in this situation as I could use this before the rectifier to get much more energy storage in them.

I was also considering putting in a dimmer switch which would allow me to control the voltage, which I could then better control the power of the welder. Any thoughts on this?
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
There has been previous threads here on battery tab welders, probabally find them with a search.
Commercial stud welders generally discharge through a large SCR.
Max.
 

crutschow

Joined Mar 14, 2008
34,285
Commercial stud welders generally discharge through a large SCR.
Yes.
If you use a mechanical switch it's likely to weld the switch contacts, along with the metal. :eek:
I was also considering putting in a dimmer switch which would allow me to control the voltage, which I could then better control the power of the welder. Any thoughts on this?
A standard dimmer switch triggers a TRIAC with a varying phase pulse that controls the turn-on time during the sine-wave.
That means the peak voltage (which is what your circuit charges the caps to) would only be affected by the dimmer when set for a phase-angle past 90°.
It may still work, but would give a rapid adjustment of the voltage up to about the half-way setting of the dimmer, and then little adjustment after.
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
There are basically two types spot welding, captive discharge that can go up to a very high voltage, usually DC, where a bolt/stud is welded to a metal base etc, these typically use a SCR to dump the large capacitive charge in one shot.
The other is the spot/resistance welder, where very low voltage, very high current is used and , the AC primary is controlled by a Triac, or a Ignitron on very high current machines, and the current is varied in voltage and time.
Both methods require the materials to be welded while under pressure.
Max.
 

tcmtech

Joined Nov 4, 2013
2,867
For more than very thin metal welding going to a low voltage high current time controlled power source like the heavy transformer based units that produce hundreds to thousands of amps for a few cycles to several tens of cycles to make the weld is far more practical.

By design they are simple, very rugged and for the most part quite safe in the electrical sense to use and work with plus something that, if you already have a good 250 HID ballast laying around to make the high current transformer from, rather easy to build too. ;)
 

Thread Starter

RogueRose

Joined Oct 10, 2014
375
For more than very thin metal welding going to a low voltage high current time controlled power source like the heavy transformer based units that produce hundreds to thousands of amps for a few cycles to several tens of cycles to make the weld is far more practical.

By design they are simple, very rugged and for the most part quite safe in the electrical sense to use and work with plus something that, if you already have a good 250 HID ballast laying around to make the high current transformer from, rather easy to build too. ;)
Thanks for the reply! I actually picked up 8 of the 250w transformers for $1 each at the Re-Strore (habitat for humanity retail store). I wanted them for copper at first until I realized they were brand new. I then wanted to see if they could be modified at all as they are much more robust than the MOT's (larger iron core by about 4-5x in mass) so I think they don't have the same problem with "saturation" or over saturation or whatever the MOT's deal with.

the probelm with the transformers, they aren't MOT shaped, they are longer with the primary for the first 5-6" and the secondary the last 2-4", so they are linear instead of stacked. They also use about 3-5x the wire by weight, and they are rated at 1/4 the wattage of the standard 1000w MOT.

I do have about 10 MOT's (5 identical "1200 watt units from a 1600 watt microwave..?) that can be wired to any config along with lots of 24g, 26g, 28g, 30g varnished wire I unwound from CRT coils (100's) and similar items - I use this to make whatever gauge wire needed for rewinding the MOT - I haven't used this wire yet but I think it should be fine it was never heated past about 120F soften up to unwind it cleanly) then rolled in spools. I then use 2 screws at whatever distance needed for the length of wire and do loops around them pulling very tight - then twist tie about every 4-6" to hold tight together then do a single wire (24g) where every inch it loops around the cable, back under the 24g wire to secure there, move an inch, repeat to the end. This keeps all the wires from moving at all. IDK if this is adequate but I have also twisted the cable with a drill (along with the looped 24g wire). All is varnished and the 24g is not hooked into the circuit. I guess I could have used dental floss or something instead of wire..?
 

Tonyr1084

Joined Sep 24, 2015
7,853
Used to work in a steel shop where they made steel tanks. During pressure testing the tank had to be at or above a certain temperature. One way was to clean the surface of the tank and to try and stick a thermocouple to it to get a decent measurement of the temperature. It was a hassle and often didn't work well. Infrared thermometers also were ineffective, especially during the winter months when cold and hot air flows were all about in the shop.

What I did was to take an old JK type thermometer which was nothing more than a dissimilar metal junction at the end of a pair of wires and cut that junction off. I'd then take a capacitor (don't remember the value but it was pretty big) and charge it up. I'd then plug it onto the wires and take the two wires to the tank. Pick a clean spot and stick the wires to the metal surface. The sudden rush of current would be enough to spot weld the wires to the tank. The one dissimilar wires would see the steel as a junction and act as a pretty close temperature sensor. No switches, just the momentary contact with the steel tank. After the capacitor was discharged I could unplug the banana jacks from the cap and plug them into the Fluke Temp meter. Worked pretty snappy if you ask me. AND it was easy to break the wires off the tank and grind out the arc strike, and nobody knew the difference.

[edit] Sometimes I had to charge the cap up twice. Usually had to stick one wire on the tank and arc the second wire to the tank. Then charge up the cap again then arc the second wire to the tank. And I kept the two wires very close together. That way there would be less interplay between the two wires and the tank itself would become the temperature sensor.
 
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