# Spot welding



## Andrewinpopayan (Sep 2, 2008)

I have been into workshops with spot welders in use. Most appear to be a clamp with copper electrodes and I was wondering if it was possible to use a 140 amp stick welder as the power supply for a home made spotter?


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## kf2qd (Sep 2, 2008)

A stick welder is not the proper power source for spot welding. Spot welding uses lower voltage and turns the current on after the electrodes are clamping the part, and turns off the current before unclamping the part.


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## Mcgyver (Sep 2, 2008)

like kf said, spot welding is resistance welding and needs low voltage, 1000's of amps. Read this : http://www.millerwelds.com/pdf/Resistance.pdf At low voltage the steel passing that many amps gets hot where as the copper, a better conductor does not. The tough part is finding a transformer that you can rewind that will deliver say 1.5 V on the secondary and 3000+ amps. I've got a complete one built, rewound a 1KVA transformer, getting 1500 amps through it, not enough.

here's some pics of mine, timer, toggle, foot operated arms and electrical switch work to perfection but only on very thin stuff 












the front of these bell cranks go to a large lever type foot switch (build now, not at the time of this pic) that also starts the cycle with a micro switch, all on a low voltage control circuit that fires a pair of hockey puck scr's.


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## Andrewinpopayan (Sep 3, 2008)

Hmmm, thousands of Amps is a lot more current than I expected. I was only going to use it to weld some custom wire mesh joints, the mesh to be made of lengths of 10g mild steel wire. MIG-ing it would be repetitive and a shop bought job out of the question. The actual weld area where the 10g wires cross would be very small and even a small amount of pressure from the tongs would result in considerable clamping force on the joint.

How about a separate overwind secondary on the stick welder transformer? 1.5 volts would only be 3-5 turns of welding cable, that is if there is space inside the bobbin.


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## Bluechip (Sep 3, 2008)

Hi Andrew

Turns ratio is not the end of it unfortunately. If you want a lot of VA out you need a BIG core, otherwise saturation occurs, and you're limited by this. 

Many smaller transformers are deliberately designed thus to stop a s/cct burning out the secondary more or less instantaneously.

One of lifes many annoyances .. unless you like destroying transformers.

Dave


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## Mcgyver (Sep 3, 2008)

if its just wire, that's a different story and needs less amps. guys have built these, for wire, i believe rewinding transformers as small as a microwaves. Mine won't do, at least until i a bigger transformer(s), 24 gage sheet steel and its putting out around 1500 or 1600 amps but there should be much less resistance with wire. oh yeah, mine looks like lots of turns, its not, its just 1.5 turns x 4 in parallel


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## Stan (Sep 3, 2008)

Yes indeed, resistance welders run a lot of current. Many years ago, I repaired a seam welder used to weld the side seam in ordinary oil barrels. The seam was run between two driven wheels with 250,000 amps going through the wheels. Even the welders that weld 10 gauge reinforcing mesh run thousands of amps. Of coarse they are welding about 20 wires with each cycle.


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## Loose nut (Sep 3, 2008)

A couple of years ago an electrician friend of mine and I decided to McGyver a spot welder, not that we needed one just wanted to see if we could, We rewound the transformer, the original windings were 1/8" square copper wire and after it was finished minus the current limiter on the primary side we gave it a try.

The timer was set to 1/2 sec. and on trying it, it drew 156 amps on the primary (220v 30 Amps circuit and it didn't blow) and the output was 3600 amps (calculated figure, didn't have equipment rated high enough to measure).

The first pieces about 30 GA. vaporised, the second were two sections of 16 GA. welded together but it left a crater on both sides about 5/8" in dia. it should weld at least 1/8" if not more.

Should give you an idea whats required.


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## Mcgyver (Sep 3, 2008)

wow, what was the transformer like - need one of those!  I mean that's like a 20-30kva transformer for it to not saturate - it would be massive.  how did you measure the amps? .....also voltage would have been way to high for a spot welder, no?


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## Loose nut (Sep 4, 2008)

Can't remember to much about it and my friend is laid up with an injury so I can't contact him at the moment but the transformer came out of a 3 phase industrial battery charger. Their are actually 3 of this type and 3 of a different type that came out of the charger (the kind of transformer you use to make a homemade spot welder is very important, some types won't work). It is a bit larger than what is usually recommended for this application. We had intended to make a current limiting devise on the input to control the power it puts out, like a shunt, but haven't got around to it since he has been injured.


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## Kludge (Sep 4, 2008)

One fairly easy type of spot welder is the capacitive discharge welder. They use about a farad (which is a really huge massive much lot) of capacitance charged to around 25-35 volts then discharged through the short circuit formed by the joint to be welded. The capacitors available for auto audio systems will work with some modification but "computer grade" caps are better suited to the task. I could probably calculate the theoretical welding current but in the real world, there are way many much lots of variables and my brain's not up to it.

Anyway, it's an alternative approach if anyone's interested.

BEst regards,

Kludge


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## Mcgyver (Sep 5, 2008)

Loose nut  said:
			
		

> Can't remember to much about it and my friend is laid up with an injury so I can't contact him at the moment but the transformer came out of a 3 phase industrial battery charger. Their are actually 3 of this type and 3 of a different type that came out of the charger (the kind of transformer you use to make a homemade spot welder is very important, some types won't work). It is a bit larger than what is usually recommended for this application. We had intended to make a current limiting devise on the input to control the power it puts out, like a shunt, but haven't got around to it since he has been injured.



sorry to hear about your friend. I'd like to learn more about the transformer, I've been going mental trying to find a a transformer that i can work with, had this thing built for 6 months with a bunch of things I'd like to use it for....all ready except the transformer. Next one i think I'll do a mitered copper bar secondary, even have the copper. lol. I guess you don't have any pics since your buddy's out? any leads on where it came from, as in might there be more?

thanks


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## Loose nut (Sep 5, 2008)

I don't have the plans anymore, I think he has them but they can be found all around the INTERNET, variations of the same thing. Basically you take a tran. of the right size (check the plans, and it depends on what you want it weld) strip off the the outer coil, Rap heavy welding cable around the inner core as tight as possible, any gap reduces output, and thats basically a spot welder. Now for the extras, current control and a timer to control the spot weld time (again check the plans), handy for not melting the wiring in your shop and necessary housing and mechanical accessories to make it work. 

And the most important thing, Please don't electrocute yourself.

I will try and locate something on this subject and let you know.

P.S. _Kludge_ if you have anymore info on that capacitance discharge welder could you post it, looks like a interesting piece of equipment to make.


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## Mcgyver (Sep 5, 2008)

hey Loose, i've got the mechanical and electrical design done, in fact got it built (as per pics above) all i need is a big enough transformer to rewind so any idea or leads you might have on where to get one are greatly appreciated, thanks


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## Stan (Sep 5, 2008)

Any big spot welders I worked on used bare copper for the secondary winding. Usually square wire but I don't know why. Some used one turn of flat copper the full width of the core. Most had copper water pipes soldered to the secondary winding for cooling water.

The other big consideration is resistance. At several thousand amps .1 (one tenth) ohms will drop your entire secondary voltage and no current will flow at the electrodes.


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## Kludge (Sep 5, 2008)

Loose nut  said:
			
		

> P.S. _Kludge_ if you have anymore info on that capacitance discharge welder could you post it, looks like a interesting piece of equipment to make.



I'm looking for it. As soon as I posted I knew someone was going to ask.

The basics are pretty much the same - a clamp thingie to hold the pieces to be welded that include the electrodes, a power source and a trigger. The trigger even works the same but the weld current comes all at once and can't be stretched out over seconds. The caps take a while to recharge and the power supply used for charging has to have a resistor in series with the load to prevent it from being short circuited, either during discharge or during recharging. 

The one I made wasn't used for anything major and there are probably some serious power limitations but nothing I welded with one ever came apart. Well, not at the weld and not due to the welding process. (Things that fall great distances that aren't intended to fall great distances often don't fare very well on landing.) 

I know I have that site somewhere ... *grumble*

Best regards,

Kludge


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## Loose nut (Sep 6, 2008)

Mcgyver  said:
			
		

> hey Loose, i've got the mechanical and electrical design done, in fact got it built (as per pics above) all i need is a big enough transformer to rewind so any idea or leads you might have on where to get one are greatly appreciated, thanks



I know where there are two more just like the one we worked on [size=10pt]*BUT*[/size] they are sitting in a cupboard at work and I can't get them, we use to be able to get "scrap" for free (a reasonable amount) now days we can't have anything, period. No doubt they will end up in the scrap bucket some day. Sorry.

If anything changes I'll let you know.


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## Kludge (Sep 6, 2008)

Loose nut  said:
			
		

> P.S. _Kludge_ if you have anymore info on that capacitance discharge welder could you post it, looks like a interesting piece of equipment to make.



Here's the original version: http://www.philpem.me.uk/elec/welder/?elec/welder

I puffed mine up a bit (higher voltage & more capacitance) because I wanted to do more than battery tabs. It and the other baby welders are put away until the shop is significantly more accessable. It probably would be a lot less expensive to build a regular spot welder but the solid electrical *_WHAM_* of all that current all at once is just so cool. 

BEst regards,

Kludge


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## Mcgyver (Sep 6, 2008)

i didn't see where he addressed it, may have, i just quickly skimmed, but those caps may not last long unless they are of the quick discharge variety

Too bad Loose, seems a waste. I understand why, but its shame the abuse by a few crooks have made so many companies ban dumpster diving.


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## Kludge (Sep 6, 2008)

Mcgyver  said:
			
		

> i didn't see where he addressed it, may have, i just quickly skimmed, but those caps may not last long unless they are of the quick discharge variety



"Computer grade" caps can handle about anything you do to them including discharge via the expedient of a short circuit. The ones used for auto stereo systems are too new to have any sort of life expectancy guestimations but given they're a much newer technology, I wouldn't doubt they can handle quite a bit of abuse as well. 

Gathering up enough photo discharge caps to make a serious weld would be a lot of fun; 250-300 volts and maybe half a farad (since we have voltage working for us as well) would make a really nice weld, I'd think.

My fatted up one had 3 farads at 48 volts worth of capacitor which I charged to 40 volts. When I can get to it, I want to rebuild it so I can select the charge voltage and capacitance using jumpers so I can customize the _zap_ to the need a lot better. 

BEst regards,

Kludge


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## Nick Hulme (Oct 21, 2008)

The last thing you want for spot welding is more volts, heat generated in a conductor (anything you are welding) is relative to current squared times resistance. 
This is why long distance power transmission is at higher voltages, lower losses for a given conductor. 
What you need is the lowest possible voltage source, this increases the current & thus heat, for any given power input, 
Regards, 
Nick


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## Kludge (Oct 21, 2008)

Nick Hulme  said:
			
		

> The last thing you want for spot welding is more volts, heat generated in a conductor (anything you are welding) is relative to current squared times resistance.



Darn - power equation. Hadn't even considered that. Thank, Nick.

BEst regards,

Kludge


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## rawblacksmith (Oct 22, 2008)

i'm so behind in learning electronics and higher math.

really thankful for the internet ..the best course for mastery comes from finding out how much ther is to know!

eh i'm glad i got someone with a math degree in the house!


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## batt-man (Nov 11, 2008)

Nick Hulme  said:
			
		

> The last thing you want for spot welding is more volts, heat generated in a conductor (anything you are welding) is relative to current squared times resistance.
> This is why long distance power transmission is at higher voltages, lower losses for a given conductor.
> What you need is the lowest possible voltage source, this increases the current & thus heat, for any given power input,
> Regards,
> Nick



I'm not sure i understand your staement above. Current is equal to volts divided by resistance; in other words for any given weld (which will have resistance, the value being dependent on the materials being welded plus the resistance between the materials where they touch) the ONLY way to get more current, and therefore heat, is to increase voltage.


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## gunboatbay (Nov 11, 2008)

If you're not locked into building your own, you might want to take a look at this:
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=45690

Every once in a whaile they have it on sale for $139.95 US


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## Mcgyver (Nov 11, 2008)

batt-man  said:
			
		

> I'm not sure i understand your staement above. Current is equal to volts divided by resistance; in other words for any given weld (which will have resistance, the value being dependent on the materials being welded plus the resistance between the materials where they touch) the ONLY way to get more current, and therefore heat, is to increase voltage.



sort of, it definitely obeys ohms law but what you're missing is that there is always a practical constraint on power and transformer behaviour means you wont necessarily drop all the open circuit potential across the arms when closed.

lets say resistance is .001 ohms.

at 1.5 volts (a realistic spot welder potential) it'll flow 1500 amps.....say about 7.5 amps on 220V circuit, more than doable.

if you increase the voltage, yes amps increase but look at what happens to power...

at 11.5 volts, it'll flow 15,000 amps, yeah that'll spot weld and then some, but where's that going to come from? you'll instantly blow the breaker.

its amps that spot weld, so you've got to step down voltage so that you can get amps high enough (for mine I'm shooting for 3000) without hitting the wall on how much power you're trying to put through the panel/wires/etc. Transformers are rated in volt/amps so in the context of spot welders, when someone says you're better of with lower voltage it kind of means because then you are increase the amps out of the transformer (I know its not 1:1). For example if I put an extra turn on I'd be doubling the voltage and halving the amps before the transformer saturated whereas what i want is to increase the amps.

Things get more complex in design as you are not necessarily dropping all the potential across the arms, there are transformer losses etc....your open and closed circuit voltage reads on say the arms can be quite different. 

the electrical engineers can elaborate but I hope that gives you some hints on it


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## Nick Hulme (Dec 10, 2008)

batt-man  said:
			
		

> I'm not sure i understand your staement above. Current is equal to volts divided by resistance; in other words for any given weld (which will have resistance, the value being dependent on the materials being welded plus the resistance between the materials where they touch) the ONLY way to get more current, and therefore heat, is to increase voltage.



Nope, 
you're making an assumption that your power supply is capable of delivering infinite current limited only by the externa load and you are completely wrong. 

While V = I*R and V/I =R and V/R =I are all true, they are in this context irrelevant, what I was pointing out was that heat generated in a conductor (including the workpeice) is relative ( or proportional ) to Current * Resistance. 

1. Heat generated in a conductor is relative to Resistance Times Current, I'm not making this up, it's a rather inconvenient Physics fact, and as I stated is why long distance power transmission is done at higher voltages. 

2. Double your voltage & you get half the curent for the same power so given that the above statement is true (and it is) if you double your voltage you need four times the power to do the same job when resistance welding - think about it. 

Your statement would be correct when the voltage delivered is insufficient to drive a resistance welding current through the work, however the problem is almost always insufficient current delivery not insufficient voltage, 

Regards, 
Nick


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## Kludge (Dec 11, 2008)

Hold on a second. Since what I was talking about was a cap discharge spot welder, the power source has a while to charge the caps between shots plus isn't providing anything during a weld. With appropriate limiting resistors to prevent a massive surge at the beginning of the charge cycle, we can set the maximum current at the wall socket to some reasonable level.

The capacitors and wiring will provide some resistance during the welding process, however slight it may be, but it will add to the total load about the same amount whether it's low voltage or high voltage. THis is independent of the power source used to charge the caps. 

One other thing that will affect the "zap" is the capacitance. Less capacitance, less stored charge at a given voltage. Crank up the voltage and a few factors change and I think one of which is the internal resistance of the capacitor. I need to review here a bit but I still think there would be a net increase in current with increased voltage though I'm not sure how much caacitance I'd need at 450 volts to match a 40v charge on one farad, taking into account circuit resistance.

Just some thoughts from a med-fuzzed brain.

Best regards,

Kludge


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## Nick Hulme (Dec 12, 2008)

Kludge, 
yes Capacator discharge is a different matter to transformer, you have a shot at very high currents, for very small light jobs I think it's do able. 
For anything structural I suspect caps with the spec required have a good chance of outstripping the cost of an off the shelf spot welder, 
Regards, 
Nick


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## Kludge (Dec 12, 2008)

Well, work at the sizes I do, cap discharge can even be overkill sometimes though it's nice for assembling battery packs. I really have no idea where the cross over would be as far as cost compared to more conventional ways. Since I can put together a fairly fat condensor (what we called them when we had saber tooth tigers for pets ) pack rated at 450 volts vs 48 volts using more current technology without spending a whole horrid amount of cash, it may be interesting to try a few experiments to see what happens. 

*sigh* ... my experiment list is growing. Is anyone keeping track of where I am on what?

Best regards,

Kludge


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## cfellows (Dec 12, 2008)

Kludge  said:
			
		

> Well, work at the sizes I do, cap discharge can even be overkill sometimes though it's nice for assembling battery packs. I really have no idea where the cross over would be as far as cost compared to more conventional ways. Since I can put together a fairly fat condensor (what we called them when we had saber tooth tigers for pets ) pack rated at 450 volts vs 48 volts using more current technology without spending a whole horrid amount of cash, it may be interesting to try a few experiments to see what happens.
> Kludge



Kludge,

What do you make your capacitors from?

Chuck


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## Bluechip (Dec 12, 2008)

Hi Folks

This seems to be going on for ages. I would have thought you had 'em all built by now. I think Kludge mentioned 'Computer Grade' capacitors before. Seen this link ??, someone with worse hand drawn schematics than me, it might have some pointers. No interest in blasting caps. off the face of the planet myself, so you're on you're own.

You need 'Low ESR' caps. of some description, and VERY expensive !! At least in UK. Sterling approaching parity with cat-crap 

http://www.philpem.me.uk/elec/welder/

PS the energy in a cap. is 'half x C x V squared'. In joules. IF you don't know.


http://www.philpem.me.uk/elec/welder/images/schematic.png

Edit Sorry, wrong link. :













Hope you survive ...

Dave


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## Kludge (Dec 12, 2008)

cfellows  said:
			
		

> What do you make your capacitors from?



Oops, bad wording. I apologize.

I don't make the caps, I happily join banks of them together to create what I need. It's fun and entertaining and keeps the ward attendants on their toes. ;D

Best regards,

Kludge


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## Kludge (Dec 12, 2008)

Bluechip  said:
			
		

> This seems to be going on for ages.



Well, yeah. Good topics don't die. They just ... ummm ... actually, I have no clue what they do. 



> You need 'Low ESR' caps. of some description, and VERY expensive !! At least in UK. Sterling approaching parity with cat-crap



They're not so very cheap here either. Well, nothing's cheap when "here" = "Hawaii" but these caps are also uncheap when "here" = "US."



> http://www.philpem.me.uk/elec/welder/



"Computer grade" caps just shrug off extremely low resistance discharge paths but I'm not convinced the "auto grade" caps can say the same thing. THis is one area for testing along with newer tech electrolytics for the HV versions. They're small, cheap and available.



> PS the energy in a cap. is 'half x C x V squared'. In joules. IF you don't know.



Ah, yes. THis looks familiar. C in farads, unless I want to play with fun decimal adjusting. Thanks!

BEst regards,

Kludge


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