MOT Spot Welder





Introduction: MOT Spot Welder

Well, I am going to attempt to make a spot welder that can join two 1.2mm bits of stainless sheet together. Lets get started.

First things first, the tools! My awesome makita 18v battery drill and it's rather odd brother, and some hand tools, very handy yup.

Step 1: Prep Your Tranny

There are a slew of microwave oven transformer instructables out now, so really I shouldn't bother with this eh? Nahh, I did it this way ...

Notes on the pics.

Step 2: Is It Safe? & Rewinding the Secondary

I found a nice bit of car audio cable, had no size marked on it but it was fat... ish. I wound as much of it as would fit nicely into the space on the former, then marked the end for my cut length with some tape.

At this point I started to wonder if I had scratched the primary coil at all and so decided to get my test meter out and have a zap. I also decided to enclose the audio cable in some heatshrink rated at 600/1000V just to make sure the chance of an insulation breakdown between primary and secondary was much less likely.

The heatshrink will also serve to stop the original insulation melting all over the place. I just KNOW this thing is gonna be overloading that wire big time.

Step 3: Juice It Up Baby

Right! Lets see what we get out of this thing.

It's humming at me, good start. Not doing much. What if I short it out... fzzazzppzzzfrrp woo neat, that burnt up a lot of the tiny strands in the car wire.

I was a bit silly and didn't take any pics as far as I can find when it came to turning this on and measuring the output, here's what I measured though.

Vp = 235Vac
Ip = 5.8A
Vs = 3Vac
Is = 400A+ overloaded my ammeter which only goes to 400.

Well, seeing as I know the primary voltage and current, and the secondary voltage, we can find out what the secondary current is in theory at least.

Vp/Vs = Is/Ip

Is = (Vp/Vs)*Ip
= (235/3)*5.8
= 454.3A

Just enough to make my meter grumpy.

I'm not 'avin it! (PhotonicInduction quote hehehahahah) I'm going to remove that secondary wire and redo it.

Step 4: Rewind, Go Reverse, Back Up.

OK, so shorting out the secondary made the lights dim a little and the tranny made a medium volume hum at me, the current it pushed out was enough to melt a few piddly bits of wire, but clamped onto the two bits of SS I want to weld, it made a loud MAAAAA noise and a dirty brown mark on the metal. SHITTY!

I think changing to some proper cable might improve things a lot. Out with the flexible stranded unknown size, in with the 16mm^2 tps. Now this stuff is usually used to wire house mains and has a rated current of 63Amps (remember that, you'll freak later).

In the pics I've gotten a bit of twin cable and stripped off the outer sheath. I wrapped some insulation tape around the inside of the tranny core because it was a bit sharp on the inside edges.

Wind the red on, no problem. Wind the black on, sweet as. Well that was piss easy, lets test.

Step 5: Woah, That's Got Grunt...

Wires wound, mains connected, woah wait, lets just try this with a choke first.

I'm limiting the primary current to roughly 1amp just to see what goes on when I short out the secondaries in different configurations.

Red shorted, mmm, nice 70.1Amps @ 3.2V. Same measurements for Black too, very good.

OK Red and Black connected in parallel, we expect to see 140Amps @ 1.6V, but we see 68.3Amps @ 1.8V, why? Limited input, in order to double the output we have to let the input pull ~2Amps instead of the ~1Amp it is stopped at.

Right, connect the windings in series, expected 35Amps @ 6.4V, we have 34.6Amps (nice!) @ 3.8V. Ah, as above not enough flux to supply the doubled amount of voltage.

That was limited to 1Amp on the primary side. REMOVE THE CHOKE!

I must admit that I got a fright when I pushed the button this time, bloody loud 50Hz hum and the whole thing jumped 2cm as the current blasted through the dead short across the parallel connected secondaries.

Before I press that button again I'll connect the ammeter and see what the primary is pulling, *eyes bulge* 31.59Amps!
Oh yeah, I gotta work out the secondary current, this'll be insane...

Vp/Vs = Is/Ip

Is = (235.8 / 3) * 31.59
=78.6 * 31.56

HOLY #(*Q!% No wonder the bit i used to short it was too hot to touch after 1 second, the windings were fair hot too. Remember that current rating I mentioned earlier? The winding are rated to carry 63Amps safely and they were shifting 1241.5Amps per winding. That's just a weeny bit much for em, I'll have to come up with a variable limiter to put on the input, I don't want to blow holes in my spot welding projects.

Well this is the 'engine' for the spot welder, I'll add the arm construction to this as I work my way through it.

Step 6: Update, and Re-hash of the Wee Beastie

I decided that the bit of flexible lead in the middle was bugging me far too much, so the whole thing got pulled apart and redone.

There is one less turn on the secondary now, so that means the potential current has gone up by a ton, meh, we can handle that I reckon.

I was wondering about how to get the contacts to line up and be in the proper place to make a weld, so I hunted about and rather than all the effort of making wood arms, I got lazy and used some high impact conduit. It was all I had laying around anyway haha!!

Measure, cut, bottom arm needs something to support the, er, 'contact', conduit end box will do nicely. Drilled a hole for the 8mm bolt and secured the 2 lugs to the lid, and poked the bolt up and tightened it all up. Screw it to the wood and there we go, nice.

Top arm was pretty much the same procedure except I wanted a handle to be able to push the top contact down, so I found a 'through box' and used that in pretty much the same way as the lower one. The hinge bit on the end is terrible at the moment, I have to rebuild it with something a lot stiffer than the pc power supply case I've used.
I put a couple of wood guides on the board to keep the arm aligned properly, but that hinge at the back is pretty damn useless, it allows forward and back movement, and the bolt threads have engaged and locked a couple of times so far. Luckily not while it was turned on.

The transformer, fuse, and stiff wire to flexible wire joint are all housed in a nice plastic box now, so no more accidental touch hazard exists. Very little left to do now, just a few cosmetic things really.

I've got a vid of me testing it out, and then one of me testing it .... again... you'll see hahahaha

test 1 =>

test 2 =>

(edit 30/Oct/2012: fixed youtube links)



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    test 1 =>

    test 2 =>

    doesn't work. Please, can you re-upload ? Tnx

    1 reply

    I'm working on it. I've got a fair bit on at the moment, so everything is going at a snails pace while tougher aspects of looking for a new house are being dealt with.

    Oops, I forgot about these links. I'll see if I can find them and reupload, if not I'll just have to make a vid of the welder making random noises and sparks

    I am making a spot welder just like yours but do you have to use a ballast or just hook it up to the mains?


    6 replies

    I've changed the internals of the box a little bit to include a 6amp circuit breaker just in case anything goes a bit wobbly, but apart from that I've got the lead connected straight to the primary of the transformer.

    The wee beast works pretty well. I have to replace the current bolts (electrodes) with copper though, because they heat up too much and end up sticking to my work sometimes.

    Thanks for the replie, i have made my spot welder on the weekend including a emf filter and a 10amp circuit breaker. I did try to weld thin tin cans and my brass bolts (electrodes) stick to my work too its imposible to get the work off the bolts with out breaking the weld, it also rips a tiny part of the brass off on to my work.

    I am wondering if i could use a harder brass for the electrodes or i could use copper, but i dont know how i could make electrodes out of copper?

    When I was using it it didn't take long to heat up 1mm stainless steel sheet and bond 2 together. Those tin cans are pretty thin so it might just be a case of closing the jaws for a shorter amount of time.

    A better idea might be to put a push button in that energises the primary coil when pressed, that way you can clamp your work into the jaws and press the button really fast. The weld would end up in the right place, and with a bit of practice you can judge just how long to hold the button down. I might give that a go actually.

    Have you filed your bolt ends to rounded points where they touch your work piece? That will make a difference. Post some pictures, let us see!

    I cant post any photos at the moment as the welder is in bits every where but i will soon, any way, im using the wall swich to turn on and off the welder and the brass bolts are not rounded at the end.

    My brass bolts glow read hot at only about 10 seconds into the weld and i put the arm down before i power it on and pull it up after it is off. If i go any lower than 10 seconds it does not weld thin can.

    The voltage coming of the secondary is about 1.5volts no load if that helps.


    Oh sorry,

    Do i have to remove those seperators inbetwen the primary and secondary coil

    Too much heat. What you're aiming for is to heat your work just past the melting point, but only juuust past. If those bolts are glowing and the tin isn't melted together you might need thicker bolts to let more current pass.

    Don't bother removing the separators for now, sounds like there is plenty of current there already.

    My thoughts about the intended function of the welder is to send high current to the work through a low resistance path. The resistance of the work is what creates the heat to weld the pieces together. Rounding the ends of the bolts will give a smaller 'pinch point' which will increase the heating effect on whatever you have clamped in there, it should also reduce the stress on all but the tips of your bolts too.

    Nice work - I watched the testing videos. Wish I had a clamp meter...

    4 replies

    I must admit, mine is a fairly cheap one. But seeing as I usually only work on residential up to medium machinery the 400Amp range is quite ample for my needs.

    For some really nice meters check out Fluke ones. Or for a big selection try

    Hmm, a lot cheaper than I thought they were, but in entirely the wrong country! I've actually started wondering how easy it would be to make a clamp meter probe for my standard multimeter. *Thinks* surely I can just put a winding on a ferrite core for this....

    Dude, don't worry about which country anything is in now! The interweb has freed us of all those restrictions *dance*

    To make your own clamp meter you'd need to be pretty good with transformer calculations, or at least have some steady current drains you can calibrate your measurements with.

    If you used a ferrite core there may be the problem that once installed as a current transformer you'd have to leave it in place, or pull apart your wiring to get it back. Have a look at this page under 'Wound Primary CTs'

    Actually found some perfectly adequate clamp meters on eBay very cheap - perfectly adequate for my very occasional and not very accurate needs! Interesting web page - I once read a book about transformer design (about 20 years ago) and realised it was something I was never going to do simply because I didn't know where to get a core with appropriate known qualities to work out the primary turns - for me it was always going to be knowing the basic ratio and thinking of a number for the primary turns - not at all satisfactory.

    Haha I love Photonicinduction and alll the stuff he dose