Picture of Spot Welder from a microwave oven transformer and water tank
I have seen a few projects which use an old microwave oven transformer re-wound with a low voltage secondary to make a hobby spot welder, and I decided to have a go at making one. I ended up designing my own using copper sheet to make the secondary, following an unsatisfactory experience attempting to use wire I made from about 100 strands of stripped down CAT5 cable, which at nearly 6mm diameter wasn't thick enough and melted its insulation, causing the winding to short circuit.

The copper strips were cut from an old domestic hot water tank, of the sort usually equipped with an immersion heater, though this one had a heat exchanger (which looks like it really needs to be made into a tesla coil primary... hmmmm....). The steel parts came mostly from an old rotary washing line which had lost it's line, and the electrodes were made from a brass arm from an old ball cock (still trying to think of something to make from the ball). The wooden parts came from an old pallet and a bit of old shelf. The brass electrodes tend to soften and deform and really need to be made of something tougher. I used the wire from my experimental winding, doubled over, to connect the secondary to the electrodes.

My original experimental winding (100 strands of CAT5) gave a cross sectional area of less than 28.3mm2. Standard wire of this thickness would be rated at only 104 amps in free air,

Using 5 x 0.5mm copper strips, 25mm wide gives a cross sectional area for the winding of 62.5mm2 , which if it were standard wire would be rated at about 185 amps. Retrospectively, I should have used 6 layers of copper, giving a rating of approximately 212 amps, however I had anticpated an extra turn of my winding and 6 layers would have taken up too much space.

Today it blew a 5 amp fuse, showing it to have been pulling at least 750 watts. Unfortunately I have no way to measure current in the primary as my meter only measures up to 1 amp, so I don't know what the actual power being delivered is.

Afterword - If I were building this thing from scratch again, I would position the transformer to the side of the jaws, so using shorter wires, and house the transformer to cover up the mains wiring. I'd connect the wires to the where the electrodes emerge from the jaws too, so the current has less electrode to go through.
rimar20001 year ago


Is there any alternative for microwave transformer? how about other appliances do you have any idea?

throbscottle (author)  Fab Muchada1 year ago

Hi fburito

The only thing I can think of straight away is a "site transformer" - the ones that are used on building sites to provide safer power for the tools. They are pretty big and beefy. I see them on Ebay a lot for very little money, though they tend to be "buyer collects". Of course they might not be available in your country. The next thing I can think of is a transformer from an arc welder - and it's already fairly close to what you want.

If you have access to some thin sheet steel you could of course cut out your own laminations. Would be a poor performer compared to proper transformer steel but it would do if you can't get anything else. I found this site to be quite informative:


Good luck!

Congratulations on an excellent piece of work. I´ve been searching on the net for a MOT spot welder diy project. This one of yours is by far the best. Thank you. Keep up the good work. No need of a video step by step demonstration. Pictures, drawing, text, just right.
throbscottle (author)  tropicaliano2 years ago
Wow - I'm deeply flattered! Thank you :D
yaser ayash2 years ago
Domenik3 years ago
cool instructables!! but what about those chisels?? like them what is the brand?
may be a video instructional and step by step, how work well done thanks...
throbscottle (author)  geraldino2503 years ago
If I re-wind another transformer like this, I will certainly try to make a video of it.
jds19693 years ago
Your ingenious solution to the secondary winding is terrific. An excellent design.
pj633 years ago
What a Great Instructable loads of pictures and well written with lots of information.....Thanks i've read lots of these as I want to make a spot welder,this is by far the best i've read..........If you want to make something out of the float ball from the cisten cock have a look at my 'Ball Mill' ...I have 4 ... I dont want to blow my own trumpet, but they are quick and easy to make and work really well......Thank for this again Phil..
throbscottle (author)  pj633 years ago
I have no idea what I would mill though! Besides, it's a copper float and has great potential to make something *shiny* :o)
rimar20003 years ago
Very good work! Congratulations.