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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.

Step 1: Tools and materials

For the welder
1 microwave oven transformer
0.5mm copper sheet for re-winding, taken from an old hot water cylinder
Scrap wood
10cm (4") hinge
19mm steel tubing from old rotary washing line
Steel rod taken from old rotary washing line
Brass or copper rod
Nuts, bolts, screws
Copper tubing offcuts
3 core mains cable, connectors
Cross dowels
Solder
Steel sheet offcuts

For the footswitch
Microswitch from microwave oven
Scrap wood
A long bolt
Spring from liquid soap bottle

Tools
Square
Wood saw
Hacksaw
Screwdrivers
Chisels
Drill
19mm drill or hole saw (same diameter as steel tube if different)
small wood drill bits
high speed drill bits
taps and dies
hammer
blowtorch

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Quick safety question. I have rewrapped the secondary of my mot with 2/0 with only a couple turns so I'm getting 2.5 v out. So at 12 amps in at 120 I have the potential of 575 amps out. So I'm just a tab bit nervous about holding the thing I want to spot weld....Even though I realize at 2.5 v it'd have a pretty hard time of poking me. You guys have had no problems?
<p>The secondary is isolated from the mains, so unless you did something wrong it can't hurt you. The current that flows is dependent on resistance - so even if you grabbed both electrodes firmly with your bare hands you wouldn't feel anything.</p>
Oh yeah that makes sense. Thanks!
I made this also, and I just cut one welding with a hacksaw and bent it open, stuck in a set of jumper cables, and super glued it back up, works like a charm!
<p>Interesting that you took the route of cutting a weld. I had thought about this but decided I didn't want to risk doing irreversible damage to the transformer. Brave you :)</p>
<p>i need technical specifications of the transformer</p>
<p>All I can tell you is that it's the transformer from an 800W microwave oven, and that the primary was wound with approximately 1 turn per volt. Everything else you need to know is in this instructable!</p>
<p>I am curious as to why most people are not using something like jumper cable or the like for secondaries.</p>
Tried it with expensive jumper cables, in about 10 seconds the outer plastic started to melt off xD
<p>Jump leads come in two types - cheap and expensive. The cheap ones tend to have very thick insulation to make them look better, so are not really a good type of wire to use, and with expensive ones probably the wire is too thick, and would you really want to cut one up anyway? Either way you would probably find it was simply too thick.</p>
<p>Brilliant!</p>
<p>Is there any alternative for microwave transformer? how about other appliances do you have any idea?</p>
<p>Hi fburito</p><p>The only thing I can think of straight away is a &quot;site transformer&quot; - 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 &quot;buyer collects&quot;. 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.</p><p>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: </p><p><a href="http://ludens.cl/Electron/trafos/trafos.html" rel="nofollow">http://ludens.cl/Electron/trafos/trafos.html</a></p><p>Good luck!</p>
Congratulations on an excellent piece of work. I&acute;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.
Wow - I'm deeply flattered! Thank you :D
taanks <br>
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...
If I re-wind another transformer like this, I will certainly try to make a video of it.
Your ingenious solution to the secondary winding is terrific. An excellent design.
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..
I have no idea what I would mill though! Besides, it's a copper float and has great potential to make something *shiny* :o)
Very good work! Congratulations.

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Bio: Loving getting back into electronics as a hobby after a break of many years. Now I work as an EPOS engineer, so I spend my ... More »
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