Picture of MOT spot welder tutorial

This instructable explains how to make a spot welder from a MOT (microwave oven transformer).

The primary coil (thick wire) will still be used as primary windings. The secondary coil (thin wire) will be replaced with very thick wire of less windings.

Do NOT plug in this transformer before it's ready. Especially not when the original secondary windings are still in there! This device outputs an extremely deadly high voltage. You have been warned!

Also, as in all my other instructables, read all warning notes! I'm not responsible for any accidents.

Test video:

Another video:
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Step 1: Requirements

Picture of Requirements
- a MOT (microwave oven transformer)
- 3 female slide connectors (not required, you can also solder the wires)
- wall plug with ground terminal
- pliers
- cutting pliers
- drill with thick iron drill bit (10mm or so)
- iron handsaw
- wood saw or jig-saw
- 1m thick 2 gauge flexible wire (1 meter) (if you don't have this, check step 5)
- a volt-meter (or multimeter)
- a wooden shelf (see step 7)
- some other small wooden pieces (see step 6 and 9)

Step 2: Removing the secondary windings

Picture of Removing the secondary windings
I first tried to remove the coil with cutting pliers but I soon realised that it was totally not possible that way.
This seems to be the best way to do it:

- recover any slide connectors (picture 1)
- saw off the windings as far as you can without damaging the primary windings (picture 2)
- take a 10mm iron drill bit and drill a few holes trough the windings (picture 2)
- do not damage the iron core of the transformer
- use long nose pliers, a hammer and a screwdriver, or whatever it takes to remove the secondary windings
- remove the two magnetic shunts (picture 3 and 4)
- clean up the hole you've made

The other steps will be easier, don't worry.

Step 3: The power connector

Picture of The power connector
The pictures should explain everything.

Step 4: Grounding the iron core

Picture of Grounding the iron core
Every touchable metal part must be grounded, for your safety!

If you don't do this, and something inside the transformer goes wrong (like a wire from the coil makes connection to the metal core) then the metal core will come under deadly voltage!

Adding a ground wire will make sure your RCD (Residual-current device, Diff-O-Jump) will make sure power gets shut down immediately.

There are lots of ways for connecting the wire to the core, I used solder. When soldering, make sure the wire doesn't come loose, because soldering to a thick metal piece isn't easy. I more suggest to use a ring connector and a screw.

If the MOT already had a connection to the iron core (picture 2), it's best to solder your ground wire on there.

Step 5: The secondary windings

Picture of The secondary windings
Note: Skip this step if you already have thick flexible wire.

- use multiple thinner wires if you don't have thick wire, like I did in the pictures
- don't damage the wires with your pliers when trying to twist them. (only use pliers at the end and at the beginning)
- the total length of the wire should be just 1 meter.

Step 6: Woodworking

Just follow the pictures ;)

Step 7: Adding the secondary windings

Picture of Adding the secondary windings
- connect the wooden piece and the MOT to the shelf (picture 1 and 2)
- make 2 windings with the thick wire, where the old secondary windings were. (picture 3 and 4)

Step 8: Adding a switch (not required)

Picture of Adding a switch (not required)
Connections for a single pole switch:
- go from the brown/red wire from the plug to the switch
- go from the other side of the switch to the MOT
- go from the other connection of the MOT back to the plug (to the black/blue wire)
- connect the ground wire (yellow/green) to the iron core of the MOT (like in step 4)

Connections for a double pole switch: (better)
- go from the brown/red wire from the plug to the switch
- go from the black/blue wire from the plug to the switch
- go with another brown/red wire from the switch to the MOT
- go with another black/blue wire from the switch to the MOT
- connect the ground wire (yellow/green) to the iron core of the MOT (like in step 4)

I strongly suggest to use a double pole switch! (so that's not like the one in the pictures).
This, because when turning a single pole switch off, there will still be voltage between the ground connection (iron core of the MOT) and the primary coil!!!
The MOT will stop operating, but it's still very dangerous when you touch it!

If you Do have a single pole switch, eject the plug from the socket before touching anything.

Step 9: Some more woodworking

Again follow the steps in the images.

Step 10: The big test!

Picture of The big test!
- make sure the output wires are NOT touching eachother
- grab your multimeter, and make sure it's in the VOLT position! else you'll destroy the multimeter
- plug in the spot welder
- do not touch any wires, also not your multimeter's wires
- measure the output voltage (AC) (it should be 1 to 3V)
- if the output voltage differs, please leave a comment

In theory, touching a 2V connection (the output from our transformer) would do no harm. You wouldn't even notice. But just for safety, don't touch it. There might always go something wrong. (example: a wire from the primary windings accidentally connects to the secondary windings)
codongolev3 years ago
also, if you left a wire poking out from either end and it pierced your skin, that high of amperage would kill you if it hit blood.