Disclaimer: Do this at your own risk. Microwave Oven Transformers are dangerous.

There are plenty of tutorial to show you how to make a MOT (microwave oven transformer) spot welder.  Just Google it.  You basically take a microwave transformer.  Remove the secondary winding (small wire), and rewind the secondary with a large gauge wire.  I recommend 4 gauge.  You will get two to three turns.  The principle is on the primary winding, you have 120V 15A and the secondary winding 3-4V and several hundreds amp.  Your goal is to make enough turns to get 3-4V.





If you just want to spot weld two piece of metal together, you can use a switch to turn the power on and off.  If you want to spot weld tab on battery, you want a very short pulse.  Pulse in the range of 80-100ms.  It is just not humanly possible to consistently achieve this.  Therefore, you want to use some kind of timer.  In my case, I am using an Arduino with a potentiometer to control the pulse duration.  If you don't want to have the ability to set pulse duration, you just need an Arduino.  In the code, you can pre-set the duration of the pulse.  If you want to change the pulse duration, you just have to change it in the code, recompile, and reload.  It is just not as elegant, but is perfect if you just want to spot weld battery.

Once you have your spot welder built and working, you will need to add a few things.  First is a rely AC to DC 25A.  You want to run AC neutral from plug straight to the MOT.  For the hot wire, you run that to AC terminal of the relay.  The other AC terminal, run a black wire to the MOT.  On the DC terminal, you will run one to D13 and the other terminal to the ground on your Arduino.  You will have to change the code to fit whatever adruino you use.

https://www.amazon.com/gp/product/B0087Z...UTF8&psc=1




Lets wire the Arduino
I ran the GRN and 5V from Adruino to a bread board.  Because there will be a few connections to these two things.  The relay, potentiometer and LCD will need GRN and 5V.

LCD https://www.amazon.com/gp/product/B01E4Y...UTF8&psc=1
The biggest thing with LCD is trying to find the address.  Be sure to purchase one with a I2C port.
From LCD to Adruino
GRN to GRN
VCC to 5V
SDA to A4
SCL to A5


Pot 10K
Three pins potentiometer.  GRN on one end and 5V on the other.  The middle pin, output voltage goes to A0 on Adruino.


Foot Pedal https://www.amazon.com/gp/product/B00GWF...UTF8&psc=1
You will see three wires (red, white and black)
Wiring from foot pedal to Adruino
Red wire to D8
White to GRN


You will need to download Adruino IDE software
https://www.arduino.cc/en/Main/Software
There is plenty of tutorial on how to load a Sketch onto your Adruino.  I am not going to get into that.  You will need the LiquidCrystal_I2C library that I included below.  The Sketch is the text file below.
If everything works, you will have see a number that changes when you turn the pot knob.  It should go from 0-5115.  I usually set it to 80 ms for spot welding battery tab.  


When you press the foot pedal, you should see "Shock" on LCD.  You will also notice that the LED on the Adruino Nano lights up for the duration.

Thanks voodoo!!

Looks dangerous - maybe add a disclaimer? "I am not responsible if you kill yourself..."

Nice one Voodoo, I actually have a few MOT's after asking the local facebook community if anyone has busted microwaves they need to get rid of. MOT is incredibly versatile, as well as other bits from a microwave (motors, fans, magnets, caps), but like Scotty implies, always dismantle them with care, there could be lethal charge stored in the massive cap, and other potentially harmful materials too! I have not yet put any of my MOT's to use, I think a spot welder might be a good place to start...

I do agree that MOTs are useful. I had my MOT for years before building the spot welder.

It is dangerous if you don't take it seriously. Although I didn't show it, but I also put a kill switch and a LED light when the welder is armed. Good idea with disclaimer. I will add it to the beginning.

It is pretty fun welding things together.