If anything should happen we (as well as anyone affiliated with this site) are not in any way responsible for any damages done to you, your vehicle, or any of your surroundings, as a result of use or misuse of these plans. Welding can be very dangerous. Unless protective equipment is worn, you could damage your eyes or catch yourself on fire. If you choose to take part in any of these activities, you do so entirely at you own risk.
How an Alternator Works
The rotor is the part of the alternator that rotates. The rotor consists of a coil of wire wrapped around an iron core. The stator is another set of windings (three sets of windings distributed evenly at 120 degrees), which are fixed to the outside of the alternator surrounding the rotor. These windings are stationary and do not turn. As the rotor turns inside the stator an electric current is produced in the stator. This current is actually three phase AC (alternating current), because of the three windings, and the way the rotor turns. The current then passes through two sets of diodes, to make a total of six diodes. This is called a rectifier and converts the AC to DC. Then the current flows through a regulator to regulate the voltage, which is then used to charge the battery in the automobile.
Welding with an Alternator
Twelve volts is hardly enough power to weld, so if we bypass the regulator, switch the output leads from the automotive circuit to an outlet, wire some heavy cable for welding, then rev the engine and you can weld or power AC/DC equipment. The higher the revs the higher the voltage.

*Welding continually for a long time with an alternator will heat up the diodes, so if you want to do a lot of welding you will have to replace the rectifier with a heavy duty one.
You can buy heavy-duty rectifiers online. I took one out of a large ford alternator and hooked it up externally. I also wired a solenoid in line to the field and hooked up a remote switch at the stinger. This way I could switch it off once in a while to let it cool if I weld for a long time.
*When you are not welding the alternator is producing current. The current has to go somewhere so it will find the shortest path through the diodes and may burn them out. That is why I wired an ordinary light bulb (100 watt, 110 volt) to the output of the alternator. When not welding the light bulb lights up and uses up the current, then when you strike an arc the light bulb goes out, because you are using up the current in welding.
*It’s a good idea to wire an amp meter and a voltage meter, so you don’t pop the diodes.
*Cautionyou can only run certain brush type 110 volt equipment, like drills, grinders, lights, etc.
*Do not try and use your original vehicles' alternator  for this purpose. Hook up another alternator to your vehicle, or use a little engine to power this alternator for welding.
Below is a diagram of how I made my welder.
Building a Welder from an Alternator

Watch this video on how I built a welder
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