Things You'll Need
The "choke" mechanism controls the amount of current, or amperage, that flows through an arc welder. Arc welders use direct current, rather than alternating current. If the welder is well constructed, the power flowing through the arc welder will be "clean," that is, relatively steady and constant. The primary means of controlling the arc welder's power, then, is via the choke mechanism.
Disassemble your existing welder. If you are rebuilding a welder, you will be able to see the choke mechanism. The choke mechanism is simply a metal rod that moves through a hole in a wire coil. The current moves through the wires toward the arc points. If you are building an arc welder from scratch, imagine you are creating a metal finger and pointing it through the hole in a copper wire doughnut, in a design similar to that of an electric magnet or guitar pickup.
Coil stripped, insulated copper wire neatly in a tight doughnut shape, leaving a hole in the center just big enough for the metal core.
Place a heat-resistant nonconductive barrier to shield the transducer rod from physical contact with the coil.
Place a ferromagnetic iron rod through the center of the coil. It is the movement of this rod through the coil that varies the amperage that the choke lets through.
Attach the positive and negative leads to either end of the wire coil and on to the arc welder.
Solder or otherwise attach an insulated knob to the ferromagnetic iron core. This knob moves the core back and forth within the coil, adjusting the amperage.
Consider adding capacitance to the system after the rectifier, which will help smooth out the roughness remaining in the AC to DC conversion process.
Ensure your wire coil is substantial enough to handle the current from your power source. In the United States, you can expect wall-generated power to be 110V alternating current. Overseas, it may be 220V alternating current. However, for DC arc welding, the power will have gone through a rectifier prior to reaching the choke, which can vary the voltage. If the wires in the coil are insufficient for the voltage, however, the choke may break down. Do not attempt to make electrical devices without substantial training in electricity and electrical safety.