GMAW (Gas Metal Arc Welding, commonly called MIG welding) uses a constant voltage (CV) power source. This is the defining feature that separates a GMAW machine from the constant current machines used for stick or TIG welding. The CV power source works in tandem with a wire feeder, welding gun, and shielding gas system to create a self-regulating arc that keeps the weld stable even as your hand moves the torch closer to or farther from the workpiece.
Why GMAW Requires a Constant Voltage Power Source
A constant voltage power source maintains a nearly fixed output voltage while allowing the amperage to fluctuate freely. This matters because GMAW feeds a consumable wire electrode into the arc at a constant speed. As you weld, small changes in your torch distance cause slight shifts in arc length. On a CV machine, that tiny voltage change triggers a large, automatic swing in current, which speeds up or slows down the rate the wire melts. The arc essentially corrects itself without any input from you.
This self-regulation is what makes MIG welding so approachable. You set your voltage and wire feed speed, and the machine handles the rest. A constant current machine can’t do this because it holds amperage steady and lets voltage swing wildly, which would cause the wire to either burn back into the contact tip or pile up against the workpiece.
Standard Polarity and Electrical Setup
GMAW machines run on direct current (DC), almost always with the electrode set to positive (DCEP). This polarity drives deeper penetration into the base metal and produces a more stable arc. Running the electrode on the negative side (DCEN) does melt wire faster and puts less heat into the workpiece, but it causes arc instability and erratic metal transfer. For that reason, DCEN rarely shows up outside of specialized research applications.
The Four Core Components
A complete GMAW system includes four major pieces of hardware:
- CV power source: Supplies direct current at a set voltage. This is the machine itself, either a standalone unit or part of a multiprocess welder.
- Wire feeder: Pulls electrode wire off a spool and pushes it through the gun at a constant speed you select. Basic feeders only control wire speed, requiring you to walk to the power source to adjust voltage. Intermediate and advanced feeders include voltage controls right at the feeder, which saves significant time when the power source is far away.
- Welding gun: Delivers the wire, electrical current, and shielding gas to the joint. It contains a contact tip that transfers current to the wire and a nozzle that directs gas flow around the arc.
- Shielding gas system: A compressed gas cylinder, a flow meter (set in cubic feet per hour), and a gas line running to the machine. The gas displaces atmospheric oxygen and nitrogen to prevent contamination of the molten weld pool.
Shielding Gas Choices by Material
The gas you use depends on the metal you’re welding and the transfer mode you need. For carbon steel, the most common options are 75% argon/25% CO₂ for all-position work, or pure CO₂ for deeper penetration at lower cost (though with more spatter). Argon blended with small amounts of oxygen (1-2%) works well for stainless steel in spray transfer. Aluminum requires pure argon, sometimes blended with helium for thicker sections where more heat input is needed.
Wire Electrode Basics
GMAW uses solid wire electrodes that range from 0.023 inches to 0.125 inches in diameter. The most common choice for mild steel is classified as ER70S-6 under the AWS system. That name breaks down simply: “E” for electrode, “R” for rod, “70” for 70,000 psi tensile strength, “S” for solid wire, and “6” for its specific chemical composition. For stainless steel, ER309L is a widely used option (the “L” indicating low carbon content). Aluminum welding typically calls for ER4043 wire.
Thinner wires like 0.023 and 0.030 inches suit sheet metal and light fabrication. Mid-range sizes of 0.035 and 0.045 inches handle the majority of structural and general-purpose work. Heavier wires are reserved for high-deposition industrial applications.
Input Power: 120V vs. 240V Machines
Household 120V machines are the entry point for GMAW. They can weld mild steel from thin sheet metal (24 gauge) up to roughly 3/16 inch. That covers auto body panels, light furniture, and hobby projects, but runs out of power quickly on anything structural. Dual-voltage machines that accept both 120V and 240V input extend your range up to about 3/8 inch on steel and aluminum, covering the vast majority of shop and field work. Industrial single-phase and three-phase 240V+ machines push well beyond that for production environments.
Duty Cycle Ratings
Duty cycle tells you how many minutes out of a 10-minute window a machine can weld at its rated output before it needs to cool down. Light-duty machines typically carry a 20% duty cycle, meaning two minutes of welding followed by eight minutes of rest. Medium-duty machines fall in the 40-60% range, suitable for repair shops and moderate fabrication. Heavy-duty industrial machines run at 60-80%, built for production lines where the arc stays lit for long stretches.
If you’re welding short beads on small projects, a 20% duty cycle is rarely an issue. If you’re running long passes on structural joints or doing repetitive production work, a higher duty cycle keeps you from waiting on the machine to cool.
Inverter vs. Transformer Machines
Older GMAW machines use heavy copper-and-iron transformers to convert incoming power. They’re durable and simple but bulky. Modern inverter-based machines use high-frequency electronic switching to do the same job in a much smaller, lighter package. Inverters also draw less electricity for the same welding output, which lowers operating costs over time. The trade-off is that inverter circuit boards can be more expensive to repair if they fail, though reliability has improved substantially in recent years.
For portable work or shops where you move equipment frequently, the weight savings of an inverter machine is a major practical advantage. For a fixed station in a production environment, either type works well.
Pulsed GMAW Machines
Standard GMAW machines offer two main transfer modes: short-circuit (lower heat, good for thin material and out-of-position work) and spray (higher heat, fast deposition, limited to flat and horizontal positions). Between these sits a gap where the arc becomes unstable and produces large, irregular droplets called globular transfer. Pulsed GMAW machines solve this problem.
A pulsed machine rapidly alternates the current between a high peak and a low background level, detaching one small droplet of metal per pulse. This produces smooth, spatter-free welds at mean currents between 50 and 150 amps, a range where standard machines would be stuck in messy short-circuit or globular transfer. Pulsed transfer also works in all positions at higher deposition rates than short-circuit mode, making it ideal for thicker sections where you need more heat than short-circuit can deliver but spray transfer would be excessive. These machines cost more and require more setup knowledge, but they open up a wider operating window on a broader range of materials and joint configurations.