Mastering the Constant Voltage Power Source GMAW Volt Amp Curve

A constant voltage (CV) power source for GMAW maintains a nearly flat volt-amp output curve, keeping arc voltage stable while allowing welding current to fluctuate freely based on wire feed speed and arc length. When you look at the constant voltage power source GMAW volt amp curve, you are looking at the electrical blueprint for arc self-regulation. Unlike other welding processes where the operator manually controls the filler metal feed rate to match the heat, Gas Metal Arc Welding (GMAW/MIG) relies on the power supply's electrical characteristics to automatically balance the melt-off rate with the mechanical wire feed speed.

The Physics of the Flat Volt-Amp Curve

To understand why GMAW requires a CV output, we have to look at the slope of the volt-amp (V-A) curve. The V-A curve graphs the relationship between the voltage output (Y-axis) and the current draw (X-axis) of the power supply under load.

A true constant voltage machine has a very flat slope, typically dropping only 1.5 to 2.0 volts per 100 amps of current increase. Because the voltage remains essentially constant across a wide range of amperages, the welding current becomes entirely dependent on the electrical stickout (the length of unmelted wire extending past the contact tip) and the arc length itself.

This flat curve changes a fundamental dynamic in the real circuit: it shifts the burden of current regulation from the power supply to the physical geometry of the arc. The power supply dictates the arc length (via voltage), while the wire feeder dictates the baseline current (via feed speed). According to the American Welding Society (AWS), this decoupling is what makes semi-automatic and robotic GMAW possible, as the machine automatically compensates for the inevitable hand tremors of a human welder or the joint variations in a robotic cell.

Worked Numeric Example: Arc Self-Regulation in Action

Let us run the numbers on a standard short-circuit transfer setup using 0.035-inch ER70S-6 solid wire with a 75% Argon / 25% CO2 shielding gas mix.

Imagine the welder's hand pushes the gun slightly closer to the workpiece, reducing the arc length by 1/8 inch. Here is the exact sequence of electrical events that occurs in milliseconds:

1. Voltage Dip: The shorter arc drops the circuit voltage from 24V down to 22V.
2. Current Spike: Because the CV power source is trying to maintain its flat curve and the electrical resistance of the shorter arc/stickout has dropped, Ohm's Law dictates a massive current surge. The current spikes from 200A up to 280A.
3. Thermal Response: The melt-off rate of the wire is proportional to the square of the current ($I^2R$ heating in the stickout plus anode heating at the arc). The jump from 200A to 280A increases the resistive heating factor by nearly 96%.
4. Self-Correction: The wire melts significantly faster than the 300 IPM feed rate can replenish it. The arc length physically lengthens until the voltage climbs back to 24V and the current settles back to 200A.

If this were a Constant Current (CC) machine, the current would have remained locked at 200A. The wire would simply stub into the puddle, the arc would go out, and the welder would have to manually pull the gun back to re-establish the arc.

What People Commonly Confuse It With

The most frequent point of confusion is mixing up CV (Constant Voltage) with CC (Constant Current) power sources, often referred to as 'droopers'.

A Constant Current (CC) power source has a steeply drooping V-A curve. It is designed for processes where the filler metal is fed manually by hand, such as Gas Tungsten Arc Welding (GTAW/TIG) or Shielded Metal Arc Welding (SMAW/Stick). In a CC circuit, the voltage varies wildly as the operator moves their hand, but the current (the heat) remains locked. If you attempt to run a wire feeder on a CC machine, the lack of current fluctuation means the arc cannot self-regulate. The wire will either burn back into the contact tip or stub violently into the workpiece.

As detailed in the Lincoln Electric Process and Theory Guide, modern inverter machines often feature switchable or software-defined V-A curves, allowing a single power supply to act as a CV source for MIG and a CC source for TIG. However, the underlying physics of the arc demands the correct curve for the specific process.

Where You Meet This in Practice

You will encounter the CV volt-amp curve in any environment relying on continuous wire welding processes:

• Robotic Welding Cells: Automotive manufacturing relies entirely on CV outputs. The robots do not have tactile feedback to adjust the torch height in real-time; they rely on the CV curve's self-regulation to maintain a stable arc over stamped parts with varying fit-up gaps.
• Shipyards and Heavy Fabrication: When running Flux-Cored Arc Welding (FCAW) or Submerged Arc Welding (SAW) at 400+ amps, the CV curve ensures that minor variations in the flux layer or joint geometry do not cause massive swings in heat input, which could lead to lack of fusion or burn-through.
• Auto Body Repair: Short-circuit MIG welding on thin 22-gauge sheet metal requires a highly responsive CV curve to prevent the wire from stubbing and blowing holes through the thin parent metal.

Decision Path: Selecting Your Power Source Output

Choosing the right machine architecture depends entirely on the processes you intend to run. Use this decision tree to select the correct V-A curve capability for your shop.

Default Recommendation: If you are setting up a general fabrication space and need to run GMAW reliably without breaking the bank on premium waveform inverters, buy a dedicated CV machine like the Miller Millermatic 255. It provides a highly stable, factory-tuned flat volt-amp curve optimized specifically for short-circuit and spray transfer MIG, eliminating the compromised arc characteristics often found in budget 'do-it-all' CC/CV switchable machines.

FAQ: Troubleshooting the CV Curve in GMAW

Why is my wire burning back into the contact tip?
Burnback occurs when the melt-off rate exceeds the wire feed speed. On a CV machine, this usually means your voltage is set too high for your WFS, or your travel speed is too fast. The arc length extends, voltage stays high, and the current melts the wire faster than the feeder can push it. Fix: Drop the voltage by 1-2V or increase the WFS by 20 IPM.

Why does the wire keep stubbing into the puddle and stopping?
Stubbing is the opposite of burnback. The voltage is set too low, meaning the CV curve is forcing a low-current state that cannot melt the wire fast enough to keep up with the mechanical feed. The physical wire crashes into the workpiece. Fix: Increase the voltage tap by 1-2V to allow the current to spike and melt the wire properly.

Can I use a CV machine for Stick (SMAW) welding?
Technically, you can strike an arc, but it will be nearly impossible to control. Because the CV curve tries to maintain voltage, any slight change in your hand's arc length will cause massive, violent swings in current. The rod will either freeze to the plate or blow through the puddle. Always use a CC (drooper) curve for Stick and TIG.

For deeper troubleshooting on specific transfer modes and voltage thresholds, refer to the Miller Welds Welding Guides for exact parameter charts matched to your wire diameter and gas mixture.