A resistance welding power supply delivers tens of thousands of amps for a few hundred milliseconds to fuse two metal sheets together at a spot. The nugget — the molten zone that solidifies into the weld — grows as a function of heat input: current squared times resistance times time (I²Rt). If the current, the resistance, or the time varies from weld to weld, the nugget size varies, and with it the joint strength. The power supply's regulation mode determines which variable it holds constant despite process disturbances, and which variables it lets float. Picking the wrong regulation mode for your application means the controller fights the wrong disturbance while the real source of weld variation goes unchecked.
Constant Current: The Default Mode — and Its Blind Spot
Constant current regulation is the most common mode in resistance welding. The controller measures the primary or secondary welding current via a Rogowski coil or current transformer, compares it to the setpoint, and adjusts the SCR firing angle or inverter PWM duty cycle to maintain the commanded current throughout the weld pulse. If the line voltage sags 5% when the neighboring press hits its weld pulse, the controller opens the SCRs earlier on the next half-cycle to compensate — and the current stays flat.
Constant current compensates for line voltage variation and transformer impedance changes with temperature. What it does not compensate for is resistance variation at the weld interface. If the contact resistance between the electrode and the workpiece rises — because the electrode cap has mushroomed, or the workpiece surface has oxidized, or the part fit-up leaves a gap — the heat generated (I²R) increases in proportion to R, even as the controller dutifully maintains constant I. A 20% increase in interface resistance with constant current produces 20% more heat — potentially causing expulsion (molten metal ejection from the nugget), which weakens the joint and creates a safety hazard for the operator.
Use constant current when: the dominant disturbance is line voltage fluctuation, the electrodes are dressed on a regular schedule, and the material surface condition is consistent — sheet metal assembly in a controlled factory environment with automated tip dressing.
Constant Power: Compensating for Resistance Changes
Constant power regulation measures both current and voltage at the welding transformer secondary (or estimates power from the primary side using the known transformer ratio) and adjusts the firing angle to maintain constant I × V product — that is, constant instantaneous power. If the contact resistance at the workpiece increases, the controller reduces the current to hold power constant. Heat input per unit time stays flat even as the load impedance changes.
This mode compensates for electrode wear — as the electrode cap deforms and the contact area grows, the current density drops and the interface resistance changes. Constant current would deliver the same total current into a larger contact area, reducing current density and producing a smaller nugget. Constant power adjusts the current to maintain the heat input rate, partially compensating for the changing electrode geometry. It also compensates for material surface condition variation — oil, mill scale, or coating thickness differences that change the initial contact resistance.
The trade-off: constant power control requires voltage sensing at the transformer secondary, which adds a pair of sense leads and a voltage measurement channel. On a pedestal welder with an integral transformer, this is straightforward. On a robotic weld gun with a remotely mounted transformer and a long secondary cable, the cable resistance and inductance add significant voltage drop that varies with cable routing and must be compensated for in the power measurement.
Use constant power when: electrode wear is the primary source of weld drift, material surface conditions vary from lot to lot, and secondary voltage sensing is practical — automotive body-in-white welding with robotic guns and automated tip dressing.
Constant Voltage: For Projection Welding and Controlled Collapse
Constant voltage regulation maintains a fixed voltage across the welding electrodes. As the workpiece heats and the resistance drops (the metal transitions from solid to mushy to molten, with resistivity falling at each phase transition), the current naturally rises to maintain V = IR — delivering more current as the weld progresses, which accelerates heating and speeds nugget formation. This is the natural compensation for positive temperature coefficient (PTC) materials like steel, whose resistivity increases with temperature — the rising voltage maintains current flow through a rising-resistance load.
Constant voltage is the standard mode for projection welding, where a small embossed feature on one part concentrates the current at a defined point. The projection collapses as it melts, the contact area increases, the resistance drops, and the current rises — the weld self-limits by the physical collapse of the projection. Constant voltage allows this natural current rise without the controller fighting it, producing consistent projection welds with a sharp end-of-weld current signature that can be used as a quality indicator.
Use constant voltage when: projection welding, cross-wire welding, and other applications where the part geometry changes during the weld in a way that a constant-current controller would misinterpret as a process disturbance. Do not use constant voltage on aluminum, copper, or other materials with very low and nearly temperature-invariant resistivity — the current will spike uncontrollably as the contact area grows.
For advanced multi-axis welding applications, controllers like the Soyer MA-400 S CNC stud welding controller provide configurable current/power/voltage regulation modes with 8-axis servo control for automated weld cell integration. Browse the broader electrical power control catalog for power supplies and controllers spanning resistance welding, heat treatment, and related high-current applications.
Constant current ignores resistance variation. Constant power compensates for it. Constant voltage exploits it. The regulation mode is not a preference — it is a decision about which process disturbance you are willing to let through and which one you cannot afford to ignore.
