Home/Technical Glossary/Inverter (HF) Inverter (HF) Inverter Welders use submillisecond pulsewidth modulation (switching) technology with closed-loop feedback to control the weld energy in submillisecond increments. Three phase input current is full wave rectified to DC and switched at (up to) 25 kHz to produce an AC current at the primary of the welding transformer. The secondary current is then rectified to produce DC welding current with an imposed, low-level, AC ripple. The high-speed feedback circuitry enables the inverter power supply to adapt to changes in the secondary loop resistance and the dynamics of the welding process. For example, a 25 kHz inverter power supply adjusts the output current every 20 microseconds after rectification, which also allows the weld time (duration of current) to be controlled accurately in increments as small as 0.1 milliseconds. The high frequency closed loop feedback can be used to control (maintain constant) either current, voltage, or power while also monitoring another of the same three parameters. Additional benefits of high frequency switching technology include reduced power consumption, smaller welding transformers, and the use of a very short pre-weld ‘check pulse’ to test electrode and parts positioning prior to executing a weld. The result of this pre-weld check can be used to inhibit the weld by setting check limits. For more information regarding how to choose the right power supply for your application, read our blog: AC, DC, CD or HF: Which Spot Welding Power Supply Should I Use? And for more information about the resistance welding process read our Fundamentals of Resistance Welding and The Benefits of Closed-loop Control for the Resistance Welding Process.