Modernizing a Resistance Welding System: From Ignitron to SCR Technology

Many continuous steel processing lines still rely on technologies installed decades ago. While groundbreaking at the time, these systems now face challenges related to maintenance, spare parts availability, and process reliability.

This case study presents the modernization of a power control system used in resistance welding for steel coil splicing, replacing an Ignitron-based solution with a HYAMP Varixx SCR power controller.

The Application

The process involves resistance seam welding with disc electrodes (rollers) to join steel coils in continuous rolling, pickling, galvanizing, and slitting lines. This operation is essential to keep production running during coil changes without interrupting the process.

Accurate power control is critical to ensure consistent weld quality while preventing cold welds, overheating, and excessive weld spatter.

The Original System

The original system, installed in 1976, used Ignitrons as the power switching device.

Developed in 1933, the Ignitron is a mercury-arc rectifier capable of controlling extremely high currents. Although it was a breakthrough technology before the advent of power semiconductors, it gradually became obsolete.

The system required frequent maintenance due to ignitor wear and the handling of liquid mercury, creating environmental and occupational risks. It also offered lower firing accuracy, was sensitive to operating conditions, and depended on components that are now extremely difficult to replace.

The Modernization

The system was upgraded with a HYAMP Varixx SCR-based power controller rated at 2,000 A, responsible for controlling the primary side of the welding transformer.

The new solution eliminated mercury-based technology and mechanically worn components while delivering significantly more precise power control.

The controller provides microsecond-level firing accuracy, precise synchronization with the AC supply, closed-loop RMS current monitoring, and integrated protection against overcurrent, short circuits, and firing failures.

Built-in diagnostics and monitoring capabilities also support predictive maintenance and improve process traceability.

Results

Replacing the nearly 50-year-old Ignitron system with a modern solid-state solution significantly improved the welding process.

The modernization delivered:

• Greater welding stability and repeatability.


• More accurate power control.


• Reduced maintenance requirements.


• Elimination of mercury-related environmental and safety risks.


• Easier access to spare parts and long-term technical support.


• Increased production line reliability and availability.

Ready for the Next 50 Years

This modernization demonstrates how legacy industrial equipment can be upgraded with today's power electronics while preserving the existing process and significantly improving efficiency, reliability, and safety.

With more than 50 years of experience in power electronics, Varixx develops engineered solutions that help keep mission-critical industrial processes operating safely, efficiently, and reliably.