Welded contacts are one of the most common failure modes in high-voltage DC contactors, occurring when excessive localized heating at the contact interface fuses the contact materials together. This application note explains the primary causes, including capacitive inrush current, short-circuit events, improper coil drive behavior, and mechanical shock, and notes that welding usually happens during contact closure even though it is typically discovered only when the contactor fails to open. Closing into an uncharged capacitive load, such as the input filter capacitors of a motor inverter, can produce thousands of amps in microseconds, so the circuit should be precharged to 5τ before the main contactors close. It also outlines oscilloscope-based diagnostic methods to determine root cause and prevent recurrence in battery systems, inverters, and DC distribution architectures.
