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This article focuses more on the application of SCRs and less on the history of rectifier devices or solid-state theory and appropriately so for the IEEE Industry Applications Society (IAS) members. Much of the importance of the SCR can be traced to the window it opened to the wider understanding of the switch-mode operation. The tremendous benefits of the switching operations in power electronics came into the public domain following the introduction of the thyratron. The theory of phase control was well advanced by the time the SCR became available. Although phase control circuit theory was widely known, the complexity of those circuits and the fragile nature of the rectifier devices greatly restricted practical applications of phase control. The SCR largely overcame both obstacles. Using phase control devices, opening doors to a wider application of switch-mode operating devices, including time-ratio-control (TRC).