SCR adjusts the output voltage, while SSR controls the frequent on-off, but the output voltage cannot be adjusted.

The relay is a contactless switch type; its control principle is simple; the control current range is wide, ranging from several tens of amperes to thousands of amperes, but there is a high heat generation due to voltage reduction, which is more severe with higher current. The solid-state cooling effect for high power is not very good, and the safety protection measures are not very comprehensive (such as: three-phase power, current imbalance, no main circuit voltage detection, over-temperature alarms, overload protection, and load abnormal detection). It is particularly susceptible to fluctuations in the main voltage imbalance, which may cause internal component damage. The trigger voltage is high but the response is slow; there may still be a small leakage current after shutdown. Phase-shifting voltage regulation generates a large slope wave; it has a significant impact on grid shock and pollution. Additionally, it has poor overload capacity; to purchase solid-state, power must be increased by more than 20%; this consequently increases unnecessary cost; it is not aesthetically pleasing, has a large volume due to high power, poor electromagnetic interference resistance, and complicated wiring or on-site maintenance...