Power controllers (SCRs, SSRs, and thyristors) are typically used for proportional control as they allow the temperature controller to use any output percentage from 0% to 100%. These are all solid state controllers, meaning they have no moving parts.
Power controllers typically function in either with either “on/off” or “phase angle” control logic. On/Off involves switching on and off at the zero-cross portion of the AC/DC sine wave, resulting in low electrical noise. This is often used with wire elements. Because phase angle systems switch at any point on the sine wave, there can be significant electrical noise, typically restricting its use to certain heating element materials such as molybdenum disilicide and silicon carbide, because their resistance changes significantly with temperature and age respectively.
Because there are no moving parts, these power controllers can switch on and off faster than any other type of switch. The result is better, “proportional” control and longer element life. As long as the power controller stays cool enough, it will last longer than most other types of relays as well.