The document discusses several power control devices: the silicon controlled rectifier (SCR), DIAC, TRIAC, and unijunction transistor (UJT). It provides details on their structure, operation principles, and equivalent circuits. The SCR operates by conducting current when its breakover voltage is reached. The DIAC operates similar to two zener diodes in series. The TRIAC can conduct in both directions depending on the gate signal polarity. The UJT is used in triggering oscillator circuits and its operation depends on the intrinsic standoff ratio.