So what is an SCR and what is its basic operation? On this slide the PN junction construction can be seen, it is basically two transistors interconnected as shown. The connections are the gate which is used to turn the SCR on and off, an anode, and a cathode. When the SCR is turned ON it becomes basically a diode, it rectifies and passes current in one direction. The rectification is shown in the diagram on the slide, passing current on only one polarity of the AC cycle.