SCR Principles And Circuits | Nuts & Volts Magazine SCR Principles And Circuits. By Ray Marston this circuit can be used to sound an alarm when water rises above a pre-set level in a bath, tank, or cistern, etc. Figure 16 shows a particularly effective yet simple variable speed-regulator circuit. This uses an SCR as the control element and feeds half-wave power to the motor (this causes SCR based pre regulator - Multisim Live SCR based pre regulator. 0. Favorite. 0. Copy. 231. Views. Open Circuit. Social Share. Circuit Description. Circuit Graph. I wanted to figure out the worj=kibg of SCR in a preregulatpr circuit to the power supply. I wanted to figure out the worj=kibg of SCR in a preregulatpr circuit to the power supply.