Measuring Power regulated by Zero Crossing SCR Controllers

Measuring Power regulated by Zero Crossing SCR Controllers

A zero crossing SCR controller is used to regulate the power delivered to a resistive load. The controller uses two Silicon Controlled Rectifiers (SCRs) connected back to back in opposite directions and a zero crossing detector. The zero crossing detector detects the zero crossing of the AC voltage input and allows the SCR’s to conduct starting with a zero crossing of the voltage. This avoids harmonic problems associated with the phase angle fired SCR controller.

A variable time base firing circuit is used to provide precise control of power delivered to a resistive load. Zero crossing controllers are not intended for use with inductive loads such as motors. A zero crossing controller is either conducting with full voltage applied to the load or it is not conducting. On or Off. The variable time base circuit controls the proportion of ON time to OFF time. This paper addresses the measurement technique necessary to accurately measure power in Zero Corssing SCR Controller applications.■