Thyristor Semiconductor Products Page 29 Thyristor

2014 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 12/14/14 Teccor brand Thyristors AN1003 Introduction Due to high-volume production techniques, Thyristors are now priced so that almost any electrical product can benefit from electronic control. A look at the fundamentals of SCR and Triac phase controls shows how this is possible. Output Power Characteristics Phase control is the most common form of Thyristor power control. The Thyristor is held in the off condition -- that is, all current flow in the circuit is blocked by the Thyristor except a minute leakage current. Then the Thyristor is triggered For full-wave AC control, a single Triac or two SCRs connected in inverse parallel may be used. One of two methods may be used for full-wave DC control -- a bridge rectifier formed by two SCRs or an SCR placed in series with a diode bridge as shown in Figure AN1003.1. Control Circuit Line Load Two SCR AC Control Control Circuit Triac AC Control Line Load Control Circuit One SCR DC Control Control Circuit Line Load Two SCR DC Control Load Figure AN1003.1 SCR/Triac Connections for Various Methods of Phase Control Figure AN1003.2 illustrates voltage waveform and shows common terms used to describe Thyristor operation. Delay angle is the time during which the Thyristor blocks the line voltage. The conduction angle is the time during which the Thyristor is on. Phase Control Using Thyristors It is important to note that the circuit current is determined by the load and power source. For simplification, assume the load is resistive; that is, both the voltage and current waveforms are identical. Full-wave Rectified Operation Voltage Applied to Load Delay (Triggering) Angle Conduction Angle Figure AN1003.2 Sine Wave Showing Principles of Phase Control Different loads respond to different characteristics of the AC waveform. For example, some are sensitive to average voltage, some to RMS voltage, and others to peak voltage. Various voltage characteristics are plotted against conduction angle for half- and full-wave phase control circuits in Figure AN1003.3 and Figure AN1003.4. Peak Voltage RMS AVG Power 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0 0 20 40 60 80 100 120 140 Conduction Angle ( ) Normalized Sine Wave RMS Voltage Power as Fraction of Full Conduction HALF WAVE 180 160 Figure AN1003.3 Half-Wave Phase Control (Sinusoidal)

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