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AN10039

Phase Control Using Thyristors

Introduction

Due to high-volume production techniques, thyristors are now

priced so that almost any electrical product can benefit from elec-

tronic 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 con-

trol. The thyristor is held in the off condition — that is, all current

flow in the circuit is blocked by the thyristor except a minute leak-

age current. Then the thyristor is triggered into an “on” condition

by the control circuitry.

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.

Figure AN1003.1

SCR/Triac Connections for Various Methods of

Phase Control

Figure AN1003.2 illustrates voltage waveform and shows com-

mon 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.

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.

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 volt-

age characteristics are plotted against conduction angle for

half- and full-wave phase control circuits in Figure AN1003.3

and Figure AN1003.4.

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

Full-wave Rectified Operation

Voltage Applied to Load

Delay (Triggering) Angle

Conduction Angle

AN1003

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