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AN1007 - 1

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Thyristor Product Catalog

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Thyristors Used as AC Static Switches and Relays

Introduction

Since the SCR and the triac are bistable devices, one of their

broad areas of application is in the realm of signal and power

switching. This application note describes circuits in which these

thyristors are used to perform simple switching functions of a

general type that might also be performed non-statically by vari-

ous mechanical and electromechanical switches. In these appli-

cations, the thyristors are used to open or close a circuit

completely, as opposed to applications in which they are used to

control the magnitude of average voltage or energy being deliv-

ered to a load. These latter types of applications are described in

detail in “Phase Control Using Thyristors” (AN1003).

Static AC Switches

Normally Open Circuit

The circuit shown in Figure AN1007.1 provides random (any-

where in half-cycle), fast turn-on (<10 µs) of AC power loads and

is ideal for applications with a high-duty cycle. It eliminates com-

pletely the contact sticking, bounce, and wear associated with

conventional electromechanical relays, contactors, and so on. As

a substitute for control relays, thyristors can overcome the differ-

ential problem; that is, the spread in current or voltage between

pickup and dropout because thyristors effectively drop out every

half cycle. Also, providing resistor R1 is chosen correctly, the cir-

cuits are operable over a much wider voltage range than is a

comparable relay. Resistor R1 is provided to limit gate current

(IGTM) peaks. Its resistance plus any contact resistance (RC) of the

control device and load resistance (RL) should be just greater

than the peak supply voltage divided by the peak gate current

rating of the triac. If R1 is set too high, the triacs may not trigger

at the beginning of each cycle, and phase control of the load will

result with consequent loss of load voltage and waveform distor-

tion. For inductive loads, an RC snubber circuit, as shown in Fig-

ure AN1007.1, is required. However, a snubber circuit is not

required when an alternistor is used.

Figure AN1007.2 illustrates an analysis to better understand a

typical static switch circuit. The circuit operation occurs when

switch S1 is closed, since the triac Q1 will initially be in the block-

ing condition. Current flow will be through load RL, S1, R1, and

gate to MT1 junction of the thyristor. When this current reaches

the required value of IGT, the MT2 to MT1 junctions will switch to

the conduction state and the voltage from MT2 to MT1 will be VT.

As the current approaches the zero crossing, the load current will

fall below holding current turning the triac Q1 device off until it is

refired in the next half cycle. Figure AN1007.3 illustrates the volt-

age waveform appearing across the MT2 to MT1 terminals of Q1.

Note that the maximum peak value of current which S1 will carry

would be 25 mA since Q1 has a 25 mA maximum IGT rating. Addi-

tionally, no arcing of a current value greater than 25 mA when

opening S1 will occur when controlling an inductive load. It is

important also to note that the triac Q1 is operating in Quadrants I

and III, the more sensitive and most suitable gating modes for tri-

acs. The voltage rating of S1 (mechanical switch or reed switch)

must be equivalent to or greater than line voltage applied.

Figure AN1007.1

Basic Triac Static Switch

Figure AN1007.2

Analysis of Static Switch

Load

100

Ω

100

Ω

Control

Device

Reed

Switch

For

Inductive

Loads

0.1 µF

Triac

R1 ≥

√2•V

(RL + RC) Where IGTM is Peak Gate Current

Rating of Triac

IGTM

VRMS

MT1

I GT

MT2

AC Voltage Input

120 V rms, 60 Hz

VIN

I GT

V GT

Load

Q2008L4

AN1007

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