Fairchild Optically Isolated Phase Controlling Circuit Solution Data Handbook

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Application Note AN-3006 Optically Isolated Phase Controlling Circuit Solution

Introduction from 500 V/µs to 2000 V/µs, protecting the device from acci- dental triggering by ac power line transients.

Optocouplers simplify logic isolation from the ac line, power supply transformations, and the control of polyphase power

The circuit in Figure 1 is the basic circuit for on–off power

systems. They help solve problems by keeping ac line noise

control. With a continuous forward current through the LED,

and transients out of sensitive logic. Fairchild’s 6–Pin Dip

the detector of the zero–crossing optocoupler switches to the

Optocoupler family, with high surge voltage capability (7500

conducting state only when the applied ac voltage passes

volts peak ac, 60 Hz, 1 second duration), allows designers to

through a point near zero. Phase control applications, such as

achieve their goals. This paper presents a power triac phase

controlling the speed of a motor or brilliance of a lamp,

control circuit and contrasts it with the traditional zero–

require triggering at points along the ac voltage wave. This

crossing circuit. The example circuit isolates the low level

necessitates a random phase triac driver optocoupler.

control circuitry from the ac line. It is able to control the speed of a universal motor or brilliance of a lamp. The

Phase Controlling a Power Triac

universal motor is capable of high starting torque and wide

speed range — commonly used in mixers, blenders, floor Using MOC3023

polishers, electric hand and woodworking tools, etc.

Design Objective The application circuit shown in figure 2 is an example of

Zero–Crossing and Random Phase

phase controlling a power triac. The random phase triac

Triac Drivers

driver is Fairchild’s MOC3023. It has an LED trigger current

The zero–crossing triac driver optocouplers are the of 5 m A and off–state output terminal voltage V of

FT DRM

MOC316X and MOC308X series. The random phase opto- 400 V. The power triac used in this example has an on–state couplers are the MOC301X, MOC302X and MOC305X (RMS) current of 15 Amps at T = 80C. The load is a 1/3 series. All families have the same type of aluminum gallium HP, single phase induction motor driving a fan with maxi- arsenide infrared light–emitting diode but optically couple to mum speed of 1750 rpm. This circuit can be applied to dif- different monolithic silicon detector chips. The zero–cross ferent power triacs and loads with larger rating. The design family is designed for interface applications between control objective is to phase control a power triac that drives a motor circuit and power loads. The advantage of using zero–cross- or a light dimmer powered from the 115 VAC line. This is ing switching is less surge current and resulting electromag- accomplished by using a variable pulse width generated netic interference (EMI). This reduces reliability problems from an optically isolated logic system with a control voltage in many applications such as solid–state relays, industrial of 5 to 15 VDC. controls, motors, solenoids and consumer appliances. The high– speed zero–crossing switch provides a minimum dv/dt

VCC HOT

MOC3162

CONTROL SIGNAL

LOAD NEUTRAL

DESIGN RULE: Vpeak /I peak = 180 /1 amp = 180 ohms (Assume the line voltage is 115 volts RMS)

Figure 1. Zero–Cross Switching Using MOC3061

REV. 4.01 6/24/02