6/15/05

Page 7 of 11

© 2005 Fairchild Semiconductor Corporation

6-PIN DIP RANDOM-PHASE

OPTOISOLATORS TRIAC DRIVERS

(600 VOLT PEAK)

MOC3051-M

MOC3052-M

APPLICATIONS GUIDE

Basic Triac Driver Circuit

The new random phase triac driver family MOC3052-M and

MOC3051-M are very immune to static dv/dt which allows

snubberless operations in all applications where external

generated noise in the AC line is below its guaranteed dv/dt

withstand capability. For these applications a snubber circuit is

not necessary when a noise insensitive power triac is used.

Figure 11 shows the circuit diagram. The triac driver is directly

connected to the triac main terminal 2 and a series Resistor R

which limits the current to the triac driver. Current limiting

resistor R must have a minimum value which restricts the

current into the driver to maximum 1A.

The power dissipation of this current limiting resistor and the

triac driver is very small because the power triac carries the

load current as soon as the current through driver and current

limiting resistor reaches the trigger current of the power triac.

The switching transition times for the driver is only one micro

second and for power triacs typical four micro seconds.

Triac Driver Circuit for Noisy Environments

When the transient rate of rise and amplitude are expected to

exceed the power triacs and triac drivers maximum ratings a

snubber circuit as shown in Figure 12 is recommended. Fast

transients are slowed by the R-C snubber and excessive

amplitudes are clipped by the Metal Oxide Varistor MOV.

Triac Driver Circuit for Extremely Noisy Environments, as

specified in the noise standards IEEE472 and IEC255-4.

Industrial control applications do specify a maximum transient

noise dv/dt and peak voltage which is superimposed onto the

AC line voltage. In order to pass this environment noise test a

modified snubber network as shown in Figure 13 is recom-

mended.

Figure 11. Basic Driver Circuit

Figure 12. Triac Driver Circuit for Noisy Environments

Figure 13. Triac Driver Circuit for Extremely Noisy

Environments

VCC

RET.

RLED

TRIAC DRIVER

POWER TRIAC

AC LINE

LOAD

R

Q

CONTROL

R

TRIAC DRIVER

POWER TRIAC

RLED

VCC

RET.

CONTROL

RS

CS

MOV

LOAD

AC LINE

R

TRIAC DRIVER

POWER TRIAC

RS

CS

MOV

LOAD

AC LINE

VCC

RET.

CONTROL

RLED

RLED = (VCC - V F LED - V sat Q)/IFT

R = Vp AC line/ITSM

Typical Snubber values RS = 33 Ω, CS = 0.01 µF

MOV (Metal Oxide Varistor) protects triac and

driver from transient overvoltages >VDRM max.

Recommended snubber to pass IEEE472 and IEC255-4 noise tests

RS = 47 W, CS = 0.01 mF