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AN1007

Application Notes

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AN1007 - 4

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Thyristor Product Catalog

Figure AN1007.8

Optocoupler Circuit for Lower Current Inductive

Loads (Triac or Alternistor)

In this circuit, the series gate resistors are increased to 180

Ω

each, since a 240 V line is applied. Note that the load is placed

on the MT1 side of the power triac to illustrate that load place-

ment is not important for the circuit to function properly.

Also note that with standard U.S. residential 240 V home wiring,

both sides of the line are hot with respect to ground (no neutral).

Therefore, for some 240 V line applications, it will be necessary

to have a triac switch circuit in both sides of the 240 V line input.

If an application requires back-to-back SCRs instead of a triac or

alternistor, the circuit shown in Figure AN1007.9 may be used.

Figure AN1007.9

Optocoupled Circuit for Heavy-duty Inductive Loads

All application comments and recommendations for optocoupled

switches apply to this circuit. However, the snubber network can

be applied only across the SCRs as shown in the illustration. The

optocoupler should be chosen for best noise immunity. Also, the

voltage rating of the optocoupler output triac must be equal to or

greater than the voltage rating of SCRs.

Summary of Random Turn-on Relays

As shown in Figure AN1007.10, if the voltage across the load is

to be phase controlled, the input control circuitry must be syn-

chronized to the line frequency and the trigger pulses delayed

from zero crossing every half cycle. If the series gate resistor is

chosen to limit the peak current through the opto-driver to less

than 1 A, then on a 120 V ac line the peak voltage is 170 V;

therefore, the resistor is 180

Ω. On a 240 V ac line the peak volt-

age is 340 V; therefore, the resistor should be 360

Ω. These gate

pulses are only as long as the device takes to turn on (typically,

5 µs to 6 µs); therefore, 0.25 W resistor is adequate.

Figure AN1007.10

Random Turn-on Triac Driver

Select the triac for the voltage of the line being used, the current

through the load, and the type of load. Since the peak voltage of

a 120 V ac line is 170 V, you would choose a 200 V (MIN) device.

If the application is used in an electrically noisy industrial envi-

ronment, a 400 V device should be used. If the line voltage to be

controlled is 240 V ac with a peak voltage of 340 V, then use at

least a 400 V rated part or 600 V for more design margin. Selec-

tion of the voltage rating of the opto-driver must be the same or

higher than the rating of the power triac. In electrically noisy

industrial locations, the dv/dt rating of the opto-driver and the

triac must be considered.

The RMS current through the load and main terminals of the triac

should be approximately 70% of the maximum rating of the

device. However, a 40 A triac should not be chosen to control a

1 A load due to low latching and holding current requirements.

Remember that the case temperature of the triac must be main-

tained at or below the current versus temperature curve specified

on its data sheet. As with all semiconductors the lower the case

temperature the better the reliability. Opto-driven gates normally

do not use a sensitive gate triac. The opto-driver can supply up to

1 A gate pulses and less sensitive gate triacs have better dv/dt

capability. If the load is resistive, it is acceptable to use a stan-

dard triac. However, if the load is a heavy inductive type, then an

alternistor triac, or back-to-back SCRs as shown in Figure

AN1007.9, is recommended. A series RC snubber network may

or may not be necessary when using an alternistor triac. Nor-

mally a snubber network is not needed when using an alternistor

because of its high dv/dt and dv/dt(c) capabilities. However,

latching network as described in Figure AN1007.8 may be

needed for low current load variations.

Zero Crossing Turn-on, Normally Open

Relay Circuits

When a power circuit is mechanically switched on and off

mechanically, generated high-frequency components are gener-

ated that can cause interference problems such as RFI. When

power is initially applied, a step function of voltage is applied to

the circuit which causes a shock excitation. Random switch

opening stops current off, again generating high frequencies. In

addition, abrupt current interruption in an inductive circuit can

lead to high induced-voltage transients.

The latching characteristics of thyristors are ideal for eliminating

interference problems due to current interruption since these

devices can only turn off when the on-state current approaches

zero, regardless of load power factor.

On the other hand, interference-free turn-on with thyristors

requires special trigger circuits. It has been proven experimen-

Rin

Vcc

180

240 V ac

MT2

MT1

180

0.1 µF

0.047 µF

3.3 k

Load

Rin

Vcc

120 V ac

0.1

µF

Load

100

NS-SCR

NS-

SCR

Triac or

Alternistor

MT2

0.1

µf

100

Ω

Load

MT1

Hot

Neutral

120/240 V ac

180

Ω for 120 V ac

360

Ω for 240 V ac

Input

Rin

Load could be here

instead of lower location

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