AC16FSM datasheet(177/223 Pages) TECCOR

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Application Notes

AN1007

AN1007 - 5

http://www.teccor.com

Thyristor Product Catalog

+1 972-580-7777

tally that general purpose AC circuits will generate minimum

electromagnetic interference (EMI) if energized at zero voltage.

The ideal AC circuit switch, therefore, consists of a contact which

closes at the instant when voltage across it is zero and opens at

the instant when current through it is zero. This has become

known as “zero-voltage switching.”

For applications that require synchronized zero-crossing turn-on,

the illustration in Figure AN1007.11 shows a circuit which incor-

porates an optocoupler with a built-in zero-crossing detector

Figure AN1007.11

Optocoupled Circuit with Zero-crossing Turn-on

(Triac or Alternistor)

Also, this circuit includes a dv/dt snubber network connected

across the power triac. This typical circuit illustrates switching the

hot line; however, the load may be connected to either the hot or

neutral line. Also, note that the series gate resistor is low in value

(22

Ω), which is possible on a 120 V line and above, since zero-

crossing turn-on is ensured in any initial half cycle.

Summary of Zero Crossing Turn-on Circuits

Zero voltage crossing turn-on opto-drivers are designed to limit

turn-on voltage to less than 20 V. This reduces the amount of RFI

and EMI generated when the thyristor switches on. Because of

this zero turn-on, these devices cannot be used to phase control

loads. Therefore, speed control of a motor and dimming of a

lamp cannot be accomplished with zero turn-on opto-couplers.

Since the voltage is limited to 20 V or less, the series gate resis-

tor that limits the gate drive current has to be much lower with a

zero crossing opto-driver. With typical inhibit voltage of 5 V, an

alternistor triac gate could require a 160 mA at -30 °C (5 V/

0.16 A = 31

Ω gate resistor). If the load has a high inrush current,

then drive the gate of the triac with as much current as reliably

possible but stay under the ITSM rating of the opto-driver. By using

Ω for the gate resistor, a current of at least 227 mA is supplied

with only 5 V, but limited to 909 mA if the voltage goes to 20 V. As

shown in Figure AN1007.12, Figure AN1007.13, and Figure

AN1007.14, a 22

Ω gate resistor is a good choice for various

zero crossing controllers.

Figure AN1007.12

Zero Crossing Turn-on Opto Triac Driver

Figure AN1007.13

Zero Crossing Turn-on Non-sensitive SCR Driver

Figure AN1007.14

Zero Crossing Turn-on Opto-sensitive Gate SCR

Driver

Rin

Vcc

120 V ac

MT2

MT1

Load

0.1 µF

Hot

Neutral

Zero

Crossing

Circuit

100

Triac or

Alternistor

MT2

0.1

µf

100

Ω

Load

MT1

Hot

Neutral

120/240 V ac

Input

Rin

Load could be here

instead of lower location

Zero

Crossing

Circuit

Rin 1

120/240 V ac

0.1

µF

Load

100

Non-sensitive Gate SCRs

Load could be here

instead of lower location

Zero

Crossing

Circuit

Input

Rin 1

120/240 V ac

0.1

µF

Load

100

Sensitive Gate SCRs

Load could be here

instead of lower location

Zero

Crossing

Circuit

Input

1 K

* Gate Diodes to Have

Same PIV as SCRs

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