AC08FSM datasheet(139/223 Pages) TECCOR

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Application Notes

AN1003

AN1003 - 7

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Phase Control from Logic (DC) Inputs

Triacs can also be phase-controlled from pulsed DC unidirec-

tional inputs such as those produced by a digital logic control

system. Therefore, a microprocessor can be interfaced to AC

load by using a sensitive gate triac to control a lamp's intensity or

a motor's speed.

There are two ways to interface the unidirectional logic pulse to

control a triac. Figure AN1003.19 illustrates one easy way if load

current is approximately 5 A or less. The sensitive gate triac

serves as a direct power switch controlled by HTL, TTL, CMOS,

or integrated circuit operational amplifier. A timed pulse from the

system's logic can activate the triac anywhere in the AC sine-

wave producing a phase-controlled load.

Figure AN1003.19

Sensitive Gate Triac Operating in

Quadrants I and IV

The key to DC pulse control is correct grounding for DC and AC

supply. As shown in Figure AN1003.19, DC ground and AC

ground/neutral must be common plus MT1 must be con-

nected to common ground. MT1 of the triac is the return for

both main terminal junctions as well as the gate junction.

Figure AN1003.20 shows an example of a unidirectional (all neg-

ative) pulse furnished from a special I.C. that is available from

LSI Computer Systems in Melville, New York. Even though the

circuit and load is shown to control a Halogen lamp, it could be

applied to a common incandescent lamp for touch-controlled

dimming.

Figure AN1003.20

Typical Touch Plate Halogen Lamp Dimmer

For a circuit to control a heavy-duty inductive load where an

alternistor is not compatible or available, two SCRs can be driven

by an inexpensive TO-92 triac to make a very high current triac or

alternistor equivalent, as shown in Figure AN1003.21. See ”Rela-

tionship of IAV, IRMS, and IPK’ in AN1009 for design calcula-

tions.

Figure AN1003.21

Triac Driving Two Inverse Parallel Non-Sensitive

Gate SCRs

Figure AN1003.22 shows another way to interface a unidirec-

tional pulse signal and activate AC loads at various points in the

AC sine wave. This circuit has an electrically-isolated input which

allows load placement to be flexible with respect to AC line. In

other words, connection between DC ground and AC neutral is

not required.

Figure AN1003.22

Opto-isolator Driving a Triac or Alternistor

Microcontroller Phase Control

Traditionally, microcontrollers were too large and expensive to be

used in small consumer applications such as a light dimmer.

Microchip Technology Inc. of Chandler, Arizona has developed a

line of 8-pin microcontrollers without sacrificing the functionality

of their larger counterparts. These devices do not provide high

drive outputs, but when combined with a sensitive triac can be

used in a cost-effective light dimmer.

Figure AN1003.23 illustrates a simple circuit using a transformer-

less power supply, PIC 12C508 microcontroller, and a sensitive

triac configured to provide a light dimmer control. R3 is connected

to the hot lead of the AC power line and to pin GP4. The ESD pro-

tection diodes of the input structure allow this connection without

damage. When the voltage on the AC power line is positive, the

protection diode form the input to VDD is forward biased, and the

input buffer will see approximately VDD + 0.7 V. The software will

read this pin as high. When the voltage on the line is negative,

the protection diode from VSS to the input pin is forward biased,

and the input buffer sees approximately VSS - 0.7 V. The software

will read the pin as low. By polling GP4 for a change in state, the

software can detect zero crossing.

Load

MT2

Sensitive Gate

Triac

MT1

VDD

Hot

Neutral

120 V

60 Hz

VDD = 15 VDC

Touch

Plate

115 V ac

220 V ac

Halogen

Lamp

LS7631 / LS7632

VDD MODE

CAP

SYNC

TRIG VSS

EXT

SENS

MT1

MT2

NOTE: As a precaution,

transformer should have

thermal protection.

C1 = 0.15 µF, 200 V

C2 = 0.22 µF, 200 V

C3 = 0.02 µF, 12 V

C4 = 0.002 µF, 12 V

C5 = 100 µF, 12 V

R1 = 270, ¼ W

R2 = 680 k, ¼ W

C1 = 0.15 µF, 400 V

C2 = 0.1 µF, 400 V

C3 = 0.02 µF, 12 V

C4 = 0.002 µF, 12 V

C5 = 100 µF, 12 V

R1 = 1 k, ¼ W

R2 = 1.5 M, ¼ W

R3 = 62, ¼ W

R4 = 1 M to 5 M, ¼ W

(Selected for sensitivity)

R5, R6 = 4.7 M, ¼ W

D1 = 1N4148

Z = 5.6 V, 1 W Zener

T = Q4006LH4 Alternistor

L = 100 µH (RFI Filter)

R3 = 62, ¼ W

R4 = 1 M to 5 M, ¼ W

(Selected for sensitivity)

R5, R6 = 4.7 M, ¼ W

D1 = 1N4148

Z = 5.6 V, 1 W Zener

T = Q6006LH4 Alternistor

L = 200 µH (RFI Filter)

115 V ac

220 V ac

Load

MT2

Hot

Neutral

MT1

Triac

Gate Pulse

Input

Non-sensitive

Gate SCRs

100

0.1

µF

250 V

Timed

Input

Pulse

Rin

MT2

MT1

Hot

120 V

60 Hz

Triac or

Alternistor

Neutral

Load could be here

instead of upper location

Load

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