AC16FSM datasheet(136/223 Pages) TECCOR

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AN1003

Application Notes

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AN1003 - 4

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Thyristor Product Catalog

Figure AN1003.9

Basic Diac-Triac Phase Control

The hysteresis (snap back) effect is somewhat similar to the

action of a kerosene lantern. That is, when the control knob is

first rotated from the off condition, the lamp can be lit only at

some intermediate level of brightness, similar to turning up the

wick to light the lantern. Brightness can then be turned down until

it finally reaches the extinguishing point. If this occurs, the lamp

can only be relit by turning up the control knob again to the inter-

mediate level. Figure AN1003.10 illustrates the hysteresis effect

in capacitor-diac triggering. As R1 is brought down from its maxi-

mum resistance, the voltage across the capacitor increases until

the diac first fires at point A, at the end of a half-cycle (conduction

angle

θi). After the gate pulse, however, the capacitor voltage

drops suddenly to about half the triggering voltage, giving the

capacitor a different initial condition. The capacitor charges to the

diac, triggering voltage at point B in the next half-cycle and giving

a steady-state conduction angle shown as

θ for the triac.

Figure AN1003.10

Relationship of AC Line Voltage and Triggering

Voltage

In the Figure AN1003.11 illustration, the addition of a second RC

phase-shift network extends the range on control and reduces

the hysteresis effect to a negligible region. This circuit will control

from 5% to 95% of full load power, but is subject to supply volt-

age variations. When R1 is large, C1 is charged primarily through

R3 from the phase-shifted voltage appearing across C2. This

action provides additional range of phase-shift across C1 and

enables C2 to partially recharge C1 after the diac has triggered,

thus reducing hysteresis. R3 should be adjusted so that the circuit

just drops out of conduction when R1 is brought to maximum

resistance.

Figure AN1003.11

Extended Range Full-wave Phase Control

By using one of the circuits shown in Figure AN1003.12, the hys-

teresis effect can be eliminated entirely. The circuit (a) resets the

timing capacitor to the same level after each positive half-cycle,

providing a uniform initial condition for the timing capacitor. This

circuit is useful only for resistive loads since the firing angle is not

symmetrical throughout the range. If symmetrical firing is

required, use the circuit (b) shown in Figure AN1003.12.

Figure AN1003.12

Wide-range Hysteresis Free Phase Control

For more complex control functions, particularly closed loop con-

trols, the unijunction transistor may be used for the triggering

device in a ramp and pedestal type of firing circuit as shown in

Figure AN1003.13.

Load

0.1 µF

Triac

(Q2010L5)

250 k

3.3 k

120 V

(60 Hz)

(For Inductive

Loads)

100

0.1 µF

Diac

HT34B

Diac Triggers at "A"

Diac Does Not

Trigger at "A"

AC Line

Capacitor

Voltage

[+Diac VBO]

[–Diac VBO]

Diac

HT34B

Triac

(Q2010L5)

68 k

3.3 k

120 V

(60 Hz)

0.1 µF

Load

250 k

100 k

Trim

0.1 µF

Diac

Triac

(Q2010L5)

15 k

1/2 W

3.3 k

120 V

(60 Hz)

0.1 µF

Load

250 k

D1, D2 = 200 V Diodes

(a)

(b)

Diac

Triac

(Q2010L5)

120 V

(60 Hz)

Load

0.1 µF

R1 = 250 k POT

R2, R3 = 15 k, 1/2 W

R4 = 3.3 k

D1, D2, D3, D4 = 200 V Diodes

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