Thyristor Semiconductor Products Page 32 Thyristor

2014 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 12/14/14 Teccor brand Thyristors AN1003 Load R 1 C 1 0.1 F Quadrac (Q4010LT) 250 k 3.3 k R 2 120 V (60 Hz) (For Inductive Loads) 100 0.1 F Diac Figure AN1003.9 Basic DIAC-Quadrac 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 intermediate level. Figure AN1003.10 illustrates the hysteresis effect in capacitor- DIAC triggering. As R 1 is brought down from its maximum resistance, the voltage across the capacitor increases until the DIAC first fires at point A, at the end of a half- cycle (conduction angle i). 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 for the Triac. Diac Triggers at "A" Diac Does Not Trigger at "A" B A AC Line Capacitor Voltage i [+Diac V BO ] [-Diac V BO ] 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 voltage variations. When R 1 is large, C 1 is charged primarily through R 3 from the phase- shifted voltage appearing across C 2 . This action provides additional range of phase-shift across C 1 and enables C 2 to partially recharge C 1 after the DIAC has triggered, thus reducing hysteresis. R 3 should be adjusted so that the circuit just drops out of conduction when R 1 is brought to maximum resistance. R 4 C 1 Diac Quadrac (Q4010LT) 68 k 3.3 k R 1 120 V (60 Hz) 0.1 F Load R 2 250 k R 3 100 k Trim C 2 0.1 F Figure AN1003.11 Extended Range Full-wave Phase Control By using one of the circuits shown in Figure AN1003.12, the hysteresis 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. R 3 C 1 Diac Quadrac (Q4010LT) 15 k 1/2 W 3.3 k R 1 120 V (60 Hz) 0.1 F Load R 2 250 k D 1 D 2 D 1 , D 2 = 400 V Diodes (a) (b) C 1 Diac Quadrac (Q4010LT) R 3 120 V (60 Hz) Load D 1 0.1 F R 1 = 250 k POT D 3 R 4 R 1 D 4 R 2 D 2 R 2 , R 3 = 15 k, 1/2 W R 4 = 3.3 k D 1 , D 2 , D 3 , D 4 = 400 V Diodes Figure AN1003.12 Wide-range Hysteresis Free Phase Control For more complex control functions, particularly closed loop controls, 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. triggered into the conduction mode for the remainder of that half-cycle. In this circuit, triggering is in Quadrants I and III. The unique simplicity of this circuit makes it suitable for applications with small control range. Phase Control Using Thyristors (continued)

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