As charging continues, V r is eventually high enough to turn the Zener on, which then fires SCR 2 . Once SCR 2 is on, the voltage divider network of R 2 and R 3 will result in a level at V 2 which is too small to turn SCR 1 on. Thus the regulator recharges the battery whenever its voltage drops and prevents overcharging when it is fully charged.
Thyristors 31 • SCRs are unidirectional devices since current can flow only in one direction. • Diacs and triacs are bidirectional thyristors, where current can flow in either direction. • A diac has two terminals (anode 1 and anode 2 ), whilst a triac has a third terminal (gate). • The triac can be viewed as a bidirectional SCR. • Most common application for the diac is as a triggering device for a triac in AC control circuits. Bidirectional Thyristors
A 1 A 1 Thyristors 32 A 2 Schematic symbol Equivalent circuit A 2 The Diac
A 1 A 2 A 1 A 2 Q 1 and Q 2 forward- biased, Q 3 and Q 4 reverse-biased. Q 1 and Q 2 reverse- biased, Q 3 and Q 4 forward-biased. I I Thyristors 33 The Diac Two possible bias conditions
Thyristors 34 The Diac V-I Characteristic
Schematic symbol Equivalent circuit Thyristors 35 The triac is basically a diac with a gate terminal for controlling the turn-on conditions in either direction. Equivalent to a bidirectional SCR. The Triac
A 1 I A 1 A 1 A 1 A 2 A 2 A 2 A 2 G G G G I 36 Thyristors The Triac
A 2 Thyristors 37 A 2 A 2 A 2 A 2 Triac operating curve The Triac V-I Characteristic
Thyristors 38 Operation in quadrant I is identical to the operation of the SCR. Operation in quadrant III is simply a reflection of quadrant I operation. The triac can be triggered into conduction by either: 1. Applying a negative gate trigger while A 2 is positive (quadrant II) or 2. Applying a positive gate trigger while A 2 is negative (quadrant IV). The Triac Opration regions
V in A 2 Basic triac phase control A 1 R L V in V G I L triac on I L Thyristors 39 The Triac Phase control
Triac phase control circuit Thyristors 40 V in D 1 D 2 R 1 R L A 1 A 2 D 1 conducts during the positive half-cycle of V in . A 1 and G are positive with respect to A 2 . D 2 conducts during the negative half-cycle of V in . A 2 and G are positive with respect to A 1 . R 1 sets the trigger point for both cycles. G The Triac Phase control
3.36 A . Thyristors 41 R L V in V triac I L Let V in = 75 V, R = 82 k Ω , C = 1 μ F, and R L = 22 Ω . Example 1: If the triac has fired, what is the current through R L , assuming the voltage across the triac is 1 V? The Triac Phase control
Thyristors 42 Let V in = 75 V, R = 82 k Ω , C = 1 μ F, and R L = 22 Ω . Example 2: The diac has a breakover voltage of 32 V. If the triac has a trigger voltage of 1 V and a trigger current of 10 mA, what is the capacitor voltage that triggers the triac? V BO ( diac ) V GT ( triac ) 33 V . The Triac Phase control
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