As charging continues V r is eventually high enough to turn the Zener on which

# As charging continues v r is eventually high enough

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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 , C = 1 μ F, and R L = 22 . 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 , C = 1 μ F, and R L = 22 . 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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