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Chapter
32
Thyristors
Topics Covered in Chapter 32
32-1: Diacs
32-2: SCRs and Their Characteristics
32-3: Triacs
32-4: Unijunction Transistors
© 2007 The McGraw-Hill Companies, Inc. All rights reserved.
32-1: Diacs
 A diac is a three-layer, two-junction semiconductor
device that has only two leads.
 A diac is also referred to as a bi-directional diode
thyristor because it can conduct current in either
direction.
 Diacs are often used in conjunction with triacs to
provide symmetrical triggering.
McGraw-Hill
© 2007 The McGraw-Hill Companies, Inc. All rights reserved.
32-1: Diacs
 Fig. 32-1 (a) shows the basic
construction of a diac, and Fig. 32-1
(b) shows the schematic symbol.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. 32-1
32-2: SCRs and Their Characteristics
 The silicon controlled rectifier (SCR) is a four-layer




pnpn device with three leads, the anode, gate, and
cathode.
An SCR will not conduct until the forward breakover
voltage is reached, even though its anode-cathode is
forward-biased.
The gate current in an SCR controls the forward
breakover voltage.
Once an SCR turns on, the gate loses all control.
The only way to turn an SCR off is to reduce the
anode current below the holding current, IH.
32-2: SCRs and Their Characteristics
 A silicon controlled rectifier
(SCR) is a four-layer pnpn
device.
 Fig. 32-3 (a) shows the basic
construction of an SCR, and Fig.
32-3 (b) shows the schematic
symbol.
 The SCR has three external
leads: the anode, cathode, and
gate.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. 32-3
32-2: SCRs and Their Characteristics
 SCRs are frequently used to control the amount of power that is
delivered to a load.
 Fig. 32-5 (a) shows a circuit where an SCR is used to control the amount
of load current supplied to a lamp.
Fig. 32-5 (a)
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32-2: SCRs and Their Characteristics
 Fig. 32-5 (b) shows the load
and SCR voltage waveforms
for R2 (in Fig. 32-5 a) set so
that the SCR fires when the
input signal reaches its peak
value at 90°.
Fig. 32-5 (b)
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32-2: SCRs and Their Characteristics
 Fig. 32-5 (c) shows the
load and SCR voltage
waveforms for R2 (in Fig. 325 a) set so that the SCR fires
when the input signal
reaches 45°.
Fig. 32-5 (c)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
32-2: SCRs and Their Characteristics
 Fig. 32-5 (d) shows the
load and SCR voltage
waveforms for R2 (in Fig. 325 a) set at its maximum
value so that the SCR fires
when the input signal is near
0°.
Fig. 32-5 (d)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
32-3: Triacs
 A triac is a bi-directional thyristor used to control the
power in ac circuits.
 A triac has two leads designated MT1, and MT2 or A1
and A2.
 A triac has a gate lead which is used to control its
conduction.
 A triac is equivalent to two SCRs in parallel.
32-3: Triacs
 The schematic symbol for
a triac is shown in Fig. 32-7
(a).
 There are two anode
terminals, A2 and A1, and a
gate lead.
 The triac is the equivalent
of two SCRs connected in
parallel, as shown in Fig.
32-7 (b).
Fig. 32-7
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32-3: Triacs
 Fig. 32-8 shows the four operating
modes for a triac.
 When the anode 2 (A2) terminal is
positive, the triac can be turned on
with either a positive or negative
gate voltage.
 When the anode 2 (A2) terminal is
negative, the triac can be turned on
with either a positive or negative
gate voltage.
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A2
Gate
Mode
+
+
1
+
−
2
−
+
3
−
−
4
Fig. 32-8
32-3: Triacs
 Fig. 32-9 shows a very
effective way to provide a wide
range of control over load
current.
 R1-C1 and R2-C2 provide the
required phase shift necessary
for full control of the load current.
Fig. 32-9
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32-4: Unijunction Transistors
 The unijunction transistor (UJT) is a three-terminal




semiconductor device that has only one p-n junction.
The unijunction transistor (UJT) has two base leads,
B1 and B2 and an emitter (E) lead.
The interbase resistance, RBB of a UJT is the
resistance of its n-type silicon bar.
The ratio RB1/(RB1 + RB2) is called the intrinsic standoff
ratio, designated η.
UJTs are used in conjunction with SCRs and Triacs to
control their conduction angle.
32-4: Unijunction Transistors
 Construction of a UJT is shown
in Fig. 32-10 (a).
 A bar of n-type silicon (Si) is
placed on two separate pieces of
ceramic.
 Each piece of ceramic is bonded
by a gold film to each end of the ntype Si bar, which forms a very low
resistance contact.
 each end of the Si bar is called a
base.
Fig. 32-10 (a)
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32-4: Unijunction Transistors
 Negative resistance is
illustrated in the emitter
characteristic curve shown in
Fig. 32-12.
 Once VP is reached, the
emitter voltage, VE, decreases
as IE increases.
Fig. 32-12
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32-4: Unijunction Transistors
 Fig. 32-13 shows
how a UJT can be
used as a relaxation
oscillator.
 Because the
voltage waveform,
VB1 is a sharp pulse
of short duration, it is
the ideal gate
triggering source for
either an SCR or
triac.
Fig. 32-13
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32-4: Unijunction Transistors
 Fig. 32-14 shows how the firing of an SCR can be controlled by a UJT.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fig. 32-14