1. 32 Thyristors Chapter Topics Covered in Chapter 32 32-1: Diacs
32-2: SCRs and Their Characteristics
32-3: Triacs
32-4: Unijunction Transistors
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2. 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.
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3. 32-1: Diacs
Fig (a) shows the basic construction of a diac, and Fig (b) shows the schematic symbol.
Fig. 32-1
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4. 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.

5. 32-2: SCRs and Their Characteristics
A silicon controlled rectifier (SCR) is a four-layer pnpn device.
Fig (a) shows the basic construction of an SCR, and Fig (b) shows the schematic symbol.
The SCR has three external leads: the anode, cathode, and gate.
Fig. 32-3
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6. 32-2: SCRs and Their Characteristics
SCRs are frequently used to control the amount of power that is delivered to a load.
Fig (a) shows a circuit where an SCR is used to control the amount of load current supplied to a lamp.
Fig (a)
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7. 32-2: SCRs and Their Characteristics
Fig (b) shows the load and SCR voltage waveforms for R2 (in Fig a) set so that the SCR fires when the input signal reaches its peak value at 90°.
Fig (b)
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8. 32-2: SCRs and Their Characteristics
Fig (c) shows the load and SCR voltage waveforms for R2 (in Fig a) set so that the SCR fires when the input signal reaches 45°.
Fig (c)
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9. 32-2: SCRs and Their Characteristics
Fig (d) shows the load and SCR voltage waveforms for R2 (in Fig a) set at its maximum value so that the SCR fires when the input signal is near 0°.
Fig (d)
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10. 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.

11. 32-3: Triacs
The schematic symbol for a triac is shown in Fig (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 (b).
Fig. 32-7
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12. 32-3: Triacs A2
Gate
Mode + 1 − 2 3 4
Fig 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.
Fig. 32-8
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13. 32-3: Triacs
Fig 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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14. 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.

15. 32-4: Unijunction Transistors
Construction of a UJT is shown in Fig (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 n-type Si bar, which forms a very low resistance contact.
each end of the Si bar is called a base.
Fig (a)
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16. 32-4: Unijunction Transistors
Negative resistance is illustrated in the emitter characteristic curve shown in Fig
Once VP is reached, the emitter voltage, VE, decreases as IE increases.
Fig
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17. 32-4: Unijunction Transistors
Fig 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
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18. 32-4: Unijunction Transistors
Fig shows how the firing of an SCR can be controlled by a UJT.
Fig
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