SIXTH EDITION
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SIXTH EDITION. MALVINO. Electronic. PRINCIPLES. Thyristors. Chapter 15. This latch is stable in either of two states. Q 1. Q 2. Off state. On state. Latch. {. Q 1. {. Q 2. The four-layer diode. p. Q 1. n. p. n. Q 2. Four-layer diode breakover. I. I H. V. V K. V B.

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Malvino

SIXTH EDITION

MALVINO

Electronic

PRINCIPLES


Malvino

Thyristors

Chapter 15


Malvino

This latch is stable in either of two states.

Q1

Q2

Off state

On state

Latch


Malvino

{

Q1

{

Q2

The four-layer diode

p

Q1

n

p

n

Q2


Malvino

Four-layer diode breakover

I

IH

V

VK

VB


Malvino

The silicon controlled rectifier (SCR)

Anode

Anode

Anode

p

Q1

n

p

Gate

Gate

Gate

n

Q2

Cathode

Cathode

Cathode


Malvino

SCRs

  • The most widely used thyristors

  • Use gate triggering for turn-on instead of breakover triggering

  • Data sheets list the gate trigger voltage (VGT) and the gate trigger current (IGT)

  • Turn-off requires reducing the current to less than the holding current (IH)


Malvino

+VAA

A B C D

A B C D

An SCR does not turn off at the end of the trigger pulse.

+VAA

Load


Malvino

Power

supply

Protected

load

SCR crowbar to protect a load from overvoltage

VZ

VT

VGT

The overvoltage that triggers the crowbar: VT = VZ + VGT


Malvino

Power

supply

Protected

load

Adding gain to a crowbar circuit

R3

R1

R2

R4

R1 is the trigger adjust


Malvino

Power

supply

Protected

load

Adding an IC amplifier to a crowbar circuit

10 kW

10 kW

A

IC

VA

10 kW

VZ

The circuit triggers when VA exceeds VZ.


Malvino

qfire

Load

qconduction

SCR phase control

vline

120 V

R1

vload

R2

C


Malvino

Load

A snubber network can be used to limit

the rate of voltage rise across the SCR.

R1

RC snubber

R2

C


Malvino

Bidirectional thyristors can conduct in either direction.

I

Four-layer diodes

in reverse parallel

Diac


Malvino

The triac is a popular bidirectional thyristor.

I

Gate

SCRs in

reverse parallel

Triac


Malvino

Load

Triac phase control

vline

120 V

R1

vload

R2

C


Malvino

Protected

load

Triac crowbar

Fuse

R1

Triac

120 V

R2

Diac

R3

R2 is the trigger adjust


Malvino

The photo-SCR is

triggered by light.

RL

RL

Open

gate

Sensitivity

adjust

Maximum

sensitivity


Malvino

A B C D

A B C D

The gate-controlled switch is turned

off with a reverse-biased trigger.

RL

RG


Malvino

The silicon-controlled switch is

a low-current device.

Anode

Anode

gate

Cathode

gate

Cathode

Either gate can be used to trigger or open this device.


Malvino

Unijunction transistor (UJT)

B2

B2

RE

E

E

p

V

n

B1

vin

B1

When vin reaches the standoff voltage, the resistance

between the emitter and B1 drops dramatically.


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