Chapter 15

Thyristors

This latch is stable in either of two states.

Latch On state Off state

Q1

Q2

Q1

Q2

p

p

n

n

{Q1

The four-layer diode

{Q2

Four-layer diode breakover

I

VIH

VK VB

The silicon controlled rectifier (SCR)

Q1

Q2

Anode

Gate

Cathode

p

p

n

n

Anode

Gate

Cathode

Anode

Gate

Cathode

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)

Load

+VAA

A B C D

A B C D

+VAA

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

Powersupply

Protectedload

SCR crowbar to protect a load from overvoltage

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

VT

VZ

VGT

Powersupply

Protectedload

Adding gain to a crowbar circuit

R2

R1

R4

R3

R1 is the trigger adjust

Powersupply

Protectedload

Adding an IC amplifier to a crowbar circuit

10 kW

10 kW 10 kW

IC

VZ

A

The circuit triggers when VA exceeds VZ.

VA

Load

120 V

SCR phase control

qconduction

vline

vload

qfire

R1

R2C

Load

R1

R2C

RC snubber

A snubber network can be used to limit the rate of voltage rise across the SCR.

Bidirectional thyristors can conduct in either direction.

Four-layer diodesin reverse parallel

Diac

I

SCRs in reverse parallel

Triac

IGate

The triac is a popular bidirectional thyristor.

Load

120 V

Triac phase control

vline

vload

R1

R2C

Protectedload

Triac crowbar

R2

R1

R3

R2 is the trigger adjust

120 V

Fuse

Triac

Diac

Sensitivityadjust

RL RL

The photo-SCR istriggered by light.

Maximumsensitivity

Opengate

A B C D

A B C D

RL

RG

The gate-controlled switch is turned off with a reverse-biased trigger.

The silicon-controlled switch is a low-current device.

Cathodegate

AnodeAnodegate

Cathode

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

Unijunction transistor (UJT)

n

p V

vin

E

B2

B1

RE

When vin reaches the standoff voltage, the resistance between the emitter and B1 drops dramatically.

E

B2

B1