MALVINO
Electronic
PRINCIPLES
SIXTH EDITION
Thyristors
Chapter 15
This latch is stable in either of two states.
Q1
Q2
Latch
On state
Off state
The four-layer diode
Q1
Q2
{{
p
Q1
n
Q2
p
n
Four-layer diode breakover
I
IH
VK
VB
V
The silicon controlled rectifier (SCR)
Anode
Anode
Anode
p
Q1
n
Gate
Gate
p
n
Q2
Cathode
Cathode
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)
An SCR does not turn off at the end of the trigger pulse.
+VAA
Load
+VAA
A B
A B
C
D
C D
SCR crowbar to protect a load from overvoltage
VZ
Power
supply
Protected
load
VT
VGT
The overvoltage that triggers the crowbar: VT = VZ + VGT
Adding gain to a crowbar circuit
R3
Power
supply
R1
R2
Protected
load
R4
R1 is the trigger adjust
Adding an IC amplifier to a crowbar circuit
10 kW
Power
supply
10 kW
A
IC
Protected
load
VA
10 kW
VZ
The circuit triggers when VA exceeds VZ.
SCR phase control
vline
Load
120 V
R1
C
qfire
R2
vload
qconduction
A snubber network can be used to limit
the rate of voltage rise across the SCR.
Load
R1
RC snubber
C
R2
Bidirectional thyristors can conduct in either direction.
I
Four-layer diodes
in reverse parallel
Diac
The triac is a popular bidirectional thyristor.
I
Gate
SCRs in
reverse parallel
Triac
Triac phase control
vline
Load
120 V
R1
C
R2
vload
Triac crowbar
Fuse
R1
120 V
Triac
Protected
load
R2
Diac
R3
R2 is the trigger adjust
The photo-SCR is
triggered by light.
RL
RL
Open
gate
Sensitivity
adjust
Maximum
sensitivity
The gate-controlled switch is turned
off with a reverse-biased trigger.
RL
A
A
B
C
D
RG
B
C
D
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.
Unijunction transistor (UJT)
B2
RE E
vin
B2
p
V
n
E
B1
B1
When vin reaches the standoff voltage, the resistance
between the emitter and B1 drops dramatically.