1. Thyristors
Chapter 15

2. This latch is stable in either of two states.Q1 Q2
Latch
On state
Off state

3. The four-layer diode { Q1 { Q2 p Q1 n p n Q2

4. Four-layer diode breakoverIH V VK VB

5. The silicon controlled rectifier (SCR)
Anode
Anode
Anode p Q1 n p
Gate
Gate
Gate n Q2
Cathode
Cathode
Cathode

6. 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)

7. An SCR does not turn off at the end of the trigger pulse.
+VAA
Load
A B C D
+VAA
A B C D

8. SCR crowbar to protect a load from overvoltageVZ
Power
supply
Protected
load VT
VGT
The overvoltage that triggers the crowbar: VT = VZ + VGT

9. Adding gain to a crowbar circuit
Power
supply
Protected
load R2 R4
R1 is the trigger adjust

10. Adding an IC amplifier to a crowbar circuit
10 kW
10 kW A
Power
supply
Protected
load IC VA
10 kW VZ
The circuit triggers when VA exceeds VZ.

11. SCR phase control
vline
Load
120 V R1
qfire
vload R2 C
qconduction

12. A snubber network can be used to limit
the rate of voltage rise across the SCR.
Load R1
RC snubber R2 C

13. I Bidirectional thyristors can conduct in either direction.
Four-layer diodes
in reverse parallel
Diac

14. I The triac is a popular bidirectional thyristor. Gate SCRs in Triac
reverse parallel
Triac

15. Triac phase control
vline
Load
120 V R1
vload R2 C

16. Triac crowbar Fuse R1 Triac Protected load 120 V R2 Diac R3
R2 is the trigger adjust

17. The photo-SCR is
triggered by light. RL RL
Open
gate
Sensitivity
adjust
Maximum
sensitivity

18. The gate-controlled switch is turned
off with a reverse-biased trigger. RL
A B C D
A B C D RG

19. 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.

20. Unijunction transistor (UJT)B2 B2 RE E E p V n
vin B1 B1
When vin reaches the standoff voltage, the resistance
between the emitter and B1 drops dramatically.