1. Power control devices
• Silicon Controlled Rectifier Once they are triggered
into conduction, they
• Triac can only be stopped by
turning off current
• Diac external to the device.
• GTO
• BJT Can be turned off while anode current is still flowing
• MOSFET
Thyristors
Revision 01 1

2. Power devices - SCRs
• SCRs can switch high currents from about 1A –
2000A
• SCRs turn ON when the +ve potential is applied to
the gate terminal provided the anode is +ve with
respect to the cathode.
• An SCR can only turn off if the anode current is
reduced below its holding value with the gate open.
Revision 01 2

3. NE185 Power Control Systems
Silicon controlled rectifier (thyristor)
Anode (A)
Gate (G)
Large Current
Small Current
to “trigger” SCR Cathode (K)
Revision 01 3

4. Silicon controlled rectifier (thyristor)
Anode (A)
Gate (G)
Large Current
Small Current
to “trigger” SCR Cathode (K)
Note: The process of turning an SCR ON is called TRIGGERING
The process of turning an SCR OFF is called COMMUTATION
Revision 01 4

5. Silicon controlled rectifier (thyristor)
Anode (A)
Gate (G)
Large Current
Small Current
to “trigger” SCR Cathode (K)
Note: The ONLY way to turn them off is to stop the current flowing.
They must then be re-triggered on.
Revision 01 5

6. Silicon controlled rectifiers
Revision 01 6

7. Revision 01 7

8. Revision 01 8

9. Silicon controlled rectifier
Revision 01 9

10. Silicon controlled rectifier
Revision 01 10

11. Silicon controlled rectifier
Anode Anode
P P
N N N
Gate P P P Gate
N N
Cathode Cathode
4-layer device
Revision 01 11

12. Silicon controlled rectifier
Anode
Q1
Load
Gate Q2
Cathode
Revision 01 12

13. SCR control: phase control
Silicon Controlled Rectifier (Thyristor)
A N
Load Current
Load
Current required for triggering SCR
Revision 01 13

14. SCR control: phase control
Silicon Controlled Rectifier (Thyristor)
A N
Load Current
Load
Current required for triggering SCR
Lower resistance on
potentiometer. Capacitor
charges quicker
Revision 01 14

15. SCR control: phase control
Silicon Controlled Rectifier (Thyristor)
A N
Load Current
Load
Current required for triggering SCR
Higher resistance on
potentiometer. Capacitor
charges slower
Revision 01 15

16. SCR control
Silicon Controlled Rectifier (Thyristor)
A N
Control
Control
Load
SCR’s only conduct in one direction. Above is one way top overcome this.
Revision 01 16

17. TRIAC
T2
GATE
T1
Revision 01 17

18. 10k
T2
500k
120V ST4
G T1
SC151D
0.27µF
T1 T2 G
TRIAC Light Dimmer
Diac
Revision 01 18

19. Phase control
SCR1 conducts
+V
90° 180° 270° 360°
-V
SCR2 conducts
Revision 01 19

20. Phase control
SCR1 conducts
+V
90° 180° 270° 360°
-V
SCR2 conducts
Revision 01 20

21. Phase control
SCR1 conducts
+V
90° 180° 270° 360°
-V
SCR2 conducts
The main disadvantage of Phase Control (indeed SCR control) is RFI.
Revision 01 21

22. Diac characteristic curves
+IA
FORWARD
VBR(R) IHOLDING
-VA +VA
IHOLDING VBR(F)
REVERSE
28-36V typical
- IA
Revision 01 22

23. Gate turn-off thyristers (GTO’S)
• A gate turn-off switch, also known as a gate-controlled switch
(GCS) or gate turn-off thyristor (GTO), is similar to an SCR
but can be turned off by a negative signal on the gate terminal.
GTOs generally handle much lower currents than SCRs
Anode
Gate
Cathode
Revision 01 23

24. Transistor
• BJT Transistors
• A transistor is a device which acts like a controlled valve. The
current flow permitted can be controlled.
• The bipolar junction transistor (BJT) is a three-terminal
electronic valve - the output (collector) terminal current-
voltage characteristics are controlled by the current injected
into the input port (base). The BJT is a semiconductor device
constructed from two pn junctions. There are two types of
BJT: pnp and npn.
Revision 01 24

25. Transistor
• A transistor may be thought of as an electronic tap
able to control a large flow of electrons with only
small variations of the 'handle'. The 'handle' in the
case of a transistor is called the "base". The in and
out 'pipes' are called the "emitter" and the
"collector".Voltage changes at the base of the
transistor result in changes to the flow of electricity
through the transistor.
• A transistor can be thought of as a 'tap'.
• Symbol for a Transistor
Revision 01 25

26. Transistor operation
1. Bi-Junction Transistor (BJT)
P Collector
N
Base NPN
P
Emitter
Emitter
N
P Base PNP
N
Collector
Transistors are three layer devices
Revision 01 26

27. Transistor operation
1. Bi-Junction Transistor (BJT)
Collector
Base Large current NPN
Small current Emitter
Small current Emitter
Base Large current PNP
Collector
BJT’s are current operated devices
Revision 01 27

28. Transistor operation
2. MOSFET Transistor
Drain Drain Drain Drain
Gate Gate Gate Gate
Source Source Source Source
N-Channel P-Channel N-Channel P-Channel
Enhancement Enhancement Enhancement Enhancement
Mode only Mode only /Depletion /Depletion
Mode Mode
Revision 01 28

29. Transistor operation
2. MOSFET Transistor
Positive
Drain
A positive voltage RDS off = MΩ
(around +5V) on the Gate
RDS on = <0.1Ω
gate turns the
transistor fully on Source
Negative
In other words, they are VOLTAGE DRIVEN DEVICES.
Remember?… BJT’s are CURRENT DRIVEN DEVICES.
Revision 01 29

30. Transistor operation
• Advantages of transistors over thyristors:
– Lower voltage drop across transistors when
conducting (0.1V as compared to 2V) This causes
less heat to be developed.
– Faster switching times
– (less time to turn on = less heat developed while
turning on)
– Can easily be turned off.
Revision 01 30

31. Revision 01 31