2. Since the development of the transistor, continuous
research in the field of semiconductors has provided many
additional semiconductor devices. Some if these devices have
been adopted to applications that were normally impossible. These
devices, now considered as innovations in military and commercial
equipment, may become as commonplace as the junction transistor
is today. Indications are that development and applications for
solid state devices will continue to expand. In all cases, where
other devices were previously used, the solid state devices are
smaller, more efficient, less expensive, and more reliable.
Two of these devices, the uni-junction transistor (UJT)
and the silicon controlled rectifier (SCR), will be discussed in
detail during this lesson.
Special Purpose DevicesSpecial Purpose Devices

3. 2 Types; Silicone Controlled Rectifier, [SCRSCR]
Unijunction Transistor, [UJTUJT]
Both are classified in the family of electronic switches called,
Thyristors.
Electronic Switches are; 1. Faster
2. Less Expensive
3. Last Longer
4. Have No Arc or Spark as do
manual switches & relays.
Applications SCR; 1. High Voltage & Current
>2000 V & >1500 A
2. Speed Controls for motors
Applications UJT; 1. Trigger for an SCR
2. Oscillator
Special Purpose DevicesSpecial Purpose Devices
SCR

4. SCR, Gated ON
G
A
K
PP
PP
NN
NNGATE, G
ANODE, A
CATHODE, K
4 Layer PNPN Device, W/3 PN Junctions.
Looks like a diode, acts like a diode, current flows like a diode, W/ an
extra element called a Gate.
A small + DC or + spike applied to the Gate will turn on [Gated On] the
device provided forward biasing is set on the Anode & Cathode. Once
ON, Stays ON.
To Turn OFF, forward biasing must be removed or reversed Biased.
The GATE WILL NOT SHUT DOWNGATE WILL NOT SHUT DOWN conduction. [Latching Current must be
removed].
Special Purpose DevicesSpecial Purpose Devices
SCR; Silicone Controlled Rectifier.

5. SCR, Power
G
A
PP
PP
NN
PP
NN
NN
K
PP
PP
NN
PP
NN
NNQ1
Q2
G
A
K
PP
PP
NN
PP
NN
NN
G
A
K
PP
PP
NN
NN
A
G
Q2
Q1
K
11
33
22
++
--
IIVV
Special Purpose Devices
SCR; Gated On, Latched On.

6. PC; 52A
Special Purpose Devices
SCR; Efficient Power use.
P = I X E
PLAMP = ILAMP X ELAMP
PLAMP = 50 ma X 11v
PPLAMPLAMP = 550 mW= 550 mW
P = I X E
PSCR = ISCR X ESCR
PSCR = 50ma X 1v
PPSCRSCR = 50 mW= 50 mW
LAMP
SCR
12VDC

7. Special Purpose Devices
SCR; Control Ckt PC 52A. Operate Mode.
OO
NN
EE
R2
10 KΩ
R1
1KΩ
R3
100KΩ
RESET
OPERATEOPERATE
S2
C1
1µf
1
2
3
4
5
+DC+DC
S1
- 9- 9
VDCVDC
+12+12
VDCVDC
SCR
++
++
L1
II
Reset

8. PC 52B
Special Purpose Devices
SCR; Control Ckt PC 52A. Reset Mode.
OO
NN
EE
R2
10 KΩ
R1
1KΩ
R3
100KΩ
OPERATE
RESETRESET
S2
C1
1µf
1
2
3
4
5
+DC+DC
S1
- 9- 9
VDCVDC
+12+12
VDCVDC
SCR1
++
--
L1
II
Closing S2 Grounds the +
plate on Cap C1 causing it
to discharge thus Reverse
Biases SCR1, ceasing
Conduction.

9. Special Purpose Devices
SCR; Ckt PC 52B. Conducting.
G
SCR1
2N1596
R2
100 Ω
R3
100 KΩ
R1
10 KΩ
C1
4.7mfd
1mfd
C2
1
2
4
5
6
7
3
L1
PC 52B, Cutoff

10. Special Purpose Devices
SCR; Ckt PC 52B. Cutoff.
G
SCR1
2N1596
R2
100 Ω
R3
100 KΩ
R1
10 KΩ
C1
4.7mfd
1mfd
C2
1
2
4
5
6
7
3
L1
UJT

11. Continue UJT
PP
NN
NN
EE
BB22
BB11
EE
BASE 1BASE 1
BASE 2BASE 2
RRB2B2
RRB1B1
Base 2Base 2
Base 1Base 1
EmitterEmitter
NITA LESSON;NITA LESSON;
Per Instructor Discretion.
Special Purpose Devices
UJT; Unijunction Transistor

12. Continue, UJT Ops
Special Purpose Devices
UJT; Unijunction Transistor
The position of the emitter fusing on the N material determines how
much voltage is required to turn on the UJT @ the Emitter.
EE
BASE 2
BASE 1BASE 1
RB2
RB1RB1
EE
RB2
RRB1B1
BASE 2
BASE 1BASE 1
PP
NN
NN
EE
BB22
BB11
EmitterEmitter toto Base 1Base 1.
RR↑↑, VV↑↑
EmitterEmitter toto Base 1Base 1.
RR↓↓, VV ↓↓
Current, UJT isCurrent, UJT is
ConductingConducting

13. Special Purpose Devices
UJT; Oscillator PC 51. Operation, C1 Charge.
0 V
0 V
0 V
C1
Charges
C1
Discharges
1
3
5
Q1
2N4891
2
4
R3
470 Ω
R4
100 Ω
R1
18 KΩ
R2
100
KΩ
C1
.0015 µF
1212
VDCVDC
The RC TC
determines
Charge
Time &
Frequency
++
C1 Discharge

14. Trouble Shoot
Special Purpose Devices
UJT; Oscillator PC 51. Operation, C1 Discharge.
Output; ++ & -- Spike. Saw tooth @ TP3.
1
3
5
Q1
2N4891
2
4
R3
470 Ω
R4
100 Ω
R1
18 KΩ
R2
100
KΩ
C1
.0015 µF
1212
VDCVDC
The RC TC
determines
Charge
Time &
Frequency
++
0 V
0 V
0 V
C1
Charges
C1
Discharges
+ SPIKE+ SPIKE
Output @Output @
TP2TP2
- SPIKE- SPIKE
Output @Output @
TP4TP4

15. Special Purpose Devices
UJT; Oscillator PC 51, 2 Ckt. What is the Malfunction?What is the Malfunction?
NO Output
R1
R2
C1
R3
R4
5
3
1
.0015µf 25µfC2
100KΩ
R5
R6
R7
R8
2N4891
Q1 Q2
2N4891
100Ω
470Ω470Ω 47KΩ
1MΩ
2
4
8
7
9
10
O
5.01 VDC5.01 VDC
10.7 VDC10.7 VDC
.214 VDC.214 VDC
10.7
VDC
0
VDC
.214
VDC
NO Output
Terms

16. SCR;SCR; Silicon Controlled Rectifier.
Referred to as a Gated Diode.
To Turn ON, F Bias + on Gate Cathode junction.[ Gated On]
To Turn OFF, R Bias Anode/ Cathode thus removes
Latching current.
Latching Current, the minimum amount of current that can flow
through an SCR and hold it in the Break over condition or
conduction.
UJT;UJT; Unijunction Transistor.
Referred to as a Double Based Diode.
Location of the emitter joined to the N material [PN Junction]
determines the amount of voltage required @ the emitter to cause
conduction through the Emitter.
3 Waveforms Produced, + & - Spike, Saw tooth.
Main Use, Oscillator & Switching Trigger.
SCR & UJT;SCR & UJT; Both in the Family called Thyristors, [Electronic Switches].
Special Purpose Devices
SCR & UJT; Terms.
Review Questions

17. 1. One of the main uses of a UJT is as a _________, __________
.
2. Why is the UJT often used instead of a conventional transistor?
__________________________________________________ .
3. The UJT is also called a _________ _____ ______ .
4. What is the Basic purpose of a SCR? __________ ________ .
5. How is an SCR turned on ? __________________________ .
6. What family of transistors does the SCR & UJT belong to ?
__________ .
7. Draw the schematic symbol for a UJT & SCR.
8. How many PN Junctions does the SCR contain ? ____ .
9. How is an SCR turned Off ? __________ _______
__________.
Special Purpose Devices
Review Questions;
Answers; 1- 9

18. Answer; 10
Special Purpose Devices
Review Questions;
10. Refer to PC 51; The following DC voltages are present: TP4 = 10.7 VDC,
TP3 = 0 VDC, TP2 = .21 VDC. What is the Malfunction ?
a. C1 Open
b. R1 Open
c. R2 Short
d. Q1 Open
11
33
55
Q1Q1
2N48912N4891
22
44
R3R3
470470 ΩΩ
R4R4
100100 ΩΩ
R1R1
18 K18 KΩΩ
R2R2
100100
KKΩΩ
C1C1
.0015.0015 µµFF
1212
VDCVDC
PC 51PC 51