Special Purpose Devices
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 Devices
2 Types;
Silicone Controlled Rectifier, [SCR]
Unijunction Transistor, [UJT]
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
SCR
Special Purpose Devices
SCR;
Silicone Controlled Rectifier.
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 DOWN conduction. [Latching Current must be
removed].
ANODE, A
A
G
GATE, G
CATHODE, K
P
N
P
N
K
SCR, Gated ON
Special Purpose Devices
SCR;
Gated On, Latched On.
A
G
A
P
N
P
N
A
P
P
N
P
G
N
N
P
P
G
Q2
N
PP
N
NN
N
P
K
K
+
A
P
Q1
N
K
2
Q2
3
G
1
Q1
I
V
-
K
SCR, Power
Special Purpose Devices
SCR;
Efficient Power use.
12VDC
P = I X E
PLAMP = ILAMP X ELAMP
LAMP
PLAMP = 50 ma X 11v
PLAMP = 550 mW
SCR
P = I X E
PSCR = ISCR X ESCR
PSCR = 50ma X 1v
PSCR = 50 mW
PC; 52A
Special Purpose Devices
SCR;
- 9
VDC
Control Ckt PC 52A. Operate Mode.
R2
10 K
O
5
N
L1
C1
3
4
R1
1K
S1
+DC
+
2
R3
100K
+
1f
SCR
RESET
S2
E
+12
VDC
OPERATE
1
I
Reset
Special Purpose Devices
SCR;
- 9
VDC
Control Ckt PC 52A. Reset Mode.
R2
10 K
O
5
N
L1
3
4
R1
1K
S1
+DC
2
I
-
C1
+12
VDC
R3
100K
+
1f
SCR1
RESET
S2
E
1
Closing S2 Grounds the +
plate on Cap C1 causing it
to discharge thus Reverse
Biases SCR1, ceasing
Conduction.
OPERATE
PC 52B
Special Purpose Devices
SCR;
Ckt PC 52B. Conducting.
G
3
4
R2
L1
100 
SCR1
2N1596
6
R3
100 K
5
7
2
R1
10 K
4.7mfd
1mfd
C2
C1
1
PC 52B, Cutoff
Special Purpose Devices
SCR;
Ckt PC 52B. Cutoff.
G
3
4
R2
L1
100 
SCR1
2N1596
6
R3
100 K
5
7
2
R1
10 K
4.7mfd
1mfd
C2
C1
1
UJT
Special Purpose Devices
UJT;
Unijunction Transistor
B2
Base 2
BASE 2
N
E
Emitter
RB2
P
E
RB1
N
BASE 1
Base 1
B1
NITA LESSON;
Per Instructor Discretion.
Continue UJT
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.
B2
E
N
E
BASE 2
BASE 2
RB2
RB2
P
N
RB1
Current, UJT is
Conducting
B1
BASE 1
Emitter to Base 1.
R, V
RB1
E
BASE 1
Emitter to Base 1.
R, V 
Continue, UJT Ops
Special Purpose Devices
UJT;
Oscillator PC 51 . Operation, C1 Charge.
12
VDC
R1
18 K
R3
470 
0 V
5
R2
100
K
The RC TC
determines
Charge
Time &
Frequency
4
3
Q1
2N4891
+
C1
.0015 F
C1
Charges
0 V
C1
Discharges
2
R4
100 
0 V
1
C1 Discharge
Special Purpose Devices
UJT;
Oscillator PC 51 . Operation, C1 Discharge.
Output; + & - Spike.
Saw tooth @ TP3.
12
VDC
R1
18 K
R3
470 
- SPIKE
Output
0 V@
TP4
5
R2
100
K
The RC TC
determines
Charge
Time &
Frequency
4
3
Q1
2N4891
+
C1
.0015 F
2
R4
100 
1
C1
Charges
0 V
C1
Discharges
+ SPIKE
Output
0 V@
TP2
Trouble Shoot
Special Purpose Devices
UJT; Oscillator PC 51, 2 Ckt. What is the Malfunction?
10
R1
10.7
VDC
470
R3
R5
5
0
VDC
10.7 VDC
.214
VDC
.0015f
R6
1M
4
9
2N4891
Q1
.214 VDC
2
Q2
C2
7
25f
R8
R4
1
470
2N4891
5.01 VDC
C1
R7
8
R2
100K
3
47K
O
100
NO Output
Terms
Special Purpose Devices
SCR & UJT; Terms.
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;
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; Both in the Family called Thyristors, [Electronic Switches].
Review Questions
Special Purpose Devices
Review Questions;
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
?
__________
_______
Answers; 1- 9
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
12
VDC
PC 51
R1
18 K
d. Q1 Open
R3
470 
5
R2
100
K
4
3
Q1
2N4891
2
C1
.0015 F
R4
100 
1
Answer; 10