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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 Special Purpose DevicesDevices
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 Special Purpose DevicesDevices
SCR
SCR, Gated ON
G
A
K
PP
PPNN
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 Special Purpose DevicesDevices
SCR; Silicone Controlled Rectifier.
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
PPNN
NN
A
G
Q2
Q1
K
11
33
22
++
--
IIVV
Special Purpose Devices
SCR; Gated On, Latched On.
PC; 52A
Special Purpose Devices
SCR; Efficient Power use.
P = I X E
PLAMP = ILAMP X ELAMP
PLAMP = 50 ma X 11v
PPLAMP LAMP = 550 mW= 550 mWP = I X E
PSCR = ISCR X ESCR
PSCR = 50ma X 1v
PPSCR SCR = 50 mW= 50 mW
LAMP
SCR
12VDC
Special Purpose Devices
SCR; Control Ckt PC 52A. Operate Mode.
OO
NN
EE
R210 K
R11K
R3100K
RESET
OPERATEOPERATES2
C1
1f
1
2
3
4
5
+DC+DCS1
- 9 - 9 VDCVDC
+12 +12 VDCVDC
SCR
++
++
L1
IIReset
PC 52B
Special Purpose Devices
SCR; Control Ckt PC 52A. Reset Mode.
OO
NN
EE
R210 K
R11K
R3100K
OPERATE
RESETRESET
S2
C1
1f
1
2
3
4
5
+DC+DCS1
- 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.
Special Purpose Devices
SCR; Ckt PC 52B. Conducting.
G
SCR12N1596
R2100
R3100 K
R110 K
C14.7mfd
1mfd
C2
1
2
4
5
6
7
3
L1
PC 52B, Cutoff
Special Purpose Devices
SCR; Ckt PC 52B. Cutoff.
G
SCR12N1596
R2100
R3100 K
R110 K
C14.7mfd
1mfd
C2
1
2
4
5
6
7
3
L1
UJT
Continue UJT
PP
NN
NN
EE
BB22
BB11
EE
BASE 1BASE 1
BASE 2BASE 2
RRB2B2
RRB1B1
Base Base 22
Base 1Base 1
EmitteEmitterr
NITA LESSON;NITA LESSON;
Per Instructor Discretion.
Special Purpose Devices
UJT; Unijunction Transistor
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
RB1RB1EE
RB2
RRB1B1
BASE 2
BASE 1BASE 1
PP
NN
NN
EE
BB22
BB11
EmitterEmitter toto Base Base 11. R R, VV
Emitter Emitter toto Base Base 11. RR, V V
Current, UJT is Current, UJT is ConductingConducting
Special Purpose Devices
UJT; Oscillator PC 51 . Operation, C1 Charge.
0 V
0 V
0 V
C1 Charges
C1 Discharge
s
1
3
5
Q12N4891
2
4
R3470
R4100
R118 K
R2100 K
C1.0015 F
12 12 VDCVDC
The RC TC determines Charge Time &
Frequency
++
C1 Discharge
Trouble Shoot
Special Purpose Devices
UJT; Oscillator PC 51 . Operation, C1 Discharge.
Output; ++ & -- Spike. Saw tooth @ TP3.
1
3
5
Q12N4891
2
4
R3470
R4100
R118 K
R2100 K
C1.0015 F
12 12 VDCVDC
The RC TC determines Charge Time &
Frequency
++
0 V
0 V
0 V
C1 Charges
C1 Discharge
s
+ SPIKE + SPIKE Output Output @ TP2@ TP2
- SPIKE- SPIKE Output Output @ TP4@ TP4
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
.0015f 25fC2
100K
R5
R6
R7
R8
2N4891
Q1 Q2
2N4891
100
470470 47K
1M
2
4
8
7
9
10O
5.01 VDC5.01 VDC
10.7 VDC10.7 VDC
.214 VDC.214 VDC
10.7 VDC
0 VDC
.214 VDC
NO Output
Terms
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
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
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
Q1Q12N48912N4891
22
44
R3R3470 470
R4R4100 100
R1R118 K18 K
R2R2100 100 KK
C1C1.0015 .0015 FF
12 12 VDCVDC
PC 51PC 51