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Since the development of the transistor, continuousresearch in the field of semiconductors has provided manyadditional semiconductor devices. Some if these devices have
been adopted to applications that were normally impossible.These devices, now considered as innovations in military andcommercial equipment, may become as commonplace as thejunction transistor is today. Indications are that developmentand applications for solid state devices will continue to expand.In all cases, where other devices were previously used, the solidstate devices are smaller, more efficient, less expensive, andmore reliable.
Two of these devices, the uni-junction transistor (UJT)and the silicon controlled rectifier (SCR), will be discussed indetail during this lesson.
Special Purpose Devices
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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 domanual switches & relays.
Applications SCR; 1. High Voltage & Current
>2000 V & >1500 A2. Speed Controls for motors
Applications UJT; 1. Trigger for an SCR
2. Oscillator
Special Purpose Devices
SCR
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SCR, Gated ON
G
A
K
P
P
N
NGATE, 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/ anextra element called a Gate.
A small + DC or + spike applied to the Gate will turn on [Gated On] thedevice 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 WIL
L NOT SHUT DOWN conduction. [Latching Current must beremoved].
Special Purpose Devices
SCR; Silicone Controlled Rectifier.
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SCR, Power
G
A
P
P
N
P
N
N
K
P
P
N
P
N
N
Q1
Q2
G
A
K
P
P
N
P
N
N
G
A
K
P
P
N
N
A
G
Q2
Q1
K
1
3
2
+
-
IV
Special Purpose Devices
SCR; Gated On, Latched On.
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PC; 52A
Special Purpose Devices
SCR; Efficient Power use.
P = I X E
PLAMP = ILAMP X ELAMP
PLAMP = 50 ma X 11v
PLAMP = 550 mW
P = I X E
PSCR = ISCR X ESCR
PSCR = 50ma X 1v
PSCR = 50 mW
LAMP
SCR
12VDC
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Special Purpose Devices
SCR; Control Ckt PC 52A. Operate Mode.
O
N
E
R210 K
R11K
R3100K
RESET
OPERATES2
C1
1
f
1
2
3
4
5
+DC
S1
- 9VDC
+12VDC
SCR
+
+
L1
I
Reset
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PC 52B
Special Purpose Devices
SCR; Control Ckt PC 52A. Reset Mode.
O
N
E
R210 K
R11K
R3100K
OPERATE
RESET
S2
C1
1
f
1
2
3
4
5
+DC
S1
- 9VDC
+12VDC
SCR1
+
-
L1
I
Closing S2 Grounds the +plate on Cap C1 causing itto discharge thus Reverse
Biases SCR1, ceasingConduction.
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Special Purpose Devices
SCR; Ckt PC 52B. Conducting.
G
SCR1
2N1596
R2100
R3100 K
R110 K
C1
4.7mfd
1mfd
C2
1
2
4
5
6
7
3
L1
PC 52B, Cutoff
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Special Purpose Devices
SCR; Ckt PC 52B. Cutoff.
G
SCR1
2N1596
R2100
R3100 K
R110 K
C1
4.7mfd
1mfd
C2
1
2
4
5
6
7
3
L1
UJT
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Continue UJT
P
N
N
E
B2
B1
E
BASE 1
BASE 2
RB2
RB1
Base 2
Base 1
Emitter
NITA LESSON;
Per Instructor Discretion.
Special Purpose Devices
UJT; Unijunction Transistor
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Continue, UJT Ops
Special Purpose Devices
UJT; Unijunction Transistor
The position of the emitter fusing on the N material determines howmuch voltage is required to turn on the UJT @ the Emitter.
E
BASE 2
BASE 1
RB2
RB1E
RB2
RB1
BASE 2
BASE 1
P
N
N
E
B2
B1
EmittertoBase 1.R, V
Emitter toBase 1.R
, V
Current, UJT isConducting
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Special Purpose Devices
UJT; Oscillator PC 51 . Operation, C1 Charge.
0 V
0 V
0 V
C1Charges
C1Discharges
1
3
5
Q12N4891
2
4
R3470
R4100
R118 K
R2100K
C1.0015 F
12VDC
The RC TCdetermines
ChargeTime &
Frequency +
C1 Discharge
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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
R2100K
C1.0015 F
12VDC
The RC TCdetermines
ChargeTime &
Frequency +
0 V
0 V
0 V
C1Charges
C1Discharges
+ SPIKEOutput @
TP2
- SPIKEOutput @
TP4
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Special Purpose Devices
UJT; Oscillator PC 51, 2 Ckt. 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 VDC
10.7 VDC
.214 VDC
10.7VDC
0VDC
.214VDC
NO Output
Terms
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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 removesLatching current.
Latching Current, the minimum amount of current that can flowthrough an SCR and hold it in the Break over condition orconduction.
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 causeconduction through the Emitter.
3 Waveforms Produced, + & - Spike, Saw tooth.
Main Use, Oscillator & Switching Trigger.
SCR & UJT; Both in the Family called Thyristors, [Electronic Switches].
Special Purpose Devices
SCR & UJT; Terms.
Review Questions
S i l P D i
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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
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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. C1Open
b. R1Open
c. R2Short
d. Q1Open
1
3
5
Q12N4891
2
4
R3470
R4100
R118 K
R2100K
C1.0015 F
12VDC
PC 51