SCR & UJT

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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


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].


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


SCR; Gated On, Latched On.

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PC; 52A


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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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


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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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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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.


UJT; Unijunction Transistor

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Continue, UJT Ops


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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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


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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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].


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 ? __________ _________________.


Review Questions;

Answers; 1- 9

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Answer; 10


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

SCR & UJT

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