06 scr & ujt

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


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].


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


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


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


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.


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


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.


UJT; Unijunction Transistor

Continue, UJT Ops


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


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


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


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].


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 ? __________ _______ __________.


Review Questions;

Answers; 1- 9

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. 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

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