Special Purpose Devices

1. Special Purpose Devices

2. Special Purpose Devices Introduction 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.

3. 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

4. Special Purpose Devices SCR; Silicone Controlled Rectifier. A 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 G P K GATE, G N P CATHODE, K N SCR, Gated ON

5. Special Purpose Devices A SCR; Gated On, Latched On. A P P P A N N N N N N P P P P P P G G G N N N K P K N A Q2 + P N 2 Q2 K 3 Q1 1 Q1 G I V - K SCR, Power

6. Special Purpose Devices SCR; Efficient Power use. 12VDC P = I X E PLAMP = ILAMP X ELAMP PLAMP = 50 ma X 11v PLAMP = 550 mW LAMP SCR P = I X E PSCR = ISCR X ESCR PSCR = 50ma X 1v PSCR = 50 mW PC; 52A

7. Special Purpose Devices SCR; Control Ckt PC 52A. Operate Mode. R2 10 K O +12 VDC - 9 VDC 5 N L1 R3 100K C1 3 + 4 R1 1K + 1f +DC 2 SCR S1 RESET S2 OPERATE E 1 I Reset

8. Special Purpose Devices SCR; Control Ckt PC 52A. Reset Mode. R2 10 K O +12 VDC - 9 VDC 5 N L1 R3 100K C1 3 I + 4 R1 1K - 1f +DC 2 SCR1 S1 RESET S2 OPERATE Closing S2 Grounds the + plate on Cap C1 causing it to discharge thus Reverse Biases SCR1, ceasing Conduction. E 1 PC 52B

9. Special Purpose Devices SCR; Ckt PC 52B. Conducting. 3 4 G R2 100  L1 SCR1 2N1596 6 R3 100 K 5 7 2 1mfd R1 10 K C2 4.7mfd C1 1 PC 52B, Cutoff

10. Special Purpose Devices SCR; Ckt PC 52B. Cutoff. 3 4 G R2 100  L1 SCR1 2N1596 6 R3 100 K 5 7 2 1mfd R1 10 K C2 4.7mfd C1 1 UJT

11. Special Purpose Devices UJT; Unijunction Transistor B2 BASE 2 Base 2 RB2 N E E P Emitter RB1 N BASE 1 Base 1 B1 NITA LESSON; Per Instructor Discretion. Continue UJT

12. 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 BASE 2 BASE 2 N E RB2 RB2 E P Emitter toBase 1.R, V  Emitterto Base 1. R,V N RB1 RB1 E Current, UJT is Conducting B1 BASE 1 BASE 1 Continue, UJT Ops

13. Special Purpose Devices UJT; Oscillator PC 51 . Operation, C1 Charge. 0 V 12 VDC R1 18 K R3 470  5 C1 Charges 0 V R2 100 K 4 The RC TC determines Charge Time & Frequency 3 Q1 2N4891 C1 Discharges + 0 V 2 C1 .0015 F R4 100  1 C1 Discharge

14. Special Purpose Devices UJT; Oscillator PC 51 . Operation, C1 Discharge. Output; + & - Spike. Saw tooth @ TP3. 12 VDC 0 V - SPIKEOutput @ TP4 R1 18 K R3 470  5 C1 Charges 0 V R2 100 K 4 The RC TC determines Charge Time & Frequency 3 Q1 2N4891 C1 Discharges + 0 V 2 + SPIKE Output @ TP2 C1 .0015 F R4 100  1 Trouble Shoot

15. Special Purpose Devices UJT; Oscillator PC 51, 2 Ckt. What is the Malfunction? 10 O 470 47K R1 R3 R5 R7 470 5 8 10.7 VDC R2 R6 0 VDC 4 9 100K 1M 2N4891 2N4891 10.7 VDC 3 .214 VDC Q1 Q2 5.01 VDC 2 7 C1 C2 .0015f 25f .214 VDC R8 R4 100 1 NO Output NO Output Terms

16. 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

17. Special Purpose Devices Review Questions; • One of the main uses of a UJT is as a _________, __________ . • Why is the UJT often used instead of a conventional transistor? • __________________________________________________ . • The UJT is also called a _________ _____ ______ . • What is the Basic purpose of a SCR? __________ ________ . • How is an SCR turned on ? __________________________ . • What family of transistors does the SCR & UJT belong to ? __________ . • Draw the schematic symbol for a UJT & SCR. • How many PN Junctions does the SCR contain ? ____ . • How is an SCR turned Off ? __________ _______ __________. Answers; 1- 9

18. Special Purpose Devices Review Questions; • 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 12 VDC PC 51 R1 18 K R3 470  5 R2 100 K 4 Q1 2N4891 3 2 C1 .0015 F R4 100  1 Answer; 10