Special Purpose Devices Since the development of the

Special Purpose Devices 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 Devices 2 Types; Silicone Controlled Rectifier, [SCR] SCR Unijunction Transistor, [UJT] 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

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

Special Purpose Devices SCR; Gated On, Latched On. A G A P N A P P N P G N N P P G Q 2 N PP N NN N P K K + A P Q 1 N K 2 Q 2 3 G 1 Q 1 I V - K SCR, Power

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

21+ CDV Special Purpose Devices SCR; Control Ckt PC 52 A. Operate Mode. + R 2 10 K O 5 1 L N RCS + 3 R 3 100 K C 1 4 I R 1 1 K 1 f S 1 +DC 2 RESET S 2 E 1 OPERATE 9 C

Special Purpose Devices SCR; Control Ckt PC 52 A. Reset Mode. - 9 VDC N R 2 10 K O 5 L 1 3 4 R 1 1 K S 1 +DC 2 I - C 1 +12 VDC R 3 100 K + 1 f SCR 1 RESET S 2 E 1 Closing S 2 Grounds the + plate on Cap C 1 causing it to discharge thus Reverse Biases SCR 1, ceasing Conduction. OPERATE PC 52 B

Special Purpose Devices SCR; Ckt PC 52 B. Conducting. G 3 4 R 2 L 1 100 SCR 1 2 N 1596 6 R 3 100 K 5 7 2 C 1 R 1 10 K 4. 7 mfd 1 1 mfd C 2

Special Purpose Devices SCR; Ckt PC 52 B. Cutoff. G 3 4 R 2 L 1 100 SCR 1 2 N 1596 6 R 3 100 K 5 7 2 C 1 R 1 10 K 4. 7 mfd 1 1 mfd C 2

Special Purpose Devices UJT; Unijunction Transistor B 2 Base 2 BASE 2 N E Emitter RB 2 P E RB 1 N BASE 1 Base 1 B 1 NITA LESSON; Per Instructor Discretion. Continue UJT

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. B 2 E N E BASE 2 RB 2 P N RB 1 Current, UJT is Conducting B 1 BASE 1 Emitter to Base 1. 1 R , V RB 1 E BASE 1 Emitter to Base 1. 1 R , V Continue, UJT Ops

Special Purpose Devices UJT; Oscillator PC 51. Operation, C 1 Charge. 12 VDC R 1 18 K R 3 470 0 V 5 R 2 100 K The RC TC determines Charge Time & Frequency 4 3 Q 1 2 N 4891 + C 1. 0015 F C 1 Charges 0 V C 1 Discharges 2 R 4 100 0 V 1 C 1 Discharge

Special Purpose Devices UJT; Oscillator PC 51. Operation, C 1 Discharge. Output; + & - Spike. Saw tooth @ TP 3. 12 VDC R 1 18 K R 3 470 - SPIKE Output 0 V@ TP 4 5 R 2 100 K The RC TC determines Charge Time & Frequency 4 3 Q 1 2 N 4891 + C 1. 0015 F 2 R 4 100 1 Charges 0 V C 1 Discharges + SPIKE Output 0 V@ TP 2 Trouble Shoot

Special Purpose Devices UJT; Oscillator PC 51, 2 Ckt. What is the Malfunction? 10 R 1 10. 7 VDC 470 R 3 R 5 5 0 VDC 10. 7 VDC. 214 VDC . 0015 f R 6 1 M 4 9 2 N 4891 Q 1 . 214 VDC 2 Q 2 C 2 7 25 f R 8 R 4 1 470 2 N 4891 5. 01 VDC C 1 R 7 8 R 2 100 K 3 47 K O 100 NO Output Terms

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

Special Purpose Devices 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. ________________________ ___. 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 ? _______ Answers; 1 - 9

Special Purpose Devices Review Questions; 10. Refer to PC 51; The following DC voltages are present: TP 4 = 10. 7 VDC, TP 3 = 0 VDC, TP 2 =. 21 VDC. What is the Malfunction ? 11. a. C 1 Open 12. b. R 1 Open 13. c. R 2 Short 14. d. Q 1 Open 12 VDC PC 51 R 1 18 K K R 3 470 5 R 2 100 K 4 3 Q 1 2 N 4891 2 C 1. 0015 F 1 R 4 100 Answer; 10