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] 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 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 P N K SCR, Gated ON Special Purpose Devices SCR; Gated On, Latched On. A G A P N P N A P P N P G N N P P G Q2 N PP N NN N P K K + A P Q1 N K 2 Q2 3 G 1 Q1 I V - K SCR, Power Special Purpose Devices SCR; Efficient Power use. 12VDC P = I X E PLAMP = ILAMP X ELAMP LAMP PLAMP = 50 ma X 11v PLAMP = 550 mW SCR P = I X E PSCR = ISCR X ESCR PSCR = 50ma X 1v PSCR = 50 mW PC; 52A Special Purpose Devices SCR; - 9 VDC Control Ckt PC 52A. Operate Mode. R2 10 K O 5 N L1 C1 3 4 R1 1K S1 +DC + 2 R3 100K + 1f SCR RESET S2 E +12 VDC OPERATE 1 I Reset Special Purpose Devices SCR; - 9 VDC Control Ckt PC 52A. Reset Mode. R2 10 K O 5 N L1 3 4 R1 1K S1 +DC 2 I - C1 +12 VDC R3 100K + 1f SCR1 RESET S2 E 1 Closing S2 Grounds the + plate on Cap C1 causing it to discharge thus Reverse Biases SCR1, ceasing Conduction. OPERATE PC 52B Special Purpose Devices SCR; Ckt PC 52B. Conducting. G 3 4 R2 L1 100 SCR1 2N1596 6 R3 100 K 5 7 2 R1 10 K 4.7mfd 1mfd C2 C1 1 PC 52B, Cutoff Special Purpose Devices SCR; Ckt PC 52B. Cutoff. G 3 4 R2 L1 100 SCR1 2N1596 6 R3 100 K 5 7 2 R1 10 K 4.7mfd 1mfd C2 C1 1 UJT Special Purpose Devices UJT; Unijunction Transistor B2 Base 2 BASE 2 N E Emitter RB2 P E RB1 N BASE 1 Base 1 B1 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. B2 E N E BASE 2 BASE 2 RB2 RB2 P N RB1 Current, UJT is Conducting B1 BASE 1 Emitter to Base 1. R, V RB1 E BASE 1 Emitter to Base 1. R, V Continue, UJT Ops Special Purpose Devices UJT; Oscillator PC 51 . Operation, C1 Charge. 12 VDC R1 18 K R3 470 0 V 5 R2 100 K The RC TC determines Charge Time & Frequency 4 3 Q1 2N4891 + C1 .0015 F C1 Charges 0 V C1 Discharges 2 R4 100 0 V 1 C1 Discharge Special Purpose Devices UJT; Oscillator PC 51 . Operation, C1 Discharge. Output; + & - Spike. Saw tooth @ TP3. 12 VDC R1 18 K R3 470 - SPIKE Output 0 V@ TP4 5 R2 100 K The RC TC determines Charge Time & Frequency 4 3 Q1 2N4891 + C1 .0015 F 2 R4 100 1 C1 Charges 0 V C1 Discharges + SPIKE Output 0 V@ TP2 Trouble Shoot Special Purpose Devices UJT; Oscillator PC 51, 2 Ckt. What is the Malfunction? 10 R1 10.7 VDC 470 R3 R5 5 0 VDC 10.7 VDC .214 VDC .0015f R6 1M 4 9 2N4891 Q1 .214 VDC 2 Q2 C2 7 25f R8 R4 1 470 2N4891 5.01 VDC C1 R7 8 R2 100K 3 47K 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. 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: TP4 = 10.7 VDC, TP3 = 0 VDC, TP2 = .21 VDC. What is the Malfunction ? a. C1 Open b. R1 Open c. R2 Short 12 VDC PC 51 R1 18 K d. Q1 Open R3 470 5 R2 100 K 4 3 Q1 2N4891 2 C1 .0015 F R4 100 1 Answer; 10