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Chapter 7 Thyristors and Other Devices Outline (Chapter 7)

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Chapter 7 Thyristors and Other Devices Outline (Chapter 7) Chapter 7 Thyristors and Other Devices Outline (Chapter 7) The d evi ces covered i n thi s ch apt er are mai nl y used i n i nd ust ri al applications where power control and switching are needed. 7. Thyri st ors 7.1 The Four Layer Diode ( p-n-p-n Diode) p-n-p-n characteristics 7.2 Silicon Controlled Rectifier (SCR) VltVolt-ampere chtitifthharacteristics of a three-tilSCRterminal SCR 7.3 Bilateral Diode Switch (DIAC) 747.4 Bilateral Triode Switch (TRIAC) 7.5 Negative Resistance Device Unijj(unction Transistor (UJT) UJT characteristic Programmable Unijunction Transistor (PUT) Basic Four Layer Device (Shoc kley di ode) 1. The 4-layer diode is constructed of four alternating layers of p and n materials forming three pn junctions. forward blockinggg region Symbol Characteristics The region prior to breakover is called the forward blocking region. Basic Four Layer Device Anode Anode 1 p p n 1 n 2 2 p p 3 n Cathode n 3 Cathode p n BiFBasic Four Layer Di Didode is Combination of pnp BJT and npn BJT. Basic Four Layer Device 1. 4-layer diode will not conduct when reverse-biased 2. 4-layer diode will not conduct when forward -biased until the forward breakover voltage is reached. 3. 4-layer diode will continue to conduct as long as the holding current is maintained . Silicon-Controlled Rectifier (SCR) The silicon-controlled rectifier (SCR) is a four layer device with three terminals a) anode ( A) (A) (A) b) cathode (K) c) gate (G) (A) p (G) n (G) (G) p n (K) (K) (K) Basic construction Symbol Circuits Silicon-Controlled Rectifier (SCR) 1. The SCR is a switching device for high voltage and current operations. 2. How to turn on the SCR ? Forward bias the anode-cathode or Apply sufficient gate voltage, Vgate and gate current, Igate 3. Once the SCR is turned it remains latched on, even if the gate signal is removed. 4. The SCR like a diode only conducts in one direction. 5. How to turn off the SCR ? Reverse bias the anode-cathode or Remove the power source from anode to cathode. SCR is a three-terminal thyristor that conducts current when triggeredSilicon-Controlled on by a voltage at the single Rectifier gate terminal ( SCRand remain) on until the anode current falls below a specified value. Anode (A) IA on IG VF Gate (G) Characteristics Cathode (K) SCR will turn on when the breakover voltage is reached. SCR Characteristics 1. The voltage across the SCR, VF is high before it turn on, but then it drops significantly once it begins conducting. 2. The SCR only conducts in one direction. 3. The SCR will turn on, if the voltage from anode to cathode is greater or equ al to the forw ard breakov er v oltage, V(BR)F. 4. The gate current, IG can be 0. 5. As more gate current is applied (IG1, IG2), less forward voltage, (VF1, VF3) is required. 6. Current, IH is the minimum required current from anode to cathode. 7. As temperature increases, the SCR requires less forward voltage, VF and gate current, IG to turn on. Silicon-Controlled Rectifier (SCR) In this circuit SW1 is pressed momentarily to turn the SCR on and SW2 is pressed momentarily to turn it off. Bilateral Diode Switch (DIAC) 1. The diac and triac unlike the SCR w ill condu ct in both directions making it ideal for ac applications. 2. The diac turns on when the breakover voltage is reached in either direction. Symbol Characteristics Bilateral Diode Switch (DIAC) n p p n n p p n DIAC is combination of 2 Four Layer Device. Bilateral Triode Switch (TRIAC) Triac is a three-terminal thyristor that can conduct current in either direction when properly activated. Symbol Characteristics TRIAC Characteristics Question 1. _________Thyristors are 4-layer semiconductor devices. 2. Give 3 examples of thyristor devices SCRs, diacs, triacs 3. A 4-layer diode conducts once the ________breakover voltage is reached. 4. The SCR conducts with a pulse at the ______gate and is turned off when the current drops below the _____________value.holding current 5. The diac can conduct either way and is turned on when _______________breakover voltage is reached. Unijunction Transistor (UJT) 1. The UJT has one pn junction 2. The symbol is similar to a JFET. 3. The UJT is used mainly as a triggering device in thyristor circuits and can also be used in oscillator circuits. 4. Note the angle of the emitter. The other terminals are called base 1 and base 2. Unijunction Transistor (UJT) 1. The resistive equivalent circuit of a UJT shown makes it easier to understand its operation. 2. The emitter current controls the value of r’B1 inversely. 3. The total resistance or interbase resistance (r’BB) equals the sum of r’B1 and r’B2. 4.The standoff ratio (η) is the ratio r’B1/r’BB. Programmable Unijunction Transistor (PUT) 1. Although it has the same name as a UJT the programmable unijunction transistor’s (PUT) structure is not the same. 2. It is actually more similar to an SCR. 3. The anode to gate voltage is used to turn it off and on. PUT SCR Programmable Unijunction Transistor (PUT) 1. The PUT can be “programmed” to turn on at a certain voltage by an external voltage divider. 2. This yields a curve similar to a UJT therefore it can used in oscillator circuits like the UJT..
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