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| SEC. 15-1 THE FOUR-LAYER
DIODE A thyristor is a semiconductor device that uses internal positive feedback to produce latching action. The four-layer diode, also called a Schockley diode, is the simplest thyristor. Breakover closes it, and lowcurrent drop-out opens it. SEC. 15-2 THE SILICON CONTROLLED RECTIFIER The silicon controlled rectifier (SCR) is the most widely used thyristor. It can switch very large currents on and off. To turn it on, we need to apply a minimum gate trigger voltage and current. To turn it off, we need to reduce the anode voltage to almost zero. SEC. 15-3 THE SCR CROWBAR One important application of the SCR is to protect delicate and expensive loads against supply overvoltages. With an SCR crowbar, a fuse or current-limiting circuit is needed to prevent excessive current from damaging the power supply. SEC. 15-4 SCR PHASE CONTROL An RC circuit can vary the lag angle of gate voltage from 0 to 90°. This allows us to control the average load current. By using more advanced phase control circuits, we can vary the phase angle from 0 to 180° and have greater control over the average load current. SEC. 15-5 BIDIRECTIONAL THYRISTORS The diac can latch current in either direction. It is open until the voltage across it exceeds the breakover voltage. The triac is a gate-controlled device similar to an SCR. With a phase controller, a triac gives us full-wave control of the average load current. SEC. 15-6 IGBTs The IGBT is a hybrid device composed of a power MOSFET on the input side and a BJT on the output side. This combination produces a device with simple input gate drive requirements and low conduction losses on the output. IGBTs have an advantage over power MOSFETs in highvoltage, high-current switching applications. SEC. 15-7 OTHER THYRISTORS The photo-SCR latches when the incoming light is strong enough. The gate-controlled switch is designed to close with a positive trigger and open with a negative trigger. The silicon controlled switch has two input trigger gates, either of which can close or open the device. The unijunction transistor has been used to build oscillators and timing circuits. SEC. 15-8 TROUBLESHOOTING When you troubleshoot a circuit to find defective resistors, diodes, transistors, and so on, you are troubleshooting at the component level. When you are troubleshooting to find a defective functional block, you are troubleshooting at the system level. |