Thyristors are a class of four-layer (PNPN) semiconductor devices that act as switches, rectifiers, or voltage regulators. When triggered, thyristors turn on and become low-resistance current paths and remain so, even after the trigger is removed, until the current is reduced to a certain level or until thyristors are triggered off, depending on the type of device. Thyristors include four-layer diodes (Shockley diode), silicon-controlled rectifiers (SCR), diacs, triacs, gate turn-off thyristors (GTO), light-activated SCR (LASCR), and silicon-controlled switches (SCS).
Shockley diodes behave as a pair of interconnected PNP and NPN transistors. Like all thyristors, Shockley diodes tend to stay on once they have been turned on (latched), and stay off once they have been turned off. There are two ways to latch a Shockley diode: exceed the anode-to-cathode breakover voltage, or exceed the anode-to-cathode critical rate of voltage rise. There is only one way to cause a Shockley diode to stop conducting, and that is to reduce the current going through it to a level below its low-current dropout threshold.
Silicon controlled rectifiers (SCR) have an input control terminal (gate), an output terminal (anode) and a terminal common to both the input and output (cathode). SCRs are mainly used where high currents and voltages are involved, and are often used to control alternating currents, where the change of sign of the current causes the device to automatically switch off. For example, a dimmer switch for lights can be implemented using an SCR, where the turn on point is controlled to occur at a particular point on the sine curve of the AC supply. The SCR stays on for the remainder of that cycle. The drawback of using SCRs is that, like diodes, they only conduct in one direction.
Silicon controlled switches (SCS) are four-layer (PNPN) devices that are similar in construction to silicon controlled rectifiers. The SCS, however, has two gate terminals; a cathode gate and an anode gate. The SCS can be turned on and off using either gate terminal. Applying a positive voltage between the cathode gate and cathode terminals turns on an SCS. It may be turned off (forced commutation) by applying a negative voltage between the anode and cathode terminals, or simply by shorting those two terminals together. The anode terminal must be kept positive with respect to the cathode in order for the SCS thyristor to latch.
Triacs are three-terminal silicon devices that function as two SCRs configured in an inverse, parallel arrangement, so as to provide load current during both halves of the AC supply voltage. A triac is generally used to control motor speed. Since load current (armature current) flows during both halves of the applied AC voltage, the motor rotates smoothly at all rotational speeds. Diacs are bi-directional diodes that are designed to trigger a triac or SCR. Basically the diac does not conduct (except for a small leakage current) until the breakover voltage is reached.
Light-activated silicon controlled rectifiers (LASCR) are silicon controlled rectifiers where the state is controlled by light falling upon the silicon semiconductor layer of the device. LASCRs conduct current in one direction when activated by a sufficient amount of light and continue to conduct until the current falls below a specified value. LASCR thyristors are most sensitive to light when the gate terminal is open. Additionally, most LASCRs have a gate terminal so that an electrical pulse can trigger the device in a similar fashion to traditional SCR thyristors.
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