Thyristors are semiconductors that are, essentially control systems for switching and controlling power. The ones used most are Silicon-Controlled Rectifiers (SCRs), Diode AC Switches (Diacs), Triode AC Switches (Triacs), and Uni-Junction Transistors (UJTs). Here’s a breakdown of each one and how they function:
- Silicon-Controlled Rectifiers (SCRs): Three-terminal semiconductors used for controlling the flow of current in electrical circuits. This device consists of four layers of alternating P-type and N-type semiconductor material. The device operates in three states: off state, forward blocking state, and forward conducting state. In the off state, the device blocks the flow of current. When a voltage is applied to the gate terminal, the device enters the forward blocking state, and the current starts flowing through the device when the voltage across the anode and cathode exceeds a certain threshold value. SCRs are widely used in power control applications, such as motor control, heating control, and lighting control.
- Diode AC Switches (Diacs): Two-terminal semiconductors that are used for controlling the flow of AC current in electrical circuits. The device consists of two layers of alternating P-type and N-type semiconductor material. Operates in two states: off state and on state. In the off state, the device blocks the flow of current. When a voltage is applied to the device, the device enters the on state and conducts current in both directions. Diacs are commonly used in AC voltage control applications, such as dimmer circuits and fan speed controllers.
- Triode AC Switches (Triacs): Three-terminal semiconductor devices that are used for controlling the flow of AC current in electrical circuits. The device consists of two SCR devices connected in parallel in an inverse parallel configuration. The device operates in four states: off state, forward blocking state, forward conducting state, and reverse blocking state. In the off state, the device blocks the flow of current. When a voltage is applied to the device, the device enters the forward blocking state, and the current starts flowing through the device when the voltage across the anode and cathode exceeds a certain threshold value. Commonly used in AC voltage control applications, such as light dimmers and motor speed controllers.
- Uni-Junction Transistors (UJTs): Three-terminal semiconductor devices that are used for generating trigger pulses in electrical circuits. The device consists of a single P-type material and two N-type material. Operates in two states: off state and on state. In the off state, the device blocks the flow of current. When a voltage is applied to the device, the device enters the on state and conducts current between the emitter and one of the base terminals. UJTs are commonly used in applications that require precise timing, such as triggering of SCRs and Triacs.
By understanding the operation of each of these devices, control system engineers can choose the appropriate device for their specific application to achieve the desired control and power handling requirements.
REFERENCES:
The Silicon-Controlled Rectifier (SCR) | Thyristors | Electronics Textbook. (n.d.). Www.allaboutcircuits.com. https://www.allaboutcircuits.com/textbook/semiconductors/chpt-7/silicon-controlled-rectifier-scr/
What is a DIAC – Operation Applications Circuits» Electronics Notes. (n.d.). Www.electronics-Notes.com. https://www.electronics-notes.com/articles/electronic_components/scr/what-is-a-diac.php
Electrical4U. (n.d.). TRIAC: What is it? (Definition, Operation & Applications) | Electrical4U. Https://Www.electrical4u.com/. https://www.electrical4u.com/triac/
Unijunction Transistor – Construction, Working Principle, and Characteristic Features. (n.d.). Www.tutorialspoint.com. https://www.tutorialspoint.com/unijunction-transistor-construction-working-principle-and-characteristic-features