Thyristor

IEEE 315-1975 (1993) 8.5.8.1.c

Nelson Bipole Thyristors

thyristor

Thyristor layers

Thyristor I-V diagram

IEEE 315-1975 (1993) 8.5.8.2.c

Thyristor is a solid-state semiconductor device with four layers of alternating N-type and P-type material. It acts as a switch, conducting when its gate receives a current trigger, and continues to conduct while the voltage across the device is forward-biased. Thyristors are used in the control of high power, as well as in the control of AC and DC motors, heating circuits, and light dimmers.

Structure and Operation

A thyristor typically consists of three electrodes: an anode, a cathode, and a gate. The device's operation is based on the principle of PN junctions in semiconductors. When a positive voltage is applied to the anode relative to the cathode, and a positive current is applied to the gate, the thyristor begins to conduct. Once conducting, the gate no longer controls the thyristor; instead, the device remains on as long as the anode-to-cathode current remains above a certain threshold.

Types of Thyristors

Several variations of thyristors exist, each designed for specific applications. The most common types include:

• Silicon Controlled Rectifier (SCR): The basic form of a thyristor, primarily used for controlling power.
• Triac: A type of thyristor that can conduct in both directions, making it useful for AC power control.
• Gate Turn-Off Thyristor (GTO): A thyristor that can be turned off by applying a negative current to its gate.
• Reverse Conducting Thyristor (RCT): Combines a thyristor and a diode in a single package, useful in power electronics.

Applications

Thyristors are widely used in power electronics due to their ability to handle high voltages and currents, as well as their robustness and reliability. Common applications include:

• Voltage regulation
• Motor control systems
• Power conversion systems
• Light dimming circuits
• Speed control of AC and DC motors

Advantages and Disadvantages

The main advantages of thyristors include their ability to control large amounts of power and their efficiency in doing so. However, they also have some disadvantages, such as their inability to turn off without an external circuit in some cases (e.g., SCRs), and their sensitivity to over-voltage conditions, which can lead to unwanted triggering.

See Also

• Semiconductor device
• Power electronics
• Electronic component