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An Overview of Thyristors

Thyristors, also known as silicon controlled rectifiers (SCR), are semiconductor devices that can act as electronic switches to control circuits with high voltage and current levels. Though thyristors are relatively uncommon components, they are highly useful in controlling power circuits. As such, they are ideal for many power control applications, especially in those where current and voltage levels are elevated. Despite this, thyristors can also be useful in low power applications such as light control, power supply protection, and others. Thyristors are simple parts and relatively inexpensive, making them a helpful solution for many circuits.

A thyristor is considered to be an unusual form of electrical component, as it consists of four layers of differently doped silicon. Conventional bipolar transistors, on the other hand, comprise only three layers. A conventional bipolar transistor is structured p-n-p or n-p-n with the electrodes being collector, base, and emitter. A thyristor has a p-n-p-n structure, with the outer layers of the electrodes known as the anode (n) and the cathode (p). The thyristor’s control terminal is called the gate and is connected to the p-type layer adjacent to the cathode.

Thyristors are frequently made from silicon, though other semiconductor materials could be used. Silicon is the ideal choice because of its innate properties. It is able to handle the voltages and currents of high power applications, and has good thermal properties. Furthemore, because silicon technology is well-established and widely used in semiconductor devices, it is very affordable.

Thyristors have a number of different applications. Among these are AC (alternating current) power control, AC power electronic switching, as overvoltage protection for power supplies, and as control elements in phase angle triggered controllers. Because thyristors are able to switch high voltages and withstand reverse voltages, they are ideal for electronic switching applications, particularly within AC scenarios.

The thyristor was conceptualized by William Shockley in 1950. It was originally described as a bipolar transistor with a p-n hook collector and was further developed by James Ebers in 1952. In 1956, the switching mechanism was developed by employees of Bell Laboratories. Development continued and the first thyristors became available in the early 1960s, immediately gaining popularity. When General Electric launched their version of the device, they called it the silicon controlled rectifier.

Apart from standard thyristors, there are four variants of the basic device, each offering different capabilities for specific applications. They are the reverse conducting thyristor, gate assisted turn-off thyristor, gate turn-off thyristor, and asymmetric thyristor.

  • Reverse Conducting Thyristor (RCT): As their name suggests, RCTs conduct current in the reverse direction. They do this through the use of an integrated reverse diode. RCTs are commonly used in frequency changers and inverters.
  • Gate Assisted Turn-Off Thyristor (GATT): GATTs are used when fast turn-off is needed. To assist in this process, a negative gate voltage is applied, in addition to reducing the anode and cathode voltage. The structure of a GATT is similar to a standard thyristor, but the narrow cathode strips are used to give the gate more control.
  • Gate Turn-Off Thyristor (GTO): GTOs are sometimes called gate turn off switches. These devices are unique because they can be turned off by simply applying a negative voltage to the gate - there is no requirement to remove the anode cathode voltage.
  • Asymmetric Thyristor: These thyristors are used in circuits where the thyristor does not see a reverse voltage and therefore a rectifier capability is not needed. As a result, the second connection is much easier to make, allowing for improved switch on/off times.

Whatever type of thyristor you need, ASAP Semiconductor can help you find it. We offer a broad range of thyristors and other electronic components including power circuits, bipolar transistors, and more. We’re always available and ready to help you find all the parts and equipment you need, 24/7-365. For a quick and competitive quote, call us at 1-714-705-4780 or email us at sales@asapsemi.com.


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