What is a Thyristor, SCR?

- a summary or overview describing what is a thyristor or silicon controlled rectifier, SCR, giving details of a thyristor circuit, and the way thyristor control can be implemented

Triac, Diac, SCR Tutorial Includes:
Thyristor basics     Thyristor device structure     Thyristor operation     Gate turn off thyristor, GTO     What is a triac     Diac overview    

Thyristors or silicon controlled rectifiers (SCR) as they are sometimes known may appear to be unusual electronics components in many ways, but they are particularly useful for controlling power circuits. As such these electronics components are often used for applications such as light dimmers, and there may be thyristor circuits used in many power supply applications. Thyristors are simple to use and cheap to buy and often thyristor circuits are easy to build and use. All these reasons make thyristors ideal components to consider for many applications.

The idea for the thyristor is not new. The idea for the device was first put forward in 1950 by William Shockley, one of the inventors of the transistor. Although some later investigation of the device was undertaken by others a couple of years later, it was not until the early 1960s when they became available. After the introduction of the thyristor, they soon became popular for power supply circuits.

What is a thyristor?

The thyristor may be considered a rather an unusual form of electronics component because it consists of four layers of differently doped silicon rather than the three layers of the conventional bipolar transistors. Whereas conventional transistors may have a p-n-p or n-p-n structure with the electrodes named collector, base and emitter, the thyristor has a p-n-p-n structure with the outer layers with their electrodes referred to as the anode (n-type) and the cathode (p-type). The control terminal of the SCR is named the gate and it is connected to the p-type layer that adjoins the cathode layer.

Basic structure of a thyristor or SCR - silicon controlled rectifier
Basic structure of a thyristor / SCR

Thyristors are usually manufactured from silicon, although, in theory other types of semiconductor could be used. The first reason for using silicon for thyistors is that silicon is the ideal choice because of its overall properties. It is able to handle the voltage and currents required for high power applications. Additionally it has good thermal properties. The second major reason is that silicon technology is well established and it is widely used for a variety of semiconductor electronics components. As a result it is very cheap and easy for semiconductor manufacturers to use.

Thyristor applications

Thyristors, or silicon controlled rectifiers, SCRs are used in many areas of electronics where they find uses in a variety of different applications. Some of the more common applications for them are outlined below:

  • AC power control (including lights, motors,etc).
  • AC power switching.
  • Overvoltage protection crowbar for power supplies.
  • Control elements in phase angle triggered controllers.
  • Within photographic flash lights where they act as the switch to discharge a stored voltage through the flash lamp, and then cut it off at the required time.

Thyristors are able to switch high voltages and withstand reverse voltages making them ideal for switching applications, especially within AC scenarios.

Thyristor symbols & basics

The thyristor or silicon controlled rectifier, SCR is a device that has a number of unusual characteristics. It has three terminals: Anode, cathode and gate, reflecting thermionic valve / vacuum tube technology. As might be expected the gate is the control terminal while the main current flows between the anode and cathode.

As can be imagined from its circuit symbol shown below, the device is a "one way device" giving rise to the GE name for it the silicon controlled rectifier. Therefore when the device is used with AC, it will only conduct for a maximum of half the cycle.

In operation, the thyristor or SCR will not conduct initially. It requires a certain level of current to flow in the gate to "fire" it. Once fired, the thyristor will remain in conduction until the voltage across the anode and cathode is removed - this obviously happens at the end of the half cycle over which the thyristor conducts. The next half cycle will be blocked as a result of the rectifier action. It will then require current in the gate circuit to fire the SCR again.

The silicon controlled rectifier, SCR or thyristor symbol used for circuit diagrams or circuit seeks to emphasis its rectifier characteristics while also showing the control gate. As a result the thyristor symbol shows the traditional diode symbol with a control gate entering near the junction.

Thyristor or SCR circuit symbol
Thyristor or SCR circuit symbol

Other types of thyristor or SCR

There is a number of different types thyristor - these are variants of the basic component, but they offer different capabilities that can be used in various instances and may be useful for certain circuits.

  • Reverse conducting thyristor, RCT:   Although thyristors normally block current in the reverse direction, there is one form called a reverse conducting thyristor which has an integrated reverse diode to provide conduction in the reverse direction, although there is no control in this direction.

    Within a reverse conducting thyristor, the device itself and the diode do not conduct at the same time. This means that they do not produce heat simultaneously. As a result they can be integrated and cooled together.

    The RCT can be used where a reverse or freewheel diode would otherwise be needed. Reverse conducting thyristors are often used in frequency changers and inverters.
  • Gate Assisted Turn-Off Thyristor, GATT:   The GATT is used in circumstances where a fast turn-off is needed. To assist in this process a negative gate voltage can sometimes be applied. In addition to reducing the anode cathode voltage. This reverse gate voltage helps in draining the minority carriers stored on the n-type base region and it ensures that the gate-cathode junction is not forward biased.

    The structure of the GATT is similar to that of the standard thyristor, except that the narrow cathode strips are often used to enable the gate to have more control because it is closer to the centre of the cathode.
  • Gate Turn-Off Thyristor, GTO:   The GTO is sometimes also referred to as the gate turn off switch. This device is unusual in the thyristor family because it can be turned off by simply applying a negative voltage to the gate - there is no requirement to remove the anode cathode voltage. See further page in this series more fully describing the GTO.
  • Asymmetric Thyristor:   This device is used in circuits where the thyristor does not see a reverse voltage and therefore the rectifier capability is not needed. As a result it is possible to make the second junction, often referred to as J2 (see page on the device structure) can be made much thinner. The resulting n-base region provides a reduced Von as well as improved turn on time and turn off time.

Thyristors are widely used in many areas of analogue electronics. Thyristor circuits can be used for many power applications as these electronics components are above to switch high currents very easily. In addition to this these electronics components are very cheap and they are widely available.

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