What is an SCR Silicon Controlled Rectifier or Thyristor

- the thyristor or silicon controlled rectifier, SCR is a device that is widely used for controlling or switching power and often high voltage AC or DC circuits.

Thyristors or silicon controlled rectifiers, SCR are find many uses in electronics, and in particular for power control. These devices have even been called the workhorse of high power electronics.

Thyristors are able to switch large levels of power are accordingly they used in a wide variety of different applications. Thyristors even finds uses in low power electronics where they are used in many circuits from light dimmers to power supply over voltage protection.

The term SCR or silicon controlled rectifier is often used synonymously with that of thyristor - the SCR or silicon controlled rectifier is actually a trade name used by General Electric.

SCR discovery

The idea for the thyristor was first described by Shockley in 1950. It was referred to as a bipolar transistor with a p-n hook-collector. The mechanism for the operation was analysed further in 1952 by Ebers.

Then in 1956 Moll investigated the switching mechanism of the thyristor. Development continued and more was learned about the device such that the first silicon controlled rectifiers became available in the early 1960s where it started to gain a significant level of popularity for power switching.

Thyristor applications

Thyristors, or silicon controleld 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).
  • Overvoltage protection crowbar for power supplies.
  • AC power switching.
  • 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 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.

Thyristor symbol

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.

The symbol used for circuit diagrams and schematics for the silicon controlled rectifier or SCR showing the gate, anode and cathode connections
SCR / Thyristor symbol for circuit diagrams and schematics

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.

By Ian Poole

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