Silicon Controlled Rectifier, SCR or thyristor: what it is, how it works, symbol . . .

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Thyristors or silicon controlled rectifiers, SCRs - the names are used interchangeably - 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 used for many power control applications, often where current and voltage levels are relatively high.

Thyristors may also be used in lower power applications including light control, as well as for power supply protection and many other applications. Thyristors are simple to use and cheap to buy, making them an ideal option for many circuits.

The idea for the SCR is not new - the concept for the device was first proposed 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 electronic switching and power supply circuits.

What is an SCR or 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 bipolar 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.

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 devices. As a result it is very cheap and easy for semiconductor manufacturers to use for their electronic components.

SCR / 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 electronic switching.
• Overvoltage protection crowbar for power supplies.
• Control elements in phase angle triggered controllers.
• Within photographic flash lights where they act as the electronic 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 electronic switching applications, especially within AC scenarios.

Thyristor 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.

When GE launched their devices, they used the term silicon controlled rectifier, or SCR, because it only conducted in one direction and was controllable. They used the name SCR as a trade mark for their products.

SCR / Thyristor operation: the basics

In operation, the thyristor / SCR has three states in which it can be at any given time:

• Reverse blocking:   In this mode or state the thyristor blocks the current in the same way as that of a reverse biased diode. The thyristor / SCR can only conduct in one direction and blocks in the reverse direction.
• Forward blocking:   In this mode or state the thyristor operation is such that it blocks forward current conduction that would normally be carried by a forward biased diode. In this state the thyristor / SCR is not in its “turn-on” state as the gate has not fired.
• Forward conducting:   In this mode the thyristor / SCR has been triggered into conduction by a current on the gate. It will remain conducting regardless of the state of the gate. Current only needs to be applied to the gate to fire the thyristor / SCR, and it will remain conducting. The device will stop conducting when the forward current drops below a threshold value known as the "holding current."

The thyristor consists of four semiconductor regions: P N P N. The outer P region forms the anode, and the outer n region forms the cathode as shown below.

To look at how a thyristor works it is helpful to use a simplified equivalent circuit. This consists of two back to back transistors as shown below.

The transistor with its emitter connected to the cathode of the thyristor is an NPN device whereas the transistor with its emitter connected to the anode of the SCR is a PNP variety. The gate is connected to the base of the NPN transistor.

This arrangement forms a positive feedback loop within the thyristor. The output of one transistor fed to the input of the second. In turn the output of the second transistor is fed back to the input of the first. As a result it can be seen that the total current gain of the device exceeds one. This means that when a current starts to flow, it quickly builds up until both transistors are fully turned on or saturated.

When a voltage is applied across a thyristor no current flows because neither transistor is conducting. As a result there is no complete path across the device.

If a small current is passed through the gate electrode, this will turn "on" the transistor TR2. When this occurs it will cause the collector of TR2 to fall towards the voltage on the emitter, i.e. the cathode of the whole device.

In turn this will cause current to flow through the base of TR1 and turn this transistor "on".

Again this will now try to pull the voltage on the collector of TR1 towards its emitter voltage. This will cause current to flow in the emitter of TR2, causing its "on" state to be maintained.

In this way it only requires a small trigger pulse on the gate to turn the thyristor on. Once switched on, the thyristor can only be turned off by removing the supply voltage.

In summary, 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. In this way the thyristor can be used as an electronic switch.

SCR / Thyristor symbols & basics

The thyristor or silicon controlled rectifier, SCR is a semiconductor 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 of silicon controlled rectifier. Therefore when the device is used with AC, it will only conduct for a maximum of half the cycle.

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.

SCR / Thyristor specifications

In order to select the correct silicon controlled rectifier, SCR / thyristor device for any circuit, it is necessary to study the datasheets and ensure that the device has the right characteristics for the intended circuit or application.

Thyristors are rather unique components and their specifications and datasheet parameters are different to other more widely used electronic components like bipolar transistors and JFETs, MOSFETs, etc.

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 V_on as well as improved turn on time and turn off time.

Silicon controlled rectifiers or thyristors are widely used in many areas of electronics acting as electronic switches. Thyristor or SCR circuits can be used for many power applications as these electronics components are able to switch high currents very easily. In addition to this these they are very cheap and they are widely available.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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