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Thyristors - SCRs

The Silicon Controlled Rectifier (SCR) is a three terminal, two junction, four layer solid state device used as a switching device and is also referred to as a unidirectional thyristor. SCRs have been around for many years as one of the earliest semiconductor switches. Although there are more advanced thyristors available, the basic SCR is still one of the most commonly used switching devices today.

The application field of SCRs is wide and includes the control of heating, lighting, welding and machines. In addition to its ability to fully control on and off switching, the SCR has many other advantages such as fast switching, bidirectional control, low voltage drop, insulated gate construction and a wide range of triggers. One of the main reasons why SCRs are so widely used is their ability to be used as a crowbar device to protect circuits from over voltage faults.

The working principle of a SCR can be explained by looking at its internal structure. It has four layers, two P-type (positive) and two N-type (negative) layers. The two N-type layers are on the outside while the two P-type layers are sandwiched in-between. The two outside N-type layers are connected together to form the anode while the two P-type layers are connected together to form the cathode. Between the two outside N-type layers is the gate. When a voltage greater than the gate threshold voltage is applied to the gate with respect to the cathode, the SCR turns on, allowing current to flow from the anode to the cathode. When this gate voltage is removed, the SCR turns off.

TheSilicon Controlled Rectifier (SCR) is a unique device in that it can be used to provide complete control of current in both directions, from either the anode or the cathode. This means that the SCR can be used to turn power on or off, stop or start current flowing or limit the amount of current flowing. It can also be used to control the phase angle of an AC power circuit, as when used in a phase-controlled rectifier. The SCR also makes a very reliable crowbar device for over voltage protection, as it has the ability to divert large amounts of current under fault conditions.

The SCR is ideal for use as a switch controller in motor control, dimming, power factor correction, and rectification circuits. It is also very well suited for pulse width modulation applications. It is usually integrated within the power control circuitry and is used to directly switch the power devices in and out of the circuit. This allows for fast switching with very low voltage drops, making it an ideal choice for controlling buses, transformers, and other power components.

In summary, SCRs are widely used in various applications as a switching device, power controller and protection device due to its fast switching, low voltage drop, bidirectional control capabilities and wide range of triggers. SCRs are usually integrated within the power control circuitry and are used to directly switch the power devices in and out of the circuit, making them the ideal choice for motor control, dimming, power factor correction and rectification circuits.