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Thyristors are solid-state semiconductor devices with three or four layers of alternating p-type and n-type materials. These layers form a junction when the thyristor is activated and the junction can allow electrical current to flow in one direction only. The thyristor has three main categories: Silicon-Controlled Rectifier (SCR), Triac and Diac. All these thyristors have the same basic structure and similar working principles. The Silicon-Controlled Rectifier (SCR) is one of the most commonly used thyristors and is commonly used in AC and DC voltage applications. This article will focus on the SCR and its application field and working principle.

A SCR is basically a three-term forward current-controlled rectifier. It has four main parts: an anode, a cathode, a gate and an SCR resistance regulation control chip. The anode and cathode are connected to the DC power source, and the gate is connected to the control source. When a small current is applied to the gate, the SCR will turn “on” and current will be allowed to flow from the anode to the cathode. Once the SCR is “on”, the current through the SCR will remain as long as the anode-cathode voltage remains above the device’s minimum firing voltage. The gate is then cut off after a certain time, based on the control signal and the SCR “off”. SCRs provide a large forward blocking voltage capability with low forward voltage drop and are used in applications such as motor control and AC power control.

The application field of SCR is quite extensive and includes the following:

• AC Voltage Control: SCRs are used to control AC voltage applications such as starting and stopping motor drives, modulating power to process loads, and over current protection.
• Inverters: Inverters are used in a variety of applications, from residential solar panel inverters to industrial uninterruptable power supplies (UPS). SCRs are used to inverters for both the control and the power switching elements.
• Battery Charging: SCRs are used in battery charger applications to regulate the charging process and protect against overload and overcharging.
• DC Voltage Control: SCRs are also used in DC voltage applications such as controlling the speed or position of DC motors, controlling the output voltage of power supplies, and protection systems against under-voltage and overload.
• Regulation Control: SCRs are used to regulate the voltage output of alternators and other power generation devices.

The working principle of an SCR is based on the fact that when a current is applied to the gate terminal, it creates an internal electric field in the device which turns the SCR “on”. Once the current is “on”, it will continue to flow from the anode to the cathode until the anode-cathode voltage drops below the device’s minimum firing voltage or the gate current is interrupted. In addition, the “off” can be triggered by either cutting the gate current or shorting the anode-cathode junction.

The SCR is a versatile device with a wide range of applications in both AC and DC voltage applications. The SCR is able to control both the anode and cathode voltages while providing low forward voltage drop and high current levels. This makes it an ideal device for motor control, AC power control, voltage regulation, and over-current protection. SCRs are also cost-effective, allowing for added cost savings in any application that requires DC or AC power switching.

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