Silicon controlled rectifier (SCR), Definition, How it Works, and Characteristics

Table of Contents

Hello everyone, this article will discuss the definition of SCR, how SCR works, and the characteristics of SCR. Please read until the end so you can better understand silicon controlled rectifiers.

Definition of Silicon Controlled Rectifier (SCR)

SCR or Silicon Controlled Rectifier (also called thyristor) is a type of diode that functions as a controller. SCRs and regular diodes are similar, but SCRs have three terminals while regular diodes only have two.

Regular diodes only have Anode and Cathode terminals, while SCRs have three terminals: Gate, Anode, and Cathode. The Gate terminal in an SCR functions as a control trigger to connect the Anode and Cathode terminals.

SCRs are widely used as controllers or switches in electronic circuits that use medium to high voltages, such as power controllers, timers, oscillators, light dimmers, electric motor speed controllers, and more.

Essentially, an SCR consists of 4 semiconductor layers in a PNPN (Positive-Negative-Positive-Negative) configuration. The Gate terminal, which functions as the controller, is located in the P-type (Positive) material near the Cathode terminal.

How Silicon Controlled Rectifier (SCR) Works

An SCR works similarly to a regular diode by allowing direct current to flow from the Anode to Cathode terminal, but the connection between the Anode and Cathode is controlled by the Gate terminal. To activate the SCR (connecting the Anode and Cathode), a positive current must first be applied to the Gate terminal as a trigger.

When current flows to the Gate terminal, the SCR activates, connecting the Anode and Cathode terminals so current can flow from Anode to Cathode. Even if the positive current to the Gate terminal is removed, the SCR will remain active.

To deactivate the SCR, the forward current from Anode to Cathode must be reduced below the SCR's holding current point. Each SCR has a different holding current depending on its specifications.

The holding current in an SCR is the minimum current that must be maintained for the SCR to remain active, with a typical minimum value of 0.7V. Essentially, to deactivate the SCR, you can eliminate or reduce the forward current from Anode to Cathode to zero.

I-V Characteristic Curve of Silicon Controlled Rectifier (SCR)

I-V Characteristic Curve of SCR

The image shows the breakover voltage (Vbo), where if the forward voltage reaches this point, the SCR will turn ON. More importantly, the gate current (Ig) can cause the Vbo to decrease. The image shows several Ig currents and their correlation to the breakover voltage.

In SCR datasheets, this gate trigger current is often written as IGT (Gate Trigger Current). The image also shows the holding current (Ih) that keeps the SCR ON.

So for the SCR to remain ON, the forward current from anode to cathode must be above this parameter. So far we've discussed how to turn an SCR ON. In reality, once an SCR is ON, it will remain ON even if the gate voltage is removed or shorted to the cathode.

The only way to turn an SCR OFF is to reduce the anode-cathode current below the holding current (Ih). In the I-V curve image, if the forward current falls below the Ih point, the SCR returns to the OFF state. The value of this holding current is typically specified in the SCR datasheet. Turning the SCR OFF is equivalent to reducing the anode-cathode voltage to zero.

Types and Classifications of SCR

SCRs can be classified based on various aspects, such as gate bias characteristics and physical form (packaging). This classification affects how SCRs are used in electronic circuits according to specific needs and conditions. Here's a more detailed explanation of SCR types.

1. SCR Based on Gate Bias Characteristics

An SCR operates when current flows through the gate terminal, creating a specific bias. The gate bias characteristics determine the SCR's response to voltage and current. Here are SCR types based on gate bias:

a. Forward Blocking Mode SCR

• In this mode, the SCR blocks forward bias current flowing from anode to cathode.
• Terminals J1 and J3 are in forward bias, while terminal J2 is in reverse bias.
• The SCR remains OFF until triggered by the gate or until the anode-cathode voltage exceeds the breakdown voltage.

b. Reverse Blocking Mode SCR

• In this mode, the SCR blocks reverse bias current with high impedance.
• Positive voltage is applied to anode and cathode, so J1 and J3 are forward biased while J2 is reverse biased.
• If breakdown occurs at low voltage, small leakage current can flow.

c. Forward Conduction Mode SCR

• After blocking mode, the SCR can switch to conduction (ON) mode if:
  • Anode-cathode voltage is increased beyond breakdown voltage.
  • A positive pulse is applied to the gate terminal to trigger conduction.
• When ON, the SCR will continue conducting current until the current drops below the holding current value.

2. SCR Based on Physical Form (Packaging)

SCRs can also be classified based on their physical form and packaging, which affects installation and applications. Here are some SCR types based on packaging:

• Diamond SCR → Diamond-shaped, often used in high-power applications.
• Press Diode SCR → Button-shaped, commonly used in compact circuits.
• Stud SCR → Bolt-shaped, easy to mount on heatsinks.
• SOT SCR (Small Outline Transistor) → Small package, suitable for SMD (Surface Mount Device) applications.
• TO SCR (Transistor Outline) → Transistor-shaped, available in various sizes.

Applications of SCR (Silicon-Controlled Rectifier)

SCR (Silicon-Controlled Rectifier) is a semiconductor component that functions as a controlled electronic switch. Due to its ability to efficiently control electrical power, SCRs are widely used in various fields, from industry to household appliances. Here are some main applications:

1. Electric Motor Control

• DC & AC Motors: SCRs are used to control motor speed by regulating input voltage and current.
• Motor Soft Starters: Prevent current surges when motors are first turned on.
• Servo & Drive Systems: Used in industrial automation for precise motion control.

2. Rectification (Voltage Conversion)

• Controlled Rectifiers: Convert alternating current (AC) to direct current (DC) by adjusting the firing angle.
• Adjustable Power Supplies: Used in variable power supplies for electronics and industrial equipment.

3. Lighting Control

• Light Dimmers: Adjust the intensity of incandescent and LED lights by controlling power supply.
• Street & Building Lighting Systems: Enable energy savings through automatic control.

4. Heating Systems

• Oven & Furnace Temperature Control: Regulate heating power to maintain stable temperatures.
• Water Heaters & Induction Cookers: Ensure efficient and safe heating.

5. Voltage Protection & High-Power Switches

• Overvoltage Protection: Cut off current during dangerous voltage spikes.
• Electronic Circuit Breakers: Act as high-speed switches for electrical system protection.

6. Inverters & UPS (Uninterruptible Power Supply)

• DC to AC Conversion: Used in inverters to provide backup power.
• Voltage Stabilizers: Maintain stable power for sensitive equipment.

7. Industrial & High-Power Applications

• Electric Welding (Welding Machines): Control welding current for precise results.
• Power Load Control: Used in industrial power distribution systems.

8. Power Electronics & Control

• Battery Chargers: Efficiently regulate battery charging.
• Fan & Water Pump Speed Controllers: Adjust speed as needed.

This concludes our discussion about Silicon Controlled Rectifiers from Electrical.Kelasteknisi.com, covering definitions, working principles, characteristics, types, and applications. We hope this is useful. .

Reference Sources:

http://tmnstudio.com/electronics/446-fungsi-scr-sillicon-controlled-rectifier.html

https://www.teknik-otomotif.com/2017/12/fungsi-dan-cara-kerja-scr-silicon.html

https://teknikelektronika.com/pengertian-thyristor-jenis-thyristor/

https://elektronika-dasar.web.id/pengertian-scr-silicon-controlled-rectifier/

https://www.jagobelanja.com/pengertian-fungsi-thyristor-dan-cara-kerja/

https://www.wikikomponen.com/pengertian-scr-dan-prinsip-kerja-komponen-silicon-control-rectifier/