VTU Module-1 | Thyristors

18EC36 | POWER ELECTRONICS & INSTRUMENTATION | Module-1 VTU Notes

Summary:

Thyristors, such as Silicon Controlled Rectifiers (SCRs), are essential components in electronics and power control systems. Understanding their static anode-cathode characteristics and gate characteristics is crucial for their effective use.

1. Static Anode-Cathode Characteristics:

SCR is a four-layer semiconductor device with an anode, cathode, and gate. It exhibits a blocking state until a specific voltage (breakover voltage) is applied between anode and cathode, allowing current flow.

2. Gate Characteristics of SCR:

The gate terminal can trigger SCR into conduction by applying a pulse of current. The gate trigger current must exceed a certain threshold for SCR to turn on.

3. Turn-ON Methods:

SCRs can be turned on using various methods, including voltage triggering, current triggering, and gate triggering. Gate triggering is the most common method and allows precise control.

4. Turn-OFF Mechanisms:

Once an SCR is conducting, it can be turned off by reducing the anode current below a holding current or by reversing the voltage across it.

5. Turn-OFF Methods:

There are two main turn-off methods:

• Natural Commutation: The current naturally falls below the holding current, turning off the SCR.
• Forced Commutation: External circuits or components are used to force the SCR to turn off.

6. Class A and Class B Types:

SCRs are categorized into Class A and Class B based on their turn-off characteristics. Class A SCRs turn off naturally, while Class B SCRs require forced commutation for turn-off.

7. Gate Trigger Circuit:

Several gate trigger circuits are used for SCR control:

• Resistance Firing Circuit: Uses resistors to control the gate current and trigger the SCR.
• Resistance-Capacitance Firing Circuit: Combines resistors and capacitors for more precise control.

8. Unijunction Transistor (UJT):

UJT is a three-layer semiconductor device with a unique triggering behavior based on its intrinsic standoff ratio.

9. Basic UJT Operation:

UJT operates as a relaxation oscillator, producing a periodic voltage waveform at its emitter terminal.

10. UJT Firing Circuit:

UJT firing circuits are used to trigger SCRs and other devices. They exploit UJT's unique characteristics for precise timing.

Understanding these concepts is crucial for designing and controlling electronic circuits involving thyristors like SCRs and utilizing UJT in various applications. These devices play a vital role in power control and electronics, making them fundamental in modern technology.