Thyristor and how it works tutorial

Summary

This script explains thyristors—four-layer (P-N-P-N) semiconductor switches with three terminals: anode, cathode, and gate. It covers their structure, operation in forward and reverse biased states, and the latching behavior that keeps them conducting after a gate pulse. The summary details forward blocking, forward conduction, and reverse blocking modes, plus trigger and breakover behavior, latching and holding currents, and avalanche breakdown in reverse bias. It lists common thyristor types (SCR, TRIAC, GTO, LASCR, ETO, etc.) and practical uses such as motor speed control, light dimmers, welding machines, camera flashes, fault current limiters, and ignition switches.

Takeaways

• 😀 A thyristor is a high-speed switching device used in AC power control and AC/DC switching, including types like triacs and SCRs (Silicon Controlled Rectifiers).
• 😀 Thyristors are three-terminal devices with anode, cathode, and gate terminals, and consist of four layers of alternating p-type and n-type materials.
• 😀 Thyristors can operate as either an open-circuit switch or a rectifying diode, depending on how the gate is triggered.
• 😀 The gate terminal controls the thyristor’s switching, but once turned on, the thyristor latches in the on-state, making it remain on even if the gate signal is removed.
• 😀 Thyristors can handle very large currents and voltages, making them suitable for high-energy power conditioning circuits and systems with voltages over 1 kV or currents above 100 A.
• 😀 Unlike diodes, which are two-layer devices, thyristors are four-layer devices (P-N-P-N) and conduct current only in one direction.
• 😀 Thyristors are typically used in power control applications and are not suitable for amplification purposes.
• 😀 Thyristors have three operational modes: Forward Blocking (OFF state), Reverse Blocking (OFF state), and Forward Conducting (ON state).
• 😀 Thyristors can conduct only in the forward direction, and their reverse blocking capability is limited to a specific reverse breakdown voltage.
• 😀 Common types of thyristors include SCRs, GTOs, ETOs, LASCRs, and MOS turn-off thyristors, each designed for specific switching or control applications.
• 😀 Thyristors are widely used in various applications, including light dimmers, motor drives, high-power electrical control, circuit breakers, and car ignition switches.

Q & A

What is a Thyristor and how does it work?

A Thyristor is a three-terminal semiconductor device made of four layers of alternating p-type and n-type material, forming three p-n junctions. It works as a switch, allowing power to flow when triggered by a gate current and blocking power in the absence of a trigger signal.

What are the main terminals of a Thyristor and what do they do?

A Thyristor has three main terminals: Anode (A), Cathode (K), and Gate (G). The Anode and Cathode are used to connect the device to the power circuit, while the Gate controls the switching behavior of the device, turning it on or off.

What is the difference between Thyristors and other semiconductor devices like diodes and transistors?

Unlike diodes, which are two-layer devices, Thyristors are four-layer (p-n-p-n) devices with three p-n junctions, allowing them to act as both rectifiers and switches. Unlike transistors, Thyristors do not amplify signals but are primarily used for switching applications in power control circuits.

How do Thyristors switch between different states?

Thyristors have three operating states: Forward Blocking, Reverse Blocking, and Forward Conducting. They remain in the blocking state until triggered by the Gate current or until the voltage reaches a critical threshold. Once triggered, the Thyristor latches in the conducting state until the current drops below a certain threshold.

What is the significance of the 'latching' property of Thyristors?

The 'latching' property means that once a Thyristor is triggered and switched to the forward conduction state, it remains in that state even after the gate signal is removed. This allows it to continue conducting until the current falls below a certain level.

What are the types of Thyristors and how do they differ?

There are several types of Thyristors, including Silicon Controlled Rectifiers (SCR), Gate Turn-Off Thyristors (GTO), and Light Activated Silicon Controlled Rectifiers (LASCR). They differ in how they are triggered and switched off. For example, GTOs can be turned off by a gate signal, while SCRs require current to fall below a certain level to turn off.

How do Thyristors behave when in a forward biased state?

In the forward biased state, when the anode is positive, the Thyristor remains in the 'off' state (Forward Blocking mode) until triggered by the Gate current or until the forward voltage reaches a critical level, at which point it switches to the 'on' state (Forward Conducting).

What happens to a Thyristor in reverse biased mode?

When a reverse voltage is applied (with the anode negative relative to the cathode), the Thyristor remains off in the Reverse Blocking state, conducting only a small leakage current. If the reverse voltage exceeds the Reverse Breakdown Voltage, avalanche breakdown occurs, and the device begins to conduct in the reverse direction.

What are some common applications of Thyristors?

Thyristors are used in various applications such as light dimmers, variable speed motor drives, controlling high-power electrical systems, welding machines, fault current limiters, photography flashes, and even car ignition switches. They are also used in controlling the speed of electric fans.

What makes Thyristors ideal for high power applications?

Thyristors are ideal for high power applications because they can handle large currents and voltages, often exceeding 1 kV or 100 A. Their ability to switch quickly and their robustness in power conditioning circuits make them essential in high-energy systems.