Controlling AC Power with Thyristors Explained

The Thyristor

In our previous discussion on diodes, we explored how they can be used to convert alternating voltage into direct voltage using a half-wave rectifier or full bridge rectifier. However, it would be beneficial to have a controllable diode that can be turned on and off at will. Luckily, such a component exists and is known as a thyristor.

Internal Structure

A thyristor consists of four semiconductor layers with an anode and cathode terminal, as well as an additional gate terminal. This is in contrast to a diode, which has only two semiconductor layers.

Practical Example

We will be using the TYN604 thyristor as an example. Its pinout can be easily found in the datasheet. Let's start by applying a small 2.7V DC voltage to the gate terminal and connecting the positive voltage to the anode, with an LED connected between the cathode and ground.

Upon applying a positive voltage to the gate, we can see that the LED lights up but only for a short duration. This is because the thyristor usually stays in its conductive state even when the gate voltage disconnects. To turn off the thyristor, we need to interrupt the current flow using a component like a MOSFET.

Turn-Off Time

One of the important characteristics of a thyristor is its turn-off time, which is around 37 microseconds in this case. This means that even if we apply a fast-changing square wave signal to the gate terminal, the thyristor will stay conductive until this critical time is reached.

Power Loss and Temperature Increase

When using a larger load like a light bulb, we can see that the temperature of the thyristor increases drastically. This is due to the voltage drop between the anode and cathode, which creates a power loss.

Alternating Voltage

To work with alternating voltage, we will use an auto-transformer to step down the mains voltage to around 9V RMS. This is safer and allows us to demonstrate the concept without risking electrical shock.

Triac

To solve some of the problems that arise when working with alternating voltage, we can use a triac. A triac is essentially two thyristors in an inverse parallel configuration.

Phase Angle Control Circuit

To create a phase angle control circuit, we need to use a microcontroller like the Arduino Nano. We will create a theoretical schematic and then build the circuit using an optocoupler and a triac.

The final circuit allows us to control the power consumption of certain AC appliances by adjusting the phase angle. This kind of power regulation is also used in applications like speed control for universal motors.

Thyristor Electronics

Background:

Thyristor electronics is a branch of electronics that deals with the study and application of thyristors, which are semiconductor devices that can be used as switches or rectifiers. The term "thyristor" was coined in 1960 by a team of engineers at General Electric Company, who derived it from the Greek word "thyris," meaning "door." This refers to the fact that thyristors can act like an electrical door, allowing current to flow through them when they are triggered.

History:

The development of thyristor electronics began in the 1950s with the invention of the silicon controlled rectifier (SCR). The SCR was the first commercial thyristor device and was used primarily for power control applications. In the 1960s, other types of thyristors were developed, including the triac and the gate turn-off thyristor (GTO).

Principle:

Thyristors are semiconductor devices that have three or more terminals. They can be used as switches, rectifiers, or amplifiers. The most common type of thyristor is the SCR, which has three terminals: an anode, a cathode, and a gate. When a small voltage is applied to the gate, the device turns on and allows current to flow from the anode to the cathode.

Applications:

Thyristor electronics has a wide range of applications in power systems, including motor control, lighting control, and power supplies. Thyristors are also used in medical devices, such as defibrillators, and in transportation systems, such as traction drives.

Controlling AC Power with Thyristors Explained

Thyristors are semiconductor devices that are widely used for controlling the flow of electrical power in various applications. One of the most common uses of thyristors is in the control of AC power, where they are used to switch on and off the power supply to a load. In this article, we will explore how thyristors work and how they can be used to control AC power.

What is a Thyristor?

A thyristor is a type of semiconductor device that has three or more terminals. The most common type of thyristor is the silicon-controlled rectifier (SCR), which has three terminals: an anode, a cathode, and a gate. The SCR can be thought of as a switch that can be turned on and off by applying a small voltage to the gate terminal.

How Does a Thyristor Work?

The operation of an SCR can be explained as follows: when a small voltage is applied to the gate terminal, it creates a flow of current between the anode and cathode. This current flow continues until the voltage across the SCR drops below a certain threshold, at which point the SCR turns off. The SCR remains in this off state until another pulse of voltage is applied to the gate terminal.

Controlling AC Power with Thyristors

To control AC power using thyristors, a circuit is designed that uses one or more SCRs to switch on and off the power supply to a load. The most common type of circuit used for this purpose is called a "thyristor phase control" circuit.

Thyristor Phase Control Circuit

A thyristor phase control circuit uses one or more SCRs to switch on and off the power supply to a load. The circuit consists of an SCR, a load (such as a resistor or inductor), and a voltage source (such as an AC power supply). The SCR is connected in series with the load and the voltage source.

Working Principle

The working principle of a thyristor phase control circuit can be explained as follows: when the voltage across the SCR is above a certain threshold, it turns on and allows current to flow through the load. The current flow continues until the voltage across the SCR drops below the threshold, at which point the SCR turns off.

Advantages

Fast switching times: Thyristors can switch on and off rapidly, making them suitable for high-frequency applications.
High current handling capability: Thyristors can handle high currents, making them suitable for use in high-power applications.
Low power losses: Thyristors have low power losses compared to other types of switches, making them efficient for use in power control applications.

Disadvantages

Gate trigger current: Thyristors require a minimum gate trigger current to turn on, which can be a limitation in some applications.
Latching current: Once a thyristor has been turned on, it requires a certain level of latching current to remain in the on state. If this current is not provided, the thyristor will turn off.

Q1: What is a thyristor and how does it control AC power?	A thyristor is a type of semiconductor device that can be used as a switch to control the flow of electrical current. It can be used to control AC power by turning on and off at specific times to regulate the voltage and current.
Q2: What are the different types of thyristors?	There are several types of thyristors, including silicon controlled rectifiers (SCRs), triacs, and gate turn-off thyristors (GTOs). Each type has its own unique characteristics and applications.
Q3: How does a thyristor control AC power?	A thyristor controls AC power by turning on and off at specific times to regulate the voltage and current. When the thyristor is turned on, it allows current to flow through it, and when it is turned off, it blocks the current.
Q4: What are the advantages of using a thyristor to control AC power?	The advantages of using a thyristor to control AC power include high efficiency, fast switching times, and low losses. Thyristors can also be used to control high-power devices with minimal heat generation.
Q5: What are some common applications of thyristors in controlling AC power?	Thyristors are commonly used in applications such as lighting controls, motor speed control, and heating and cooling systems. They are also used in power supplies, rectifiers, and inverters.
Q6: How do I choose the right thyristor for my application?	To choose the right thyristor for your application, consider factors such as voltage rating, current rating, switching frequency, and power handling capability. You should also consider the type of load you are controlling and the level of control required.
Q7: What is the difference between a thyristor and a transistor?	A thyristor is a type of semiconductor device that can be used as a switch to control the flow of electrical current, while a transistor is a type of amplifier or switch that uses a small input signal to control a larger output signal.
Q8: Can thyristors be used in DC power applications?	Yes, thyristors can be used in DC power applications, but they are typically more suited for AC power control due to their ability to turn on and off at specific times.
Q9: What is the purpose of a gate signal in a thyristor?	The gate signal in a thyristor is used to trigger the device into conduction, allowing current to flow through it. The gate signal can be a voltage or current pulse.
Q10: Can thyristors be used in high-frequency applications?	Yes, some types of thyristors, such as GTOs and IGCTs, are designed for high-frequency applications and can switch on and off at frequencies up to several hundred kilohertz.

Rank	Pioneers/Companies	Contribution	Description
1	General Electric (GE)	Development of the first commercially available thyristor	In the 1950s, GE developed the first commercially available thyristor, which paved the way for modern power electronics.
2	Frank W. Gutzwiller (GE)	Invention of the silicon-controlled rectifier (SCR)	Gutzwiller's invention of the SCR in 1956 revolutionized power electronics and led to the development of modern thyristors.
3	International Rectifier (IRF)	Development of high-power thyristors	IRF's innovations in thyristor design and manufacturing enabled the creation of high-power devices capable of handling large currents.
4	ABB (ASEA Brown Boveri)	Development of advanced thyristor-based power electronics	ABB's work on high-performance thyristors and their applications in power systems contributed significantly to the advancement of modern power electronics.
5	Toshiba	Development of insulated gate bipolar transistors (IGBTs)	Toshiba's introduction of IGBTs in the 1980s marked a significant improvement in power electronics, offering high efficiency and reliability.
6	Hitachi	Development of high-power thyristor modules	Hitachi's innovations in thyristor module design enabled the creation of compact, high-power devices suitable for a wide range of applications.
7	Siemens	Development of advanced power electronics systems	Siemens' work on integrated power electronic systems, including thyristor-based solutions, has contributed significantly to the advancement of modern industrial and transportation applications.
8	Mitsubishi Electric	Development of high-reliability thyristors for industrial applications	Mitsubishi Electric's focus on developing highly reliable thyristors has enabled the creation of robust power electronic systems suitable for demanding industrial environments.
9	Fuji Electric	Development of compact, high-power thyristor devices	Fuji Electric's innovations in thyristor design have led to the creation of compact, high-power devices suitable for a wide range of applications, from industrial power supplies to medical equipment.
10	Infineon Technologies	Development of advanced thyristor-based solutions for automotive and industrial applications	Infineon's work on high-performance, highly reliable thyristors has enabled the creation of advanced power electronic systems suitable for demanding applications in the automotive and industrial sectors.

Introduction	Thyristors are semiconductor devices that can be used as switches to control AC power. They have three terminals: anode, cathode, and gate. The gate terminal is used to trigger the device into conduction.
How Thyristors Work	A thyristor can be thought of as a switch that can be turned on or off by applying a small voltage to the gate terminal. When the gate is triggered, the device begins to conduct and allows current to flow from the anode to the cathode.
Types of Thyristors	There are two main types of thyristors: Silicon-Controlled Rectifiers (SCRs) and Triacs. SCRs can only conduct in one direction, while triacs can conduct in both directions.
Thyristor Characteristics	Some key characteristics of thyristors include: Vgt (Gate Trigger Voltage): the voltage required to trigger the device into conduction. Ih (Holding Current): the minimum current required to keep the device in conduction. Id (On-State Current): the maximum current that can flow through the device when it is conducting.
Circuit Configurations	Thyristors can be used in a variety of circuit configurations to control AC power, including: : uses one thyristor to rectify half of the AC waveform. : uses two thyristors to rectify the entire AC waveform. : uses four thyristors to rectify the entire AC waveform.
Gate Drive Circuits	A gate drive circuit is required to provide the necessary voltage and current to trigger the thyristor into conduction. This can be achieved using a variety of circuits, including: : uses an RC network to filter out high-frequency noise. : uses a pulse transformer to provide isolation and step-up the gate voltage.
Protection Circuits	Thyristors require protection circuits to prevent damage from overcurrent, overheating, and dv/dt stress. This can be achieved using: : limits the current flowing through the device. : protects the device from high voltage transients. : monitors the device temperature and turns off the gate drive if it exceeds a certain threshold.
Applications	Thyristors are used in a wide range of applications, including: : controls the speed and torque of AC motors. : controls the intensity and color of lighting systems. : improves the power factor of AC loads.