What exactly is a thyristor?
A thyristor is actually a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure contains four quantities of semiconductor components, including 3 PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These 3 poles are the critical parts in the thyristor, letting it control current and perform high-frequency switching operations. Thyristors can operate under high voltage and high current conditions, and external signals can maintain their functioning status. Therefore, thyristors are commonly used in various electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of the silicon-controlled rectifier is usually represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The functioning condition in the thyristor is the fact each time a forward voltage is applied, the gate should have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage is utilized between the anode and cathode (the anode is linked to the favorable pole in the power supply, and also the cathode is linked to the negative pole in the power supply). But no forward voltage is applied for the control pole (i.e., K is disconnected), and also the indicator light will not glow. This demonstrates that the thyristor is not conducting and it has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, and a forward voltage is applied for the control electrode (referred to as a trigger, and also the applied voltage is known as trigger voltage), the indicator light switches on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is excited, even when the voltage around the control electrode is taken off (which is, K is excited again), the indicator light still glows. This demonstrates that the thyristor can continue to conduct. At the moment, in order to shut down the conductive thyristor, the power supply Ea must be shut down or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is applied for the control electrode, a reverse voltage is applied between the anode and cathode, and also the indicator light will not glow at this time. This demonstrates that the thyristor is not conducting and can reverse blocking.
- In summary
1) When the thyristor is put through a reverse anode voltage, the thyristor is at a reverse blocking state whatever voltage the gate is put through.
2) When the thyristor is put through a forward anode voltage, the thyristor will only conduct when the gate is put through a forward voltage. At the moment, the thyristor is in the forward conduction state, which is the thyristor characteristic, which is, the controllable characteristic.
3) When the thyristor is excited, as long as you will find a specific forward anode voltage, the thyristor will stay excited regardless of the gate voltage. That is, following the thyristor is excited, the gate will lose its function. The gate only serves as a trigger.
4) When the thyristor is on, and also the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The problem for that thyristor to conduct is the fact a forward voltage ought to be applied between the anode and also the cathode, as well as an appropriate forward voltage ought to be applied between the gate and also the cathode. To change off a conducting thyristor, the forward voltage between the anode and cathode must be shut down, or even the voltage must be reversed.
Working principle of thyristor
A thyristor is essentially an exclusive triode made from three PN junctions. It can be equivalently thought to be comprising a PNP transistor (BG2) as well as an NPN transistor (BG1).
- If a forward voltage is applied between the anode and cathode in the thyristor without applying a forward voltage for the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains turned off because BG1 has no base current. If a forward voltage is applied for the control electrode at this time, BG1 is triggered to create basics current Ig. BG1 amplifies this current, and a ß1Ig current is obtained in the collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current is going to be introduced the collector of BG2. This current is delivered to BG1 for amplification and after that delivered to BG2 for amplification again. Such repeated amplification forms an essential positive feedback, causing both BG1 and BG2 to enter a saturated conduction state quickly. A large current appears inside the emitters of these two transistors, which is, the anode and cathode in the thyristor (the size of the current is in fact based on the size of the stress and the size of Ea), so the thyristor is entirely excited. This conduction process is finished in a very short period of time.
- After the thyristor is excited, its conductive state is going to be maintained from the positive feedback effect in the tube itself. Even if the forward voltage in the control electrode disappears, it really is still inside the conductive state. Therefore, the purpose of the control electrode is only to trigger the thyristor to change on. Once the thyristor is excited, the control electrode loses its function.
- The best way to shut off the turned-on thyristor is always to reduce the anode current that it is inadequate to maintain the positive feedback process. The best way to reduce the anode current is always to shut down the forward power supply Ea or reverse the link of Ea. The minimum anode current necessary to maintain the thyristor inside the conducting state is known as the holding current in the thyristor. Therefore, strictly speaking, as long as the anode current is less than the holding current, the thyristor may be turned off.
What is the distinction between a transistor and a thyristor?
Transistors usually include a PNP or NPN structure made from three semiconductor materials.
The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The task of the transistor depends on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor demands a forward voltage and a trigger current in the gate to change on or off.
Transistors are commonly used in amplification, switches, oscillators, as well as other facets of electronic circuits.
Thyristors are mainly used in electronic circuits including controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.
Means of working
The transistor controls the collector current by holding the base current to accomplish current amplification.
The thyristor is excited or off by manipulating the trigger voltage in the control electrode to understand the switching function.
The circuit parameters of thyristors are based on stability and reliability and usually have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors can be utilized in similar applications sometimes, because of their different structures and functioning principles, they have noticeable variations in performance and use occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be utilized in frequency converters, motor controllers, welding machines, power supplies, etc.
- Within the lighting field, thyristors can be utilized in dimmers and lightweight control devices.
- In induction cookers and electric water heaters, thyristors may be used to control the current flow for the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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