What is a thyristor?
A thyristor is a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure contains four levels of semiconductor elements, including 3 PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These 3 poles are definitely the critical parts in the thyristor, allowing it to 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 a variety of electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of the Thyristor is normally represented through 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 light-controlled thyristors. The functioning condition in the thyristor is the fact that whenever a forward voltage is applied, the gate will need to 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 attached to the favorable pole in the power supply, and also the cathode is connected to the negative pole in the power supply). But no forward voltage is applied towards the control pole (i.e., K is disconnected), and also the indicator light will not illuminate. This implies that the thyristor will not be conducting and has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, as well as a forward voltage is applied towards the control electrode (called a trigger, and also the applied voltage is called trigger voltage), the indicator light switches on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, after the thyristor is switched on, whether or not the voltage on the control electrode is removed (that is certainly, K is switched on again), the indicator light still glows. This implies that the thyristor can continue to conduct. Currently, to be able to cut off the conductive thyristor, the power supply Ea has to be cut off or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is applied towards the control electrode, a reverse voltage is applied between the anode and cathode, and also the indicator light will not illuminate at this time. This implies that the thyristor will not be conducting and will reverse blocking.
- To sum up
1) When the thyristor is subjected to a reverse anode voltage, the thyristor is within a reverse blocking state no matter what voltage the gate is subjected to.
2) When the thyristor is subjected to a forward anode voltage, the thyristor will only conduct if the gate is subjected to a forward voltage. Currently, the thyristor is in the forward conduction state, which is the thyristor characteristic, that is certainly, the controllable characteristic.
3) When the thyristor is switched on, provided that you will find a specific forward anode voltage, the thyristor will always be switched on no matter the gate voltage. That is, after the thyristor is switched on, the gate will lose its function. The gate only functions as a trigger.
4) When the thyristor is on, and also the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The problem for the thyristor to conduct is the fact that a forward voltage ought to be applied between the anode and also the cathode, and an appropriate forward voltage also need to be applied between the gate and also the cathode. To turn off a conducting thyristor, the forward voltage between the anode and cathode has to be cut off, or even the voltage has to be reversed.
Working principle of thyristor
A thyristor is essentially an exclusive triode made up of three PN junctions. It can be equivalently viewed as composed of a PNP transistor (BG2) and an NPN transistor (BG1).
- When a forward voltage is applied between the anode and cathode in the thyristor without applying a forward voltage towards the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains turned off because BG1 has no base current. When a forward voltage is applied towards the control electrode at this time, BG1 is triggered to create basics current Ig. BG1 amplifies this current, as well as a ß1Ig current is obtained in its collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current is going to be brought in the collector of BG2. This current is sent to BG1 for amplification and then sent to BG2 for amplification again. Such repeated amplification forms a vital positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A sizable current appears in the emitters of these two transistors, that is certainly, the anode and cathode in the thyristor (how big the current is really determined by how big the burden and how big Ea), therefore the thyristor is totally switched on. This conduction process is done in a very limited time.
- Following the thyristor is switched on, its conductive state is going to be maintained through the positive feedback effect in the tube itself. Even if the forward voltage in the control electrode disappears, it is still in the conductive state. Therefore, the purpose of the control electrode is only to trigger the thyristor to transform on. When the thyristor is switched on, the control electrode loses its function.
- The only way to switch off the turned-on thyristor is to decrease the anode current so that it is not enough to keep the positive feedback process. The way to decrease the anode current is to cut off the forward power supply Ea or reverse the connection of Ea. The minimum anode current necessary to keep your thyristor in the conducting state is called the holding current in the thyristor. Therefore, strictly speaking, provided that the anode current is less than the holding current, the thyristor could be turned off.
What exactly is the distinction between a transistor as well as a thyristor?
Transistors usually consist of a PNP or NPN structure made up of three semiconductor materials.
The thyristor is composed of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of the transistor relies on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor requires a forward voltage as well as a trigger current on the gate to transform on or off.
Transistors are commonly used in amplification, switches, oscillators, along with other elements of electronic circuits.
Thyristors are mainly used in electronic circuits like controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.
Way of working
The transistor controls the collector current by holding the base current to attain current amplification.
The thyristor is switched on or off by managing the trigger voltage in the control electrode to comprehend the switching function.
The circuit parameters of thyristors are related to stability and reliability and in most cases have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors may be used in similar applications in some instances, because of the different structures and functioning principles, they may have noticeable differences in performance and make use of occasions.
Application scope of thyristor
- In power electronic equipment, thyristors may be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- In the lighting field, thyristors may be used in dimmers and light control devices.
- In induction cookers and electric water heaters, thyristors could be used to control the current flow towards the heating element.
- In electric vehicles, transistors may be used in motor controllers.
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