Just what is a thyristor?
A thyristor is a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure contains 4 levels of semiconductor materials, including three PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These three poles are the critical parts from 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 working status. Therefore, thyristors are popular in a variety of electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of a silicon-controlled rectifier is normally represented by the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The working condition from the thyristor is that each time a forward voltage is used, 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 connected to the favorable pole from the power supply, as well as the cathode is attached to the negative pole from the power supply). But no forward voltage is used towards the control pole (i.e., K is disconnected), as well as the indicator light will not glow. This implies that the thyristor is not really conducting and has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, along with a forward voltage is used towards the control electrode (known as a trigger, as well as the applied voltage is known as trigger voltage), the indicator light switches on. Because of this the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is excited, even if the voltage around the control electrode is taken away (which is, K is excited again), the indicator light still glows. This implies that the thyristor can continue to conduct. At the moment, in order to cut off the conductive thyristor, the power supply Ea should be cut off or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is used towards the control electrode, a reverse voltage is used between the anode and cathode, as well as the indicator light will not glow at this time. This implies that the thyristor is not really conducting and will reverse blocking.
- In summary
1) When the thyristor is exposed to a reverse anode voltage, the thyristor is in a reverse blocking state regardless of what voltage the gate is exposed to.
2) When the thyristor is exposed to a forward anode voltage, the thyristor will simply conduct if the gate is exposed to a forward voltage. At the moment, the thyristor is within the forward conduction state, which is the thyristor characteristic, which is, the controllable characteristic.
3) When the thyristor is excited, provided that you will find a specific forward anode voltage, the thyristor will remain excited regardless of the gate voltage. That is, right after the thyristor is excited, the gate will lose its function. The gate only serves as a trigger.
4) When the thyristor is on, as well as the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The disorder for the thyristor to conduct is that a forward voltage should be applied between the anode as well as the cathode, plus an appropriate forward voltage also need to be applied between the gate as well as the cathode. To turn off a conducting thyristor, the forward voltage between the anode and cathode should be cut off, or the voltage should be reversed.
Working principle of thyristor
A thyristor is essentially a unique triode made up of three PN junctions. It can be equivalently thought to be composed of a PNP transistor (BG2) plus an NPN transistor (BG1).
- In case a forward voltage is used between the anode and cathode from 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. In case a forward voltage is used towards the control electrode at this time, BG1 is triggered to create a base current Ig. BG1 amplifies this current, along with a ß1Ig current is obtained in their 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 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 into a saturated conduction state quickly. A large current appears within the emitters of these two transistors, which is, the anode and cathode from the thyristor (the size of the current is really based on the size of the load and the size of Ea), therefore the thyristor is entirely excited. This conduction process is completed in a really short period of time.
- Following the thyristor is excited, its conductive state is going to be maintained by the positive feedback effect from the tube itself. Whether or not the forward voltage from the control electrode disappears, it really is still within the conductive state. Therefore, the purpose of the control electrode is just to trigger the thyristor to transform on. When the thyristor is excited, the control electrode loses its function.
- The only method to shut off the turned-on thyristor is to decrease the anode current so that it is insufficient to keep up the positive feedback process. How you can decrease the anode current is to cut off the forward power supply Ea or reverse the link of Ea. The minimum anode current necessary to maintain the thyristor within the conducting state is known as the holding current from the thyristor. Therefore, strictly speaking, provided that the anode current is lower than the holding current, the thyristor can be turned off.
What is the distinction between a transistor along with a thyristor?
Transistors usually consist of a PNP or NPN structure made up of three semiconductor materials.
The thyristor is made up of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The task of a transistor relies upon electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor requires a forward voltage along with a trigger current at the gate to transform on or off.
Transistors are popular in amplification, switches, oscillators, and other aspects of electronic circuits.
Thyristors are mainly utilized in electronic circuits such as controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.
Method of working
The transistor controls the collector current by holding the base current to achieve current amplification.
The thyristor is excited or off by controlling the trigger voltage from 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 cases, because of the different structures and working principles, they have noticeable variations in performance and utilize occasions.
Application scope of thyristor
- In power electronic equipment, thyristors may be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- Within the lighting field, thyristors may be used in dimmers and lightweight control devices.
- In induction cookers and electric water heaters, thyristors can 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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