Just what is a thyristor?
A thyristor is actually a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure consists of four quantities of semiconductor materials, including three PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These three 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 working status. Therefore, thyristors are widely used in different electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of any Thyristor is usually represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-weight-controlled thyristors. The working condition in the thyristor is that each time 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 used 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 to the control pole (i.e., K is disconnected), and also the indicator light fails to light up. This demonstrates that the thyristor is not conducting and has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, and a forward voltage is applied to the control electrode (called a trigger, and also the applied voltage is referred to as trigger voltage), the indicator light switches on. This means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is excited, even if the voltage on the control electrode is removed (that is, K is excited again), the indicator light still glows. This demonstrates that the thyristor can carry on and conduct. At the moment, in order to cut off the conductive thyristor, the power supply Ea must be cut off or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is applied to the control electrode, a reverse voltage is applied between the anode and cathode, and also the indicator light fails to light up at the moment. This demonstrates that the thyristor is not conducting and may reverse blocking.
- In conclusion
1) When the thyristor is put through a reverse anode voltage, the thyristor is at a reverse blocking state regardless of what voltage the gate is put through.
2) When the thyristor is put through a forward anode voltage, the thyristor will only conduct once the gate is put through a forward voltage. At the moment, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, that is, the controllable characteristic.
3) When the thyristor is excited, as long as there exists a specific forward anode voltage, the thyristor will stay excited whatever the gate voltage. That is certainly, 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 seal to zero, the thyristor turns off.
5) The condition for your thyristor to conduct is that a forward voltage needs to be applied between the anode and also the cathode, plus an appropriate forward voltage ought to be applied between the gate and also the cathode. To turn off a conducting thyristor, the forward voltage between the anode and cathode must be cut off, or even the voltage must be reversed.
Working principle of thyristor
A thyristor is basically an exclusive triode composed of three PN junctions. It can be equivalently regarded as composed of a PNP transistor (BG2) plus an NPN transistor (BG1).
- If a forward voltage is applied between the anode and cathode in the thyristor without applying a forward voltage to the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor continues to be switched off because BG1 has no base current. If a forward voltage is applied to the control electrode at the moment, BG1 is triggered to produce basics current Ig. BG1 amplifies this current, and a ß1Ig current is obtained in its collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current will be introduced the collector of BG2. This current is brought to BG1 for amplification and then brought to BG2 for amplification again. Such repeated amplification forms a crucial positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A large current appears in the emitters of the two transistors, that is, the anode and cathode in the thyristor (how big the current is actually determined by how big the load and how big Ea), therefore the thyristor is completely excited. This conduction process is finished in a really limited time.
- Following the thyristor is excited, its conductive state will be maintained through the positive feedback effect in the tube itself. Whether or not the forward voltage in the control electrode disappears, it is actually still in the conductive state. Therefore, the function of the control electrode is only to trigger the thyristor to change on. After the thyristor is excited, the control electrode loses its function.
- The only way to switch off the turned-on thyristor is always to reduce the anode current that it is not enough to keep up the positive feedback process. The best way to reduce the anode current is always to cut off the forward power supply Ea or reverse the link of Ea. The minimum anode current necessary to keep your thyristor in the conducting state is referred to as the holding current in the thyristor. Therefore, as it happens, as long as the anode current is under the holding current, the thyristor may be switched off.
What is the distinction between a transistor and a thyristor?
Transistors usually consist of a PNP or NPN structure composed of three semiconductor materials.
The thyristor is made up of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of any transistor relies on electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor needs a forward voltage and a trigger current in the gate to change on or off.
Transistors are widely used in amplification, switches, oscillators, and other elements of electronic circuits.
Thyristors are mainly found in electronic circuits such as 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 achieve current amplification.
The thyristor is excited 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 often have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors can be utilized in similar applications in some instances, because of their different structures and working principles, they have noticeable differences in performance and make use of 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 light-weight control devices.
- In induction cookers and electric water heaters, thyristors may be used to control the current flow to the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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