What exactly is a thyristor?
A thyristor is really a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure contains 4 quantities of semiconductor elements, including three PN junctions corresponding for 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 operating status. Therefore, thyristors are commonly used in various electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of a Thyristor is normally represented by 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 operating condition in the thyristor is that when 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 can be used in between the anode and cathode (the anode is linked to the favorable pole in the power supply, as well as the cathode is linked to the negative pole in the power supply). But no forward voltage is used for the control pole (i.e., K is disconnected), as well as the indicator light does not illuminate. This demonstrates that the thyristor will not be conducting and it has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, along with a forward voltage is used for the control electrode (known as a trigger, as well as the applied voltage is called trigger voltage), the indicator light turns on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is switched on, even if the voltage on the control electrode is removed (which is, K is switched on again), the indicator light still glows. This demonstrates that the thyristor can still conduct. Currently, so that you can shut down the conductive thyristor, the power supply Ea should be shut down or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is used for the control electrode, a reverse voltage is used in between the anode and cathode, as well as the indicator light does not illuminate at the moment. This demonstrates that the thyristor will not be conducting and will reverse blocking.
- To sum up
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 once the gate is exposed to a forward voltage. Currently, the thyristor is within the forward conduction state, which is the thyristor characteristic, which is, 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 whatever the gate voltage. That is certainly, right after the thyristor is switched on, 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 condition for that thyristor to conduct is that a forward voltage needs to be applied in between the anode as well as the cathode, plus an appropriate forward voltage also need to be applied in between the gate as well as the cathode. To turn off a conducting thyristor, the forward voltage in between the anode and cathode should be shut down, or perhaps the voltage should be reversed.
Working principle of thyristor
A thyristor is basically a distinctive triode made up of three PN junctions. It could be equivalently thought to be consisting of a PNP transistor (BG2) plus an NPN transistor (BG1).
- When a forward voltage is used in 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 is still turned off because BG1 has no base current. When a forward voltage is used for the control electrode at the moment, BG1 is triggered to create basics current Ig. BG1 amplifies this current, along with a ß1Ig current is obtained in the collector. This current is precisely the base current of BG2. After amplification by BG2, a ß1ß2Ig current will likely be brought in 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 enter a saturated conduction state quickly. A sizable current appears in the emitters of these two transistors, which is, the anode and cathode in the thyristor (the size of the current is in fact dependant on the size of the load and the size of Ea), and so the thyristor is entirely switched on. This conduction process is done in a very short period of time.
- After the thyristor is switched on, its conductive state will likely be maintained by the positive feedback effect in the tube itself. Whether or not the forward voltage in the control electrode disappears, it is still in the conductive state. Therefore, the function of the control electrode is just 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 always to lessen the anode current that it is insufficient to maintain the positive feedback process. How you can lessen the anode current is always to shut down the forward power supply Ea or reverse the bond of Ea. The minimum anode current required 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 lower than the holding current, the thyristor could be turned off.
What exactly is the difference 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 composed of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The job of a transistor depends on 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 commonly used in amplification, switches, oscillators, as well as other elements of electronic circuits.
Thyristors are mainly utilized in electronic circuits like 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 switched on or off by managing the trigger voltage in the control electrode to understand 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 used in similar applications sometimes, due to their different structures and operating principles, they may have noticeable variations in performance and utilize occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- In the lighting field, thyristors can be used in dimmers and light-weight control devices.
- In induction cookers and electric water heaters, thyristors can be used to control the current flow for the heating element.
- In electric vehicles, transistors can be used in motor controllers.
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