So what is a thyristor?
A thyristor is a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure includes 4 levels of semiconductor materials, including 3 PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These 3 poles are definitely the critical parts from 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 different electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of the silicon-controlled rectifier is normally 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 light-controlled thyristors. The operating condition from the thyristor is that whenever 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 can be used between the anode and cathode (the anode is linked to the favorable pole from the power supply, and also the cathode is connected to the negative pole from the power supply). But no forward voltage is applied to the control pole (i.e., K is disconnected), and also 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, as well as a forward voltage is applied to the control electrode (known as a trigger, and also the applied voltage is called trigger voltage), the indicator light turns on. This means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is turned on, even when the voltage in the control electrode is taken off (that is certainly, K is turned on again), the indicator light still glows. This implies that the thyristor can still 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 will not glow currently. This implies that the thyristor is not really conducting and will reverse blocking.
- In summary
1) If the thyristor is put through a reverse anode voltage, the thyristor is at a reverse blocking state no matter what voltage the gate is put through.
2) If the thyristor is put through a forward anode voltage, the thyristor will simply 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, that is certainly, the controllable characteristic.
3) If the thyristor is turned on, as long as you will find a specific forward anode voltage, the thyristor will always be turned on no matter the gate voltage. Which is, following the thyristor is turned on, the gate will lose its function. The gate only works as a trigger.
4) If the thyristor is on, and also the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The condition for the thyristor to conduct is that a forward voltage needs to be applied between the anode and also the cathode, as well as 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 must be cut off, or even the voltage must be reversed.
Working principle of thyristor
A thyristor is actually a distinctive triode composed of three PN junctions. It could be equivalently thought to be comprising a PNP transistor (BG2) as well as an NPN transistor (BG1).
- In case a forward voltage is applied between the anode and cathode from the thyristor without applying a forward voltage to the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor is still switched off because BG1 has no base current. In case a forward voltage is applied to the control electrode currently, 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 will 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 an essential 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 from the thyristor (how big the current is actually determined by how big the stress and how big Ea), so the thyristor is entirely turned on. This conduction process is done in a very short time.
- After the thyristor is turned on, its conductive state will be maintained from the positive feedback effect from the tube itself. Whether or not the forward voltage from the control electrode disappears, it is still in the conductive state. Therefore, the function of the control electrode is simply to trigger the thyristor to change on. Once the thyristor is turned on, the control electrode loses its function.
- The only method to turn off the turned-on thyristor would be to decrease the anode current that it is insufficient to keep up the positive feedback process. The way to decrease the anode current would be to cut off the forward power supply Ea or reverse the link of Ea. The minimum anode current necessary to maintain the thyristor in the conducting state is called the holding current from the thyristor. Therefore, strictly speaking, as long as the anode current is under the holding current, the thyristor can be switched off.
Exactly what is the difference between a transistor as well as a thyristor?
Transistors usually contain 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 task of the transistor relies upon electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor needs a forward voltage as well as a trigger current in the gate to change on or off.
Transistors are commonly used in amplification, switches, oscillators, along with other elements of electronic circuits.
Thyristors are mostly 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 accomplish current amplification.
The thyristor is turned on or off by controlling the trigger voltage from the control electrode to comprehend 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 sum up, although transistors and thyristors can be utilized in similar applications sometimes, due to their different structures and operating principles, they may have noticeable differences 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.
- In the lighting field, thyristors can be utilized in dimmers and light control devices.
- In induction cookers and electric water heaters, thyristors could 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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