So what is a thyristor?
A thyristor is really a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure contains 4 quantities of semiconductor elements, including 3 PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These 3 poles are 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 popular in a variety of electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of a semiconductor device is generally represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-weight-controlled thyristors. The functioning condition in the thyristor is the fact 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 involving 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 to the control pole (i.e., K is disconnected), and also the indicator light fails to illuminate. This implies that the thyristor is not 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 applied to the control electrode (known as a trigger, and also the applied voltage is known as trigger voltage), the indicator light turns on. This means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is excited, even if the voltage in the control electrode is removed (which is, K is excited again), the indicator light still glows. This implies that the thyristor can still conduct. At the moment, in order to shut down the conductive thyristor, the power supply Ea must be shut down 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 involving the anode and cathode, and also the indicator light fails to illuminate at the moment. This implies that the thyristor is not conducting and can reverse blocking.
- In summary
1) Once the thyristor is exposed to a reverse anode voltage, the thyristor is at a reverse blocking state whatever voltage the gate is exposed to.
2) Once the thyristor is exposed to a forward anode voltage, the thyristor is only going to conduct if the gate is exposed to a forward voltage. At the moment, the thyristor is in the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) Once the thyristor is excited, provided that there is a specific forward anode voltage, the thyristor will remain excited regardless of the gate voltage. Which is, right after the thyristor is excited, the gate will lose its function. The gate only works as a trigger.
4) Once the thyristor is on, and also the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The disorder for that thyristor to conduct is the fact a forward voltage needs to be applied involving the anode and also the cathode, as well as an appropriate forward voltage ought to be applied involving the gate and also the cathode. To turn off a conducting thyristor, the forward voltage involving the anode and cathode must be shut down, or perhaps the voltage must be reversed.
Working principle of thyristor
A thyristor is basically a distinctive triode composed of three PN junctions. It can be equivalently viewed as comprising a PNP transistor (BG2) as well as an NPN transistor (BG1).
- In case a forward voltage is applied involving 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 remains turned off because BG1 has no base current. In case a forward voltage is applied to the control electrode at the moment, BG1 is triggered to produce basics 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 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 a crucial positive feedback, causing both BG1 and BG2 to get in a saturated conduction state quickly. A large current appears inside the emitters of these two transistors, which is, the anode and cathode in the thyristor (the size of the current is really dependant on the size of the stress and the size of Ea), and so the thyristor is completely excited. This conduction process is completed in a really limited time.
- After 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 still inside the conductive state. Therefore, the function of the control electrode is only to trigger the thyristor to turn on. Once the thyristor is excited, the control electrode loses its function.
- The only method to turn off the turned-on thyristor is always to lessen the anode current that it is not enough to maintain the positive feedback process. The way to lessen the anode current is always to shut down the forward power supply Ea or reverse the connection of Ea. The minimum anode current necessary to maintain the thyristor inside the conducting state is known as the holding current in the thyristor. Therefore, strictly speaking, provided that the anode current is less than the holding current, the thyristor may be turned off.
What exactly is the difference between a transistor along with a thyristor?
Transistors usually contain a PNP or NPN structure composed of three semiconductor materials.
The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The task of a transistor depends on electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor needs a forward voltage along with a trigger current in the gate to turn on or off.
Transistors are popular in amplification, switches, oscillators, as well as other elements of electronic circuits.
Thyristors are mostly used 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 attain current amplification.
The thyristor is excited or off by controlling the trigger voltage in the control electrode to comprehend the switching function.
The circuit parameters of thyristors are based on stability and reliability and in most cases have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors can be utilized in similar applications in some cases, due to their different structures and functioning principles, they have got noticeable differences in performance and utilize 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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