So what is a thyristor?
A thyristor is a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure contains four quantities of semiconductor elements, 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 popular in a variety of electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of the Thyristor is normally represented by the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The working 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 is utilized involving the anode and cathode (the anode is connected to the favorable pole in the power supply, and the cathode is linked to the negative pole in the power supply). But no forward voltage is used to the control pole (i.e., K is disconnected), and the indicator light does not light up. This shows 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, and a forward voltage is used to the control electrode (known as a trigger, and 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, following the thyristor is excited, even when the voltage around the control electrode is removed (that is certainly, K is excited again), the indicator light still glows. This shows that the thyristor can still conduct. At the moment, in order to cut off the conductive thyristor, the power supply Ea has to be cut off or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is used to the control electrode, a reverse voltage is used involving the anode and cathode, and the indicator light does not light up at the moment. This shows 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 within a reverse blocking state whatever voltage the gate is exposed to.
2) When the thyristor is exposed to a forward anode voltage, the thyristor will only conduct if the gate is exposed to a forward voltage. At the moment, the thyristor is incorporated in the forward conduction state, the thyristor characteristic, that is certainly, the controllable characteristic.
3) When the thyristor is excited, so long as you will find a specific forward anode voltage, the thyristor will always be excited whatever the gate voltage. That is certainly, following the thyristor is excited, the gate will lose its function. The gate only works as a trigger.
4) When the thyristor is on, and the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The problem for the thyristor to conduct is that a forward voltage should be applied involving the anode and the cathode, as well as an appropriate forward voltage ought to be applied involving the gate and the cathode. To change off a conducting thyristor, the forward voltage involving the anode and cathode has to be cut off, or perhaps the voltage has to be reversed.
Working principle of thyristor
A thyristor is actually a distinctive triode made from three PN junctions. It could be equivalently regarded as composed of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- When a forward voltage is used 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 is still turned off because BG1 has no base current. When a forward voltage is used to the control electrode at the moment, BG1 is triggered to generate a base 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 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 crucial positive feedback, causing both BG1 and BG2 to get into a saturated conduction state quickly. A big current appears within the emitters of these two transistors, that is certainly, the anode and cathode in the thyristor (the size of the current is really determined by the size of the burden and the size of Ea), and so the thyristor is totally excited. This conduction process is done in a very limited time.
- Right after the thyristor is excited, its conductive state is going to be maintained by the positive feedback effect in the tube itself. Even when the forward voltage in the control electrode disappears, it is still within the conductive state. Therefore, the purpose of the control electrode is only to trigger the thyristor to turn on. After the thyristor is excited, the control electrode loses its function.
- The best way to turn off the turned-on thyristor would be to lessen the anode current that it is inadequate to keep up the positive feedback process. How you can lessen the anode current would be to cut off the forward power supply Ea or reverse the connection of Ea. The minimum anode current required to maintain the thyristor within the conducting state is known as the holding current in the thyristor. Therefore, strictly speaking, so long as the anode current is less than the holding current, the thyristor could be turned off.
What exactly is the distinction between a transistor and a thyristor?
Transistors usually include a PNP or NPN structure made from three semiconductor materials.
The thyristor is made up of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of the transistor relies on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor needs a forward voltage and a trigger current in the gate to turn on or off.
Transistors are popular in amplification, switches, oscillators, along with other facets of electronic circuits.
Thyristors are mainly found in electronic circuits including 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 excited 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 in most cases have higher turn-off voltage and larger on-current.
To sum up, although transistors and thyristors may be used in similar applications in some cases, because of their different structures and working principles, they have noticeable variations in performance and use occasions.
Application scope of thyristor
- In power electronic equipment, thyristors may be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- In the lighting field, thyristors may be used in dimmers and lightweight 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 may be used in motor controllers.
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