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 four quantities of semiconductor elements, including 3 PN junctions corresponding towards the Anode, Cathode, and control electrode Gate. These 3 poles are the critical parts from the thyristor, letting it 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 commonly used 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 usually represented by the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors also have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-weight-controlled thyristors. The working condition from the thyristor is the fact that when a forward voltage is used, the gate should have a trigger current.
Characteristics of thyristor
- Forward blocking
As shown in Figure a above, when an ahead voltage is utilized in between the anode and cathode (the anode is linked to the favorable pole from the power supply, as well as the cathode is attached to the negative pole from the power supply). But no forward voltage is used towards the control pole (i.e., K is disconnected), as well as the indicator light will not light up. This shows that the thyristor is not conducting and contains forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, along with a forward voltage is used towards the control electrode (referred to as a trigger, as well as 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, after the thyristor is excited, even if the voltage on the control electrode is taken away (that is certainly, K is excited again), the indicator light still glows. This shows that the thyristor can carry on and conduct. At this time, 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 towards the control electrode, a reverse voltage is used in between the anode and cathode, as well as the indicator light will not light up currently. This shows that the thyristor is not conducting and may reverse blocking.
- To sum up
1) When the thyristor is put through a reverse anode voltage, the thyristor is in 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 when the gate is put through a forward voltage. At this time, the thyristor is within the forward conduction state, which is the thyristor characteristic, that is certainly, the controllable characteristic.
3) When the thyristor is excited, so long as there is a specific forward anode voltage, the thyristor will remain excited whatever the gate voltage. That is, after the thyristor is excited, 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 seal to zero, the thyristor turns off.
5) The problem for that thyristor to conduct is the fact that a forward voltage needs to be applied in between the anode as well as the cathode, and an appropriate forward voltage also need to be applied in between the gate as well as the cathode. To change off a conducting thyristor, the forward voltage in between the anode and cathode has to be cut off, or even the voltage has to be reversed.
Working principle of thyristor
A thyristor is actually an exclusive triode made up of three PN junctions. It may be equivalently viewed as composed of a PNP transistor (BG2) and an NPN transistor (BG1).
- In case a forward voltage is used in between the anode and cathode from the thyristor without applying a forward voltage towards the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor is still turned off because BG1 has no base current. In case a forward voltage is used towards the control electrode currently, BG1 is triggered to create a base 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 delivered to BG1 for amplification then delivered to BG2 for amplification again. Such repeated amplification forms a vital positive feedback, causing both BG1 and BG2 to enter a saturated conduction state quickly. A sizable current appears inside the emitters of the two transistors, that is certainly, the anode and cathode from the thyristor (the size of the current is in fact determined by the size of the stress and the size of Ea), and so the thyristor is entirely excited. This conduction process is completed in a very limited time.
- Right after the thyristor is excited, its conductive state will be maintained by the positive feedback effect from the tube itself. Even when the forward voltage from the control electrode disappears, it is actually still inside the conductive state. Therefore, the purpose of the control electrode is simply to trigger the thyristor to turn on. When the thyristor is excited, the control electrode loses its function.
- The only way to switch off the turned-on thyristor would be to lessen the anode current that it is insufficient to keep 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 needed to maintain the thyristor inside the conducting state is referred to as the holding current from the thyristor. Therefore, strictly speaking, so long as the anode current is less than the holding current, the thyristor can be turned off.
Exactly what is the distinction between a transistor along with a thyristor?
Transistors usually include a PNP or NPN structure made up 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 on electrical signals to control its closing and opening, 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 commonly used in amplification, switches, oscillators, along with other elements of electronic circuits.
Thyristors are mostly used in electronic circuits including controlled rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversions.
Method 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 from the control electrode to understand 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 instances, due to their different structures and working principles, they have got 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.
- Inside the lighting field, thyristors can be utilized in dimmers and light-weight control devices.
- In induction cookers and electric water heaters, thyristors could be used to control the current flow towards the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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