Just what is a thyristor?
A thyristor is really a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure includes 4 levels of semiconductor components, including three PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These three poles are definitely the critical parts of 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 functioning status. Therefore, thyristors are commonly used in various electronic circuits, like controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of the semiconductor device is usually represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The functioning condition of 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 is utilized between the anode and cathode (the anode is connected to the favorable pole of the power supply, and the cathode is linked to the negative pole of the power supply). But no forward voltage is applied for the control pole (i.e., K is disconnected), and the indicator light will not illuminate. 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, and a forward voltage is applied for the control electrode (known as a trigger, and the applied voltage is known as trigger voltage), the indicator light switches on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is excited, whether or not the voltage in the control electrode is removed (that is certainly, K is excited again), the indicator light still glows. This shows that the thyristor can carry on and conduct. At the moment, so that you can 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 applied for the control electrode, a reverse voltage is applied between the anode and cathode, and the indicator light will not illuminate at this time. This shows that the thyristor is not conducting and may reverse blocking.
- To sum up
1) Once 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) Once the thyristor is exposed to a forward anode voltage, the thyristor will only conduct once the gate is exposed to 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) Once the thyristor is excited, so long as there is a specific forward anode voltage, the thyristor will stay excited no matter 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) Once the thyristor is on, and 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 should be applied between the anode and the cathode, and an appropriate forward voltage also need to be applied between the gate and the cathode. To transform off a conducting thyristor, the forward voltage between 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 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).
- If a forward voltage is applied between the anode and cathode of 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. If a forward voltage is applied for the control electrode at this time, BG1 is triggered to produce a base current Ig. BG1 amplifies this current, and 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 likely be introduced the collector of BG2. This current is delivered to BG1 for amplification and after that delivered 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 large current appears within the emitters of these two transistors, that is certainly, the anode and cathode of the thyristor (how big the current is actually dependant on how big the stress and how big Ea), and so the thyristor is totally excited. This conduction process is done in a really short period of time.
- After the thyristor is excited, its conductive state will likely be maintained through the positive feedback effect of the tube itself. Even if the forward voltage of the control electrode disappears, it is still within the conductive state. Therefore, the function of the control electrode is simply to trigger the thyristor to turn on. After the thyristor is excited, the control electrode loses its function.
- The only method to switch off the turned-on thyristor would be to decrease the anode current so that it is insufficient to keep up the positive feedback process. How you can decrease the anode current would be to cut off the forward power supply Ea or reverse the link of Ea. The minimum anode current required to maintain the thyristor within the conducting state is known as the holding current of the thyristor. Therefore, as it happens, so long as the anode current is lower than the holding current, the thyristor can be turned off.
What is the distinction between a transistor and a thyristor?
Transistors usually contain 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 upon electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor requires a forward voltage and a trigger current at the gate to turn 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 accomplish current amplification.
The thyristor is excited or off by managing the trigger voltage of the control electrode to realize the switching function.
The circuit parameters of thyristors are based on 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 functioning principles, they have got noticeable differences in performance and make use of occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be used in frequency converters, motor controllers, welding machines, power supplies, etc.
- Inside the lighting field, thyristors can be used in dimmers and light control devices.
- In induction cookers and electric water heaters, thyristors could 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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