What exactly is a thyristor?
A thyristor is actually a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure contains 4 levels of semiconductor components, including three PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These three poles are the critical parts of 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 widely used in various electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of the Thyristor is normally represented from 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-weight-controlled thyristors. The functioning condition of the thyristor is the fact whenever a forward voltage is used, the gate needs 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 linked to the favorable pole of the power supply, as well as the cathode is connected to the negative pole of the power supply). But no forward voltage is used for the control pole (i.e., K is disconnected), as well as the indicator light fails to illuminate. This shows that the thyristor will not be conducting and contains forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, and a forward voltage is used for the control electrode (called a trigger, as well as 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, after the thyristor is turned on, even when the voltage around the control electrode is removed (which is, K is turned on again), the indicator light still glows. This shows that the thyristor can still conduct. Currently, in order to shut down the conductive thyristor, the power supply Ea should be shut down or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is used for the control electrode, a reverse voltage is used involving the anode and cathode, as well as the indicator light fails to illuminate currently. This shows that the thyristor will not be conducting and can reverse blocking.
- To sum up
1) If the thyristor is subjected to a reverse anode voltage, the thyristor is within a reverse blocking state no matter what voltage the gate is subjected to.
2) If the thyristor is subjected to a forward anode voltage, the thyristor is only going to conduct when the gate is subjected to a forward voltage. Currently, the thyristor is in the forward conduction state, which is the thyristor characteristic, which is, the controllable characteristic.
3) If the thyristor is turned on, as long as there is a specific forward anode voltage, the thyristor will stay turned on regardless of the gate voltage. That is certainly, after the thyristor is turned on, the gate will lose its function. The gate only works as a trigger.
4) If the thyristor is on, as well as the primary circuit voltage (or current) decreases to seal to zero, the thyristor turns off.
5) The condition for the thyristor to conduct is the fact a forward voltage needs to be applied involving the anode as well as the cathode, as well as an appropriate forward voltage should also be applied involving the gate as well as the cathode. To transform off a conducting thyristor, the forward voltage involving the anode and cathode should be shut down, or perhaps the voltage should be reversed.
Working principle of thyristor
A thyristor is basically a unique triode made from three PN junctions. It may be equivalently thought to be consisting of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- When a forward voltage is used involving 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 switched off because BG1 has no base current. When a forward voltage is used for the control electrode currently, 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 will likely be brought in the collector of BG2. This current is brought to BG1 for amplification and then brought 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 of the thyristor (the dimensions of the current is actually determined by the dimensions of the stress and the dimensions of Ea), and so the thyristor is totally turned on. This conduction process is done in a very short period of time.
- Right after the thyristor is turned on, its conductive state will likely be maintained from the positive feedback effect of the tube itself. Even if the forward voltage of the control electrode disappears, it is actually still inside the conductive state. Therefore, the function of the control electrode is just to trigger the thyristor to turn on. When the thyristor is turned on, the control electrode loses its function.
- The best way to turn off the turned-on thyristor is always to lessen the anode current so that it is inadequate to keep up 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 required to maintain the thyristor inside the conducting state is known as the holding current of the thyristor. Therefore, as it happens, as long as the anode current is under the holding current, the thyristor could be switched off.
Exactly what is the difference between a transistor and a thyristor?
Transistors usually contain a PNP or NPN structure made from three semiconductor materials.
The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The task of the transistor depends on electrical signals to control its opening and closing, allowing fast switching operations.
The thyristor demands a forward voltage and a trigger current in the gate to turn on or off.
Transistors are widely used in amplification, switches, oscillators, along with other facets of electronic circuits.
Thyristors are mainly utilized 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 turned on or off by managing the trigger voltage of the control electrode to realize the switching function.
The circuit parameters of thyristors are related to 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 in some cases, due to their different structures and functioning 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 may be used to control the current flow for the heating element.
- In electric vehicles, transistors can be utilized in motor controllers.
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