Just what is a thyristor?
A thyristor is a high-power semiconductor device, also known as a silicon-controlled rectifier. Its structure consists of four quantities of semiconductor components, including 3 PN junctions corresponding to 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 functioning status. Therefore, thyristors are commonly used in different electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of the silicon-controlled rectifier 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 from the thyristor is that each time 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 can be used between the anode and cathode (the anode is connected to the favorable pole from the power supply, as well as the cathode is linked to the negative pole from the power supply). But no forward voltage is used to the control pole (i.e., K is disconnected), as well as the indicator light does not light up. This demonstrates that the thyristor will not be conducting and 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, as well as the applied voltage is known as trigger voltage), the indicator light turns on. Which 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 around the control electrode is taken away (that is, K is excited again), the indicator light still glows. This demonstrates that the thyristor can continue to conduct. Currently, in order to stop the conductive thyristor, the power supply Ea must be stop 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 between the anode and cathode, as well as the indicator light does not light up currently. This demonstrates that the thyristor will not be conducting and may reverse blocking.
- To sum up
1) If the thyristor is subjected to a reverse anode voltage, the thyristor is at 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 will simply conduct when the gate is subjected to a forward voltage. Currently, the thyristor is within the forward conduction state, the thyristor characteristic, that is, the controllable characteristic.
3) If 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. Which is, right after the thyristor is excited, the gate will lose its function. The gate only functions 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 disorder for the thyristor to conduct is that a forward voltage ought to be applied between the anode as well as the cathode, as well as an appropriate forward voltage should also be applied between the gate as well as the cathode. To change off a conducting thyristor, the forward voltage between the anode and cathode must be stop, or perhaps the voltage must be reversed.
Working principle of thyristor
A thyristor is essentially a distinctive triode made from three PN junctions. It can be equivalently regarded as consisting of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- If a forward voltage is used between the anode and cathode from 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. If a forward voltage is used to the control electrode currently, BG1 is triggered to produce basics current Ig. BG1 amplifies this current, and a ß1Ig current is obtained in the 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 sent to BG1 for amplification then sent to BG2 for amplification again. Such repeated amplification forms an essential positive feedback, causing both BG1 and BG2 to get into a saturated conduction state quickly. A big current appears in the emitters of the two transistors, that is, the anode and cathode from the thyristor (the dimensions of the current is really determined by the dimensions of the stress and the dimensions of Ea), therefore the thyristor is entirely excited. This conduction process is done in a really short time.
- Right after the thyristor is excited, its conductive state will likely be maintained from the positive feedback effect from the tube itself. Even when the forward voltage from the control electrode disappears, it is actually still in the conductive state. Therefore, the purpose 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 switch off the turned-on thyristor is to reduce the anode current that it is inadequate to keep the positive feedback process. The best way to reduce the anode current is to stop the forward power supply Ea or reverse the link of Ea. The minimum anode current needed to keep the thyristor in the conducting state is known as the holding current from the thyristor. Therefore, strictly speaking, so long as the anode current is less than the holding current, the thyristor may be turned off.
What exactly is the distinction between a transistor and a thyristor?
Transistors usually consist of 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 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 on the gate to turn on or off.
Transistors are commonly used in amplification, switches, oscillators, as well as other aspects of electronic circuits.
Thyristors are mainly utilized 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 achieve current amplification.
The thyristor is excited or off by controlling the trigger voltage from the control electrode to realize the switching function.
The circuit parameters of thyristors are related to stability and reliability and often have higher turn-off voltage and larger on-current.
To summarize, although transistors and thyristors can be utilized in similar applications sometimes, because of their different structures and functioning principles, they have 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 can 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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