So what is a thyristor?
A thyristor is actually a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure includes 4 quantities of semiconductor materials, including three PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These three poles are the critical parts in 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 popular in various electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency alteration.
The graphical symbol of any semiconductor device is usually represented from the text symbol “V” or “VT” (in older standards, the letters “SCR”). Additionally, derivatives of thyristors include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-weight-controlled thyristors. The functioning condition in the thyristor is the fact that whenever a forward voltage is applied, 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 between the anode and cathode (the anode is linked to the favorable pole in the power supply, as well as the cathode is connected to the negative pole in the power supply). But no forward voltage is applied to the control pole (i.e., K is disconnected), as well as the indicator light fails to glow. This demonstrates that the thyristor is not conducting and it has forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, and a forward voltage is applied to the control electrode (called a trigger, as well as the applied voltage is called 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 switched on, even when the voltage in the control electrode is removed (that is certainly, K is switched on again), the indicator light still glows. This demonstrates that the thyristor can continue to conduct. At this time, so that you can shut down the conductive thyristor, the power supply Ea must be shut down or reversed.
- Reverse blocking
As shown in Figure d above, although a forward voltage is applied to the control electrode, a reverse voltage is applied between the anode and cathode, as well as the indicator light fails to glow at this time. This demonstrates that the thyristor is not conducting and may reverse blocking.
- To sum up
1) If the thyristor is subjected to a reverse anode voltage, the thyristor is within a reverse blocking state whatever voltage the gate is subjected to.
2) If the thyristor is subjected to a forward anode voltage, the thyristor will only conduct once the gate is subjected to a forward voltage. At this time, the thyristor is within the forward conduction state, the thyristor characteristic, that is certainly, the controllable characteristic.
3) If the thyristor is switched on, so long as there is a specific forward anode voltage, the thyristor will remain switched on whatever the gate voltage. That is certainly, after the thyristor is switched on, 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 problem for your thyristor to conduct is the fact that a forward voltage needs to be applied between the anode as well as the cathode, as well as an appropriate forward voltage also need to be applied between the gate as well as the cathode. To transform off a conducting thyristor, the forward voltage between the anode and cathode must be shut down, or even the voltage must be reversed.
Working principle of thyristor
A thyristor is actually a unique triode made from three PN junctions. It could be equivalently viewed as composed of a PNP transistor (BG2) as well as an NPN transistor (BG1).
- In case a forward voltage is applied between 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 continues to be turned off because BG1 has no base current. In case a forward voltage is applied to the control electrode at this time, BG1 is triggered to create basics 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 get in 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), therefore the thyristor is totally switched on. This conduction process is completed in an exceedingly limited time.
- Following the thyristor is switched on, its conductive state will likely be maintained from the positive feedback effect in the tube itself. Even when the forward voltage in the control electrode disappears, it really is still within the conductive state. Therefore, the function of the control electrode is just to trigger the thyristor to change on. After the thyristor is switched on, the control electrode loses its function.
- The only method to shut off the turned-on thyristor is to decrease the anode current that it is insufficient to maintain the positive feedback process. How you can decrease the anode current is to shut down the forward power supply Ea or reverse the bond of Ea. The minimum anode current needed to maintain the thyristor within the conducting state is called the holding current in the thyristor. Therefore, strictly speaking, so long as the anode current is lower than the holding current, the thyristor can be turned off.
What exactly is the difference between a transistor and a thyristor?
Transistors usually consist of a PNP or NPN structure made from three semiconductor materials.
The thyristor is composed of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The work of any transistor depends 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 change on or off.
Transistors are popular in amplification, switches, oscillators, as well as other facets of electronic circuits.
Thyristors are mostly 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 attain current amplification.
The thyristor is switched on or off by manipulating the trigger voltage in 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 summarize, although transistors and thyristors can be used in similar applications sometimes, because of their different structures and functioning principles, they have got noticeable variations 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-weight 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 can be used in motor controllers.
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