Just what is a thyristor?
A thyristor is a high-power semiconductor device, also referred to as a silicon-controlled rectifier. Its structure includes four levels of semiconductor materials, including 3 PN junctions corresponding for the Anode, Cathode, and control electrode Gate. These 3 poles are definitely 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 working status. Therefore, thyristors are popular in various electronic circuits, such as controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of any semiconductor device is normally represented by the text symbol “V” or “VT” (in older standards, the letters “SCR”). Furthermore, derivatives of thyristors also include fast thyristors, bidirectional thyristors, reverse conduction thyristors, and lightweight-controlled thyristors. The working condition in the thyristor is the fact each time 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 used involving the anode and cathode (the anode is connected to the favorable pole in the power supply, and the cathode is attached to the negative pole in the power supply). But no forward voltage is applied for the control pole (i.e., K is disconnected), and the indicator light does not illuminate. This implies that the thyristor is not really 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 called trigger voltage), the indicator light turns on. Which means that the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, following the thyristor is excited, even when the voltage in the control electrode is removed (which is, K is excited again), the indicator light still glows. This implies that the thyristor can carry on and conduct. At this time, so that you can 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 applied for the control electrode, a reverse voltage is applied involving the anode and cathode, and the indicator light does not illuminate currently. This implies that the thyristor is not really conducting and can reverse blocking.
- In summary
1) If the thyristor is put through a reverse anode voltage, the thyristor is within a reverse blocking state no matter what voltage the gate is put through.
2) If the thyristor is put through a forward anode voltage, the thyristor is only going to conduct if the gate is put through a forward voltage. At this time, the thyristor is in the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) If the thyristor is excited, provided that there is a specific forward anode voltage, the thyristor will stay excited whatever the gate voltage. That is, following the thyristor is excited, the gate will lose its function. The gate only serves as a trigger.
4) If the thyristor is on, and the primary circuit voltage (or current) decreases to close to zero, the thyristor turns off.
5) The problem for your thyristor to conduct is the fact a forward voltage needs to be applied involving the anode and the cathode, and an appropriate forward voltage also need to be applied involving the gate and the cathode. To change off a conducting thyristor, the forward voltage involving the anode and cathode should be shut down, or the voltage should be reversed.
Working principle of thyristor
A thyristor is basically a distinctive triode made from three PN junctions. It could be equivalently viewed as comprising a PNP transistor (BG2) and an NPN transistor (BG1).
- In case a forward voltage is applied involving the anode and cathode in the thyristor without applying a forward voltage for the control electrode, although both BG1 and BG2 have forward voltage applied, the thyristor remains switched off because BG1 has no base current. In case a forward voltage is applied for 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 is going to be introduced the collector of BG2. This current is brought to BG1 for amplification and after that brought to BG2 for amplification again. Such repeated amplification forms a crucial positive feedback, causing both BG1 and BG2 to get into a saturated conduction state quickly. A large current appears in the emitters of the two transistors, which is, the anode and cathode in the thyristor (the size of the current is actually based on the size of the burden and the size of Ea), therefore the thyristor is totally excited. This conduction process is completed in an exceedingly short time.
- Following the thyristor is excited, its conductive state is going to be maintained by the positive feedback effect in the tube itself. Even when the forward voltage in the control electrode disappears, it really is still in the conductive state. Therefore, the function of the control electrode is only to trigger the thyristor to transform on. Once the thyristor is excited, the control electrode loses its function.
- The best way to shut off the turned-on thyristor is always to lessen the anode current so that it is inadequate to keep the positive feedback process. The way to lessen the anode current is always to shut down the forward power supply Ea or reverse the bond of Ea. The minimum anode current necessary to maintain the thyristor in the conducting state is called the holding current in the thyristor. Therefore, as it happens, provided that the anode current is lower than the holding current, the thyristor could be switched off.
What is the distinction between a transistor and a thyristor?
Transistors usually include 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 job of any transistor depends on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor demands a forward voltage and a trigger current in the gate to transform on or off.
Transistors are popular in amplification, switches, oscillators, along with other facets of electronic circuits.
Thyristors are mostly utilized in electronic circuits such as 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 attain current amplification.
The thyristor is excited or off by manipulating the trigger voltage in the control electrode to understand 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 in some cases, due to their different structures and working principles, they have noticeable differences in performance and use 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 lightweight 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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