What is a thyristor?
A thyristor is really a high-power semiconductor device, also called a silicon-controlled rectifier. Its structure consists of 4 quantities of semiconductor elements, including 3 PN junctions corresponding to the Anode, Cathode, and control electrode Gate. These 3 poles are the critical parts from 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 a variety of electronic circuits, including controllable rectification, AC voltage regulation, contactless electronic switches, inverters, and frequency conversion.
The graphical symbol of any silicon-controlled rectifier is generally represented through the text symbol “V” or “VT” (in older standards, the letters “SCR”). In addition, derivatives of thyristors also have fast thyristors, bidirectional thyristors, reverse conduction thyristors, and light-controlled thyristors. The functioning condition from the thyristor is that each time 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 can be used between the anode and cathode (the anode is linked to the favorable pole from the power supply, as well as the cathode is attached to the negative pole from 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 will not be conducting and contains forward blocking capability.
- Controllable conduction
As shown in Figure b above, when K is closed, along with a forward voltage is applied to the control electrode (known as a trigger, as well as the applied voltage is called trigger voltage), the indicator light turns on. Because of this the transistor can control conduction.
- Continuous conduction
As shown in Figure c above, right after the thyristor is switched on, even when the voltage around the control electrode is taken off (which is, K is switched on again), the indicator light still glows. This demonstrates that the thyristor can still conduct. Currently, so that you can cut off the conductive thyristor, the power supply Ea must be cut off 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 will not be conducting and may reverse blocking.
- In summary
1) If 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) If the thyristor is exposed to a forward anode voltage, the thyristor will only conduct once the gate is exposed to a forward voltage. Currently, the thyristor is in the forward conduction state, the thyristor characteristic, which is, the controllable characteristic.
3) If the thyristor is switched on, so long as you will find a specific forward anode voltage, the thyristor will always be switched on regardless of the gate voltage. Which is, right after the thyristor is switched 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 close to zero, the thyristor turns off.
5) The disorder for your thyristor to conduct is 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 cut off, or perhaps the voltage must be reversed.
Working principle of thyristor
A thyristor is actually a distinctive triode composed of three PN junctions. It can be equivalently thought to be comprising a PNP transistor (BG2) as well as an NPN transistor (BG1).
- When a forward voltage is applied 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 switched off because BG1 has no base current. When a forward voltage is applied to the control electrode at this time, BG1 is triggered to generate a base current Ig. BG1 amplifies this current, along with 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 be brought in 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 vital positive feedback, causing both BG1 and BG2 to get into a saturated conduction state quickly. A large current appears within the emitters of the two transistors, which is, the anode and cathode from the thyristor (how big the current is actually dependant on how big the stress and how big Ea), so the thyristor is completely switched on. This conduction process is finished in a really short period of time.
- Following the thyristor is switched on, its conductive state will be maintained through the positive feedback effect from the tube itself. Whether or not the forward voltage from the control electrode disappears, it is actually still within the conductive state. Therefore, the purpose of the control electrode is simply to trigger the thyristor to turn on. Once the thyristor is switched on, the control electrode loses its function.
- The only 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 cut off the forward power supply Ea or reverse the link of Ea. The minimum anode current necessary to keep the thyristor within the conducting state is called the holding current from the thyristor. Therefore, strictly speaking, so long as the anode current is under the holding current, the thyristor can be switched off.
What exactly is the distinction between a transistor along with a thyristor?
Transistors usually contain a PNP or NPN structure composed of three semiconductor materials.
The thyristor consists of four PNPN structures of semiconductor materials, including anode, cathode, and control electrode.
The job of any transistor relies on electrical signals to control its closing and opening, allowing fast switching operations.
The thyristor needs a forward voltage along with a trigger current on the gate to turn on or off.
Transistors are widely used in amplification, switches, oscillators, as well as other facets of electronic circuits.
Thyristors are mainly used 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 accomplish current amplification.
The thyristor is switched on or off by manipulating the trigger voltage from the control electrode to comprehend 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 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 noticeable differences in performance and utilize occasions.
Application scope of thyristor
- In power electronic equipment, thyristors can be utilized in frequency converters, motor controllers, welding machines, power supplies, etc.
- In the lighting field, thyristors can be utilized in dimmers and light 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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