ⅠIntroductionThyristors are high-speed solid-state devices that can control motors, heaters, and lighting. Before we get into Thyristor Circuits. We'll look at the basic construction and operation of the Silicon Controlled Rectifier, also known as a Thyristor. Next, we'll look at how we can use thyristors and thyristor switching circuits to control much larger loads like lamps, motors, or heaters, among other things. CatalogⅠIntroductionⅡ Thyristors Circuits Related VideoⅢ What Is Silicon Controlled Rectifier?Ⅳ Construction of Silicon Controlled RectifierⅤ What is a Thyristor?Ⅵ Thyristor Switching Circuits6.1 Thyristor Circuit in DC6.2 AC Thyristor CircuitⅦ How an SCR Circuit Works with Thyristor Circuits?7.1 DC Thyristor / SCR Circuit7.2 Basic AC Thyristor / SCR Circuit7.3 AC SCR Circuit with Gate Phase ControlⅧ FAQ Ⅱ Thyristors Circuits Related VideoSilicon Control Rectifier SCR Basic AC Circuit Thyristors Circuits Video Description:  Silicon Control Rectifier SCR Basic AC Circuit Ⅲ What Is Silicon Controlled Rectifier?The Silicon Controlled Rectifier (SCR) is one of the most popular devices in the market. SCR can be found in a variety of applications such as rectification, power regulation, and inversion, among others. SCR, like a diode, is a unidirectional device that allows current in one direction but opposes it in the other gate. SCRs have the ability to turn ON or OFF, and their switching is controlled by biasing conditions and the gate input terminal.By varying the ON periods of the SCR, the average power delivered at the load can be varied. It is capable of handling tens of thousands of voltages and currents. Figure depicts the SCR symbol and its terminals.Figure1 :Silicon Controlled Rectifier   Ⅳ Construction of Silicon Controlled RectifierAs shown in the figure, an SCR has three terminals: anode, cathode, and gate. SCRs have the ability to turn ON or OFF, and their switching is controlled by biasing conditions and the gate input terminal.By varying the ON periods of the SCR, the average power delivered at the load can be varied. It is capable of handling tens of thousands of voltages and currents. Figure depicts the SCR symbol and its terminals.Figure2:Construction The SCR is manufactured using three different types of constructions: planar, Mesa, and press pack. Planar construction, in which all junctions in an SCR are diffused, is used for low-power SCRs. In a mesa type construction, junction J2 is formed by diffusion and the outer layers are alloyed to it as a result. This design is primarily used in high-power Silicon Controlled Rectifiers. The SCR is braced with plates made of molybdenum or tungsten to provide high mechanical strength. One of these plates is soldered to a copper stud, which is threaded to connect to the heat sink. Ⅴ What is a Thyristor?A thyristor is a four-layer solid-state semiconductor device made of P and N materials. When a gate receives a triggering current, it begins to conduct until the voltage across the thyristor device is biased forward. In this case, it functions as a bistable switch. To control a large amount of current flowing through the two leads, we must create a three-lead thyristor by combining the small amount of current with that current. This is referred to as control lead. If the potential difference between the two leads is less than the breakdown voltage, a two-lead thyristor is used to turn the device on.Figure3:Thyristor Ⅵ Thyristor Switching CircuitsDC Thyristor CircuitAC Thyristor circuit 6.1 Thyristor Circuit in DCWhen connected to a DC supply, we use a thyristor to control larger DC loads and current. The main advantage of using a thyristor in a DC circuit as a switch is that it provides a high current gain. Because a small gate current can control a large anode current, the thyristor is classified as a current-operated device.Figure4:Thyristor Circuit in DC 6.2 AC Thyristor CircuitWhen connected to an alternating current supply, the thyristor behaves differently because it is not the same as a DC-connected circuit. A thyristor is used as an AC circuit during one half of a cycle, causing it to turn off automatically due to its reverse biased condition. Figure6:AC Thyristor Circuit Ⅶ How an SCR Circuit Works with Thyristor Circuits?7.1 DC Thyristor / SCR CircuitMany applications call for an SCR circuit to control the operation of a DC load. This can be used for switching DC motors, lamps, or any other load.The basic SCR circuit shown below can control power to a load by using a small switch to initiate power application to the load.Figure7:Basic DC thyristor / SCR circuit With S1 closed and S2 open, no current will flow at first. The SCR circuit will turn on and the current will flow in the load only when S2 is closed and it triggers the gate by causing the gate current to flow.Until the anode circuit is broken, the current will continue to flow. S1 can be used for this. Another method is to place the switch S1 across the SCR and briefly close it, causing the voltage across the SCR to disappear and the SCR to stop conducting.Because of their functions in this SCR circuit, S1 and S2 may be referred to as the Off switch and the ON switch, respectively. In this configuration, S1 must be able to carry the full load current, while S2 must only carry the gate current. Once the SCR is turned on, the switch can be released and remain open because the SCR's action maintains the current flow through the device and thus the load.R1 connects the gate to the power supply via the switch. When S2 is closed, current flows through the resistor enters the gate and activates the SCR. The resistor R1 must be calculated to provide enough gate current to turn on the SCR circuit.R2 is included to reduce the SCR's sensitivity so that it does not fire on any noise that is detected. 7.2 Basic AC Thyristor / SCR CircuitWhen using a thyristor circuit with AC, a few changes must be made, as shown below.This is because alternating current reverses polarity throughout the cycle. This means that the SCR will become reverse-biased, effectively lowering the anode voltage to zero and causing it to turn OFF for one-half of each cycle. As a result, there is no need for an off switch because this is accomplished as part of the use of an alternating current supply.When using a thyristor circuit with AC, a few changes must be made, as shown below.This is because alternating current reverses polarity throughout the cycle. This means that the SCR will become reverse-biased, effectively lowering the anode voltage to zero and causing it to turn OFF for one-half of each cycle. As a result, there is no need for an off switch because this is accomplished as part of the use of an alternating current supply.\Figure8: AC thyristor / SCR circuitThe circuit operates in a slightly different manner than the DC SCR circuit. When the switch is turned on, the circuit must wait for sufficient anode voltage to be available as the AC waveform progresses along its path. In addition, the SCR circuit will have to wait until the voltage within the gate section of the circuit is high enough to trigger the SCR. The switch must be in a closed position for this to work.Once triggered, the SCR will remain to conduct for the duration of the positive half of the cycle. As the voltage falls, the anode-cathode voltage will become insufficient to support conduction. At this point, the SCR will come to a halt.The SCR will then not operate during the negative half of the cycle. The process will only be repeated when the next positive half of the cycle returns. As a result, this circuit will only operate when the gate switch is closed.One disadvantage of using this type of SCR circuit is that it cannot supply more than 50% power to the load because it does not conduct during the negative half of the AC cycle because the SCR is reverse biased. 7.3 AC SCR Circuit with Gate Phase ControlBy varying the proportion of the half-cycle over which the SCR conducts, the amount of power reaching the load can be controlled. This can be accomplished by using an SCR circuit with phase control of the input gate signal.Figure9:AC thyristor circuit waveformsThe SCR gate signal is derived from an RC circuit consisting of R1, VR1, and C1 before the diode D1 when using the SCR circuit with phase control.Because the SCR is forward biased, only the positive half cycle of the waveform is of interest, as with the basic AC SCR circuit. During this half-cycle, the capacitor, C1, charges up from the AC supply voltage via the resistor network of R1 and VR1. The waveform at the positive end of C1 is seen to lag behind the input waveform, and the Gate is only triggered when the voltage at the capacitor's high end has risen sufficiently to trigger the SCR via D1. As a result, the SCR's turn-on time is delayed compared to what it would be if the RC network was not present. The VR1 value changes the delay and thus the proportion of the cycle over which the SCR operates. The power into the load can thus be adjusted in this manner. Figure10: AC thyristor circuit with gate phase control R1 is a series resistor that has been included to limit the minimum value for the resistor network to a value that will provide an acceptable gate current level for the SCR. The phase angle of the gate waveform must typically vary between 0° and 180° to provide complete control of the 50% of the cycle available for conduction with an SCR. These circuits demonstrate some of the fundamental concepts underlying the design of SCR thyristor circuits. They show how they work and how they can be used in their most basic form. One of the most important considerations when designing thyristor circuits is power dissipation. Because these circuits frequently handle high voltages and high power levels, power dissipation can be a significant factor in circuit design and operation. Ⅷ FAQ1. What does a thyristor do in a circuit?The primary function of a thyristor is to control electric power and current by acting as a switch. For such a small and lightweight component, it offers adequate protection to circuits with large voltages and currents (up to 6000 V, 4500 A).2. How thyristor acts as a switch?When connected to a direct current DC supply, the thyristor can be used as a DC switch to control larger DC currents and loads. When using the Thyristor as a switch it behaves like an electronic latch because once activated it remains in the “ON” state until manually reset3. What is difference between SCR and thyristor?Thyristor is a four semiconductor layer or three PN junction device. It is also known as “SCR” (Silicon Control Rectifier). The term “Thyristor” is derived from the words of thyratron (a gas fluid tube which works as SCR) and Transistor. Thyristors are also known as PN PN Devices.4. Is thyristor convert AC to DC?A single-phase thyristor rectifier converts an AC voltage to a DC voltage at the output. The power flow is bidirectional between the AC and the DC side.5. What are the advantages of thyristor?Advantages of Thyristor :It is easy to turn on. It is able to control AC power. It can switch high voltage, a high current device. It cost is very low.