SCR BASIC OPERATIONS AND THEORY

[Audio] SCR BASIC OPERATIONS AND THEORY By: Jennifer and Chris Meloche.

[Audio] OBJECTIVE In this educational demonstration we will discuss: What is a SCR Construction of an SCR Working/operation of an SCR Triggering Application Conclusion Quiz References.

[Audio] INTRODUCTION SCRs are mainly used in devices where the control of high power, possibly coupled with high voltage, is demanded. Their operation makes them suitable for use in medium- to high-voltage AC power control applications, such as lamp dimming, power regulators and motor control..

[Audio] What is a Silicon Controlled Rectifier (SCR)? Definition: A solid state-operated device with a four-layered structure, its flow of current in the one direction just like a diode where it has three junctions along with the three terminals. These devices are defined as silicon-controlled rectifiers (SCR). The other name for this is Thyristor. These are the devices that are operated with the current. A large value of the current at the anode terminal is controlled by the value of the current applied at the terminal gate. Hence these are the current-controlled devices..

[Audio] Construction of SCR An SCR is constructed with the four layers that consist of the P-type and the N-type semiconductor material. These are layered in such a way that it tends to form three junctions that are J1, J2, and J3. The three terminals that are attached to it are known as anode, cathode, and gate. The anode is the basic terminal through which the current flows or enters the device. Where the cathode is the terminal through which the entered current leaves the device. The current entering terminal is of positive polarity and the terminal through which the current is leaving is of negative polarity. In between the flow of current among the terminals, there must be a terminal that can provide the control. This can be provided by the terminal gate. This terminal is sometimes also referred to as the terminal of control..

[Audio] Working/Operation of SCR The basic working principle in the SCR is that as the triggering or the biasing is applied at the terminal gate then the conduction begins. As it is a unidirectional device the current will be in a single direction. It resembles the operation of the diode but the only difference is that this can withstand the high amount of voltages and powers..

[Audio] Triggering SCR triggering is a method of making the device turn ON. It needs to be applied with a sufficient amount of biasing to the terminal gate. Hence it is known as SCR triggering or Thyristor triggering. Finally as the device moves to the ON state or the conducting state, the maximum amount of the current flows through the terminal anode. There are various types of triggering in SCR involved. They are:.

[Audio] 1). Voltage Triggering 2). Thermal Triggering 3). Light Triggering 4). dv/dt Triggering 5). Gate Triggering These are some of the methods involved in triggering the Thyristors..

[Audio] Lab Exercise 17 In this exercise it is demonstrating the SCR phase shifting The light bulb on the left hand side is connected to the SCR and the light bulb on the right hand side is connected to a 120 volt power supply so you can visually see the difference. Right now the SCR is OFF.

[Audio] Voltage applied to the gate in phase with the voltage applied to the anode An SCR that has not been phase shifted has the voltage that is applied to the gate locked in phase with the voltage applied to the anode. SCR fires when the gate current reaches a 5mA level (trigger point)..

[Audio] SCR fires at the peak of the waveform The variable resistor is adjusted so that the gate current will not reach 5 mA level until the applied voltage reaches its peak value. The SCR fires when the voltage applied to the anode is at the peak level also..

[Audio] SCR fires before the waveform reaches the peak If the variable resistor is decreased in value, the gate current can reach 5 mA sooner, and the SCR will fire sooner during the cycle. Notice the voltage decreased and the light bulb got brighter, that's because the meter is connected to the SCR which is opposite of the load. . If the meter was connected to the load the voltage would increase as the light bulb got brighter..

[Audio] SCR fires very early in the waveform Each decrease of the variable resistor will cause the gate current to reach 5 mA earlier in the cycle and cause the SCR to fire sooner. If the variable resistor is adjusted so that the gate current leads the voltage by a greater amount, the SCR fires later. The SCR therefore drops a greater amount of voltage and the load has less voltage applied to it. By choosing the correct values of resistance and capacitance, the voltage applied to the load can be controlled from 0 to its full value.

[Audio] APPLICATION SCRs are capable of controlling the devices with the higher value of AC powers; they can be utilized efficiently where the requirement is of controlling the devices with the high value of the voltage and the power is present. It is capable of converting the huge amounts of power of type AC into DC. Hence making this an efficient Rectifier that can withstand high voltages. In the controlling of Relays, these devices are used. In the elements where the induction heating concept is used they require the control of power in such equipment. Hence these Thyristors are applicable in such concepts..

[Audio] CONCLUSION SCR's can be used to control AC power for lamp dimmers, electric heaters, and electric motors. A half-wave, variable-resistance, phase-controlled circuit was utilized to vary the voltage to the load. We used a standard household light bulb so that we could visually represent the amount of voltage going to it. The light bulb got brighter as the voltage to the load was increased. A phase shift is necessary for the SCR to gain complete control of the waveform. To separate the voltages (voltage applied to gate, voltage applied to anode) a capacitor and potentiometer were used in the circuit. Since the current in a capacitive circuit leads the voltage, the gate current is shifted out of phase with the voltage supplied to the anode of the SCR. The potentiometer is used to determine the amount of phase shifting. The variable resistor will determine when the gate current reaches the Gate Trigger Current thus controlling when the SCR fires. By adjusting the potentiometer the SCR can be controlled to fire at any point during the cycle. This is done by a simple voltage divider circuit. By choosing the correct values of resistance and capacitance, the voltage applied to the load can be controlled from zero to its full value. SCR's can also be used as an ON-OFF control of current and due to their latching capabilities they are often used in motor control circuits and soft starters..

[Audio] QUIZ Question 1 How many layers does an SCR have? 1 layer 2 layers 3 layers 4 layers.

[Audio] ANSWER D) 4 layers. ANSWER. D) 4 layers. 19.

[Audio] Question 2 What is another name for an SCR? A) resistor B) capacitor C) Thyristor D) Conductor.

[Audio] Answer C) Thyristor. Answer. C) Thyristor.

[Audio] Question 3 What do the 4 layers consist of? A) P and N type B) Q and A type C) B and B type D) R and B type.

[Audio] Answer A) P and N type. Answer. A) P and N type.

[Audio] Question 4 How many directions does an SCR have? A) All directions B) Unidirectional C) Directional D) Semi directional.

[Audio] Answer B) Unidirectional. Answer. B) Unidirectional.

[Audio] Question 5 What is a method to turn ON the SCR? A) Bigger B) Trigger C) Digger D) Ligger.

[Audio] Answer B) Trigger. Answer. B) Trigger. 27.

[Audio] References. References. https://studylib.net/doc/6773152/lab-5--phase-shifting-an-scr https://swedishcr.weebly.com/uploads/1/5/2/2/15223984/pe_lab_experiment_23.pdf https://www.studocu.com/row/document/cosmats-university-sahiwal/industrial-electronics/experiment-no-02-industrial-electronics/10239269 https://electronicspost.com/explain-the-construction-and-working-of-an-silicon-controlled-rectifier/ https://industrial-electronics.com/ind-mtr-cntrl_061.html#:~:text=The%20SCR%20can%20control%20all,voltage%20applied%20to%20the%20anode . Electronics for Electricians 7 th Edition Industrial Motor Control 7 th Edition.