 Hello. Myself, Sunil Karshati, Ascent Professor, Department of Electronics Engineering, Vouchen Institute of Technology, Sulapur. Today, I am going to discuss Thyristor Devices, learning.com. At the end of this session, students can describe construction, characteristics and operation of silicon bilateral switch and silicon controlled switch. It is bilateral device which can conduct in both directions. Again, it is the one of the member from Thyristor family. SPS is a device consisting of two identical structures of SES arranged in anti-parallel. SPS is another precoher device which is capable of triggering tracks with alternate positive and negative gate pulses. It is popular in low voltage trigger control circuits. Precoher voltage of SPS is lower than DAX, that is plus or minus 8 volt. So, being the most popular rating, SPS conduct in both direction when applied voltage breaks internal zener diode. It operates as a switch with both polarity of applied voltage. No external gate drive is required to turn on SPS. For line synchronization and forced switching, pulse signal may be applied to the gate terminal. This is the schematic symbol of SPS having the three terminals, anode 2, A2, anode 1, A1 and gate terminal. This is the equivalent circuit of SPS. This is the VI characteristics of SPS. This characteristics is similar to the DAG, VI characteristics of SPS. VI characteristics curve of an SPS is similar to that of the DAG, but it has more pronounced negative resistance region. During positive half cycle device triggers into conduction when applied input is greater than forward switching voltage VS. SPS is turned on at fixed voltage VS because of breakdown of internal built-in zener diode Z2, which causes current flow from anode 1 to anode 2. During negative half cycle device triggers into conduction when applied input voltage is greater than forward switching voltage, that is minus VS. SPS is turned on at fixed voltage minus VS because of breakdown of internal built-in zener diode Z1, which causes current flow from anode 2 to anode 1. Silicon bilateral switches are symmetrical to within about 0.3V, that is the difference in magnitude between plus VBO and minus VBO is less than 0.3V. How to trigger triac using SPS? This is the triggering circuit of triac using SPS. Initially assume that triac and SPS both are in non-conducting state. So, during positive half cycle the capacitor C start charges and the charging path of C is the source RL, R1, R2, C1. As long as voltage across capacitor is less than switching voltage, SPS remains in non-conducting state. When the voltage across capacitor C1 crosses the switching voltage, SPS conducts because of breakdown of internal zener diode. Once the SPS conducts, capacitor C discharges and the discharging path is C1, SPS and gate of the triac. So, this current acts as a triggering current for triac. So, if you top this triac conducts and the current flows in the clockwise direction and the direction of load current is the source RL triac and the triac continuously conduct up to the end of the half cycle. After end of the half cycle triac and SPS both are turns off. Again during negative half cycle this capacitor C start charges but now this time the charging path is exactly opposite of previous one. So, capacitor charges through the source C1, R2, R1, RL. So, this time the lower plate is positive and upper plate is negative. Again as long as capacitor voltage is less than minus Vs that is switching voltage. SPS remains in non-conducting state. Once the capacitor voltage crosses minus Vs, SPS conducts and now this time the current flows through the gate of triac SPS C1. In this direction the capacitor discharges and this current acts as a triggering current and the triac conducts and current flows in the anticlockwise direction. In this way triac conducts in positive half cycle as well as in the negative half cycle. Silicon controlled switch. The silicon controlled switch is a four layer PNPN device. It is the one of the member from thyristor family. And SPS is tetrod. It is four electrode thyristor. And SPS is four layer, four terminal, low power PNPN device having four terminals anode, cathode, anode gate and cathode gate. It is unilateral device. It has two gates. One gate like PUT and another cathode gate like SCR. And SPS can be turned on by either gate. SPS differs from an SCR in the following aspects. It handles currents in milli amperes rather than amperes. It has an additional gate that is anode gate. It is physically smaller than SCR. It has smaller location holding current than SCR. It needs small triggering signals. It gives more uniform triggering characteristics from sample to sample. This is the physical diagram of SCR. And figure B shows the equivalent schematic of silicon controlled switch having four terminals anode, cathode, cathode gate and anode gate. This is the schematic symbol of SCS. SCS exhibits negative resistance characteristics that makes it useful in oscillator circuit. Application of SCS, timing circuits, logic and triggering circuits, pulse generators, voltage sensors and oscillators. Let us see how to turn on SCS. To turn on SCS, apply the positive trigger pulse to the cathode gate or negative trigger pulse to anode gate operation. When negative pulse is applied at anode gate G2 causes Q1 to switch on and transistor Q1 supplies base current to transistor Q2 and both transistor switches on and SCS turns on. When positive pulse is applied to cathode gate G1 causes transistor Q2 to switch on, transistor Q2 supplies base current to transistor Q1 and both transistor switches on and SCS turns on. Turn off process of SCS. To turn off SCS, apply negative going trigger pulse to cathode gate or positive going trigger pulse to anode gate. Turn off operation. When negative pulse is applied at cathode gate G2 causes transistor Q2 to switch off. Effect of this transistor Q1 turns off and SCS turns off. When positive pulse is applied to anode gate G1 causes transistor Q1 to switch off. Effect of this transistor Q2 turns off and SCS turns off. This is the VI characteristics of SCS. It is similar to the SCR. How SCS can be used to start or stop DC motor? This is the circuit diagram to start and stop the DC motor. The upper switch, this switch is used to turn on the SCS and this switch is used to turn off SCS. When on switch is closed, the current flows through the input voltage source R1 on switch. So, this current acts as a trigger current. So, effect of this SCS turns on. And once the SCS turns on, the motor rotates and the load current flows through the source motor SCS. And to turn off the motor, close the off switch. Effect of this, it diverges the motor current in this direction. So, SCS turns off and motor turns off. These are references. Thank you.