 Hello, myself Sunil Kalshetti, Ascent Professor, Department of Electronics Engineering, Valchan Institute of Technology, Solapur. Today, I am going to discuss Silicon Unilateral Switch, yes, yes, learning.com. At the end of this session, students can describe construction characteristics and operation of Silicon Unilateral Switch. The Silicon Unilateral Switch is a four-layer PNPN device of the Thyristor family. SES is the one of the member from Thyristor family. It is useful in various industrial circuits like oscillators, waveform generators, gate control circuits for SCRs and other power devices like IGBT MOSFET GTO. It exhibits negative resistance characteristics that makes it useful in oscillator circuits. The SCS is also known as a four-layer diode. It can be treated as low current SCR without a gate terminal. This is the symbol of SCS. SCS is having three terminals, anode, cathode and gate. This is the schematic symbol of SCS. In between gate and cathode, the generator is connected. The equivalent circuit of Silicon Unilateral Switch consists of option A, diode in series with a PUT, option B, diode in parallel with PUT, option C, diode in anti-parallel with PUT, option D, two diodes. It is similar to the PUT except that in between gate and cathode, a general diode is connected. So the option C is correct answer. Figure C is the internal diagram of SCS. It is also called as a N gate SCR. In the SCR, the gate is connected to the P region, but in SCS, the gate is connected to the N region adjacent to the anode. That's why SCS is called as a N gate SCR. It is similar to PUT except it has an internally built-in low voltage general diode in between gate and cathode. In between gate and cathode, the generator is there. It is low voltage, low current device. Unlike the other device, the SCS only turns on for a fixed value of anode to cathode voltage. No external gate drive is required to turn on SCS. To turn on the SCS, it requires fixed anode to cathode voltage, means VPN plus VG. Means it requires 0.7 plus VZ to turn on the SCS. For forced switching, bias or pulse signal may be applied to the gate of SCS. SCS NT6404 specifications are as follows, rating 20 volt 0.5 ampere, on-site drop 1.5 ampere, switching current IS 200 microampere, holding current 0.75 milliampere. Most of SCS have a break-over voltage of 8 volt and current limit of less than 1 ampere. This is the VI characteristics of SCS. VS switching voltage, VH holding voltage, IH holding current, IS switching current, we have forward voltage drop across SCS when SCS is in conducting state. If input voltage is less than VS, SCS remains in off-state. The device triggers into conduction when a forward switching voltage VS is applied. SCS is turned on at fixed voltage VS because of breakdown of internal built-in general diode which causes current to flow out of N1. This current acts as a gate current and hence no external gate drive is required. When supply voltage is more than VS, SCS turns on and suddenly the voltage is reduces and current encourages rapidly and the device exhibits in the negative resistance region and the negative resistance region is represented by the region SH. So because of this reason SCS can be used as a relaxation oscillator comparison between SCS and UJT. SCS is turned on at fixed ANO2 cathode voltage determined by its internal general diode whereas UJT is turned on at fixed fraction of eta inter-base voltage. The switching current IS of SCS is much higher than that of UJT and is very close to holding current IH that is the valley point current. The negative resistance region is very much restricted in SCS as switching current and holding current are very close. For line synchronization and forced switching pulse signal may be applied to the gate terminal that is operating SCS as N gate SCR. How SCS can be acts as an over voltage sensor when over voltage is appears across SCS that is across ANO2 cathode the internal general diode breaks down and once the internal general diode breaks down SCS conducts and it maintains the safe low voltage in this way SCS acts as an over voltage sensor. Relaxation oscillator using SCS this is the one of the important application of SCS initially assume that SCS is in non conducting set and capacitor C is in fully discharging state when voltage is applied across RC circuit capacitor C start charges through the source R and C. As long as capacitor voltage is less than switching voltage SCS remains off when the voltage across capacitor crosses the switching voltage the internal general diode breaks down. So because of that breakdown of general diode SCS turns on and once the SCS turns on the current flows through the SCS and capacitor C starts discharges and the discharging path of C is the C SCS R0. So because of this pulse is produced across R0. As long as voltage across capacitor is greater than holding voltage SCS remains on once the capacitor voltage crosses the valley point voltage SCS turns off and again the capacitor C starts charges and the process continuously repeated triggering circuit for SCR using SCS. Here SCS is used to trigger the SCR assume that initially SCR SCS both are in non conducting state and capacitor C is in fully discharging state when the voltage is applied capacitor starts charges and the charging path of C is the input source voltage RC. As long as voltage across capacitor is less than switching voltage SCS remains off. When the voltage across capacitor crosses the switching voltage the breakdown of internal general diode takes place because of that SCS turns on and once the SCS turns on the current flows through the SCS and the capacitor C starts discharges and the discharging path of C's capacitor is C SCS. This discharging current acts as a trigger current of SCR. So because of this SCR conducts and the direction of load current is the VCC load SCR. In this way this circuitry acts as a triggering circuit for SCR or any power device. These are references thank you.