 Hello. Myself Sunil Kalshatti, Ascent Professor, Department of Electronics Engineering, Walchen Institute of Technology, Singapore. Today, I am going to discuss the uni-junction transistor, learning outcome. At the end of this session, students can describe construction, characteristics and operation of UJT. UJT is break-over type switching device. It is useful in many industrial circuits like timers, oscillators, waveform generators, gate control circuit for SCRs and tracks. The UJT, as the name implies, is characterized by single p-n junction. It exhibits negative resistance characteristics that makes it useful in oscillator circuit. This is the UJT symbol and this is the construction diagram of UJT. It is special transistor that has two bases and one emitter. It is also called as double base diode. It is two layer, three terminal, solid set, silicon switching device. UJT specification for 2N2646. Maximum voltage between two bases, VB2B1 is 35 volt. Maximum emitter reverse voltage is 30 volt. Maximum RMS emitter current IE, 50 milliampere. Maximum peak emitter current IE, 2 ampere. Operating temperature range from minus 65 degree centigrade to plus 150 degree centigrade and maximum power dissipation is 300 milliwatts. The UJT is having three terminals, base 1, V1, base 2, V2 and emitter. The UJT is made up of an n-type silicon bar which acts as a base, as shown in figure. It is a very lightly doped. It consists of lightly doped n-type silicon bar with small piece of heavily doped p-type material alloyed to its one end to produce single p-n junction. The single p-n junction accounts for the terminology unijunction. The silicon bar at its two ends has two ohmic contact designated B1, B2 and p-type region termed emitter E. The emitter junction is usually closer to B2 than B1 so that device is not symmetrical. Two paths for current flow one is from BT to B1 and another is from emitter to base 1. The emitter terminal is represented by an arrow pointing from p-type emitter to n-type base like this. This is an arrow from emitter to base 1. Simplified equivalent circuit of UJT. On the equivalent circuit the n-type channel basically consists of two registers R-B1 and R-B2 these are in series. The diode D representing the p-n junction connected to their center point. The emitter p-n junction is fixed in position along the ohmic channel during manufacture and can therefore not be changed. The resistance R-B1 represents internal dynamic resistances between the emitter E and terminal B1. R-B1 vary from 5 kilo ohm to 50 ohm for corresponding change in emitter current from 0 to 50 micro ampere. This R-B2 represents dynamic resistance between the emitter E and terminal B2 that is R-B2 is equal to 2.8 kilo ohm. These two series resistances produces a voltage divider network between the two base terminals of the unijunction transistor. Internal resistance between B1 and B2 is called as a inter-base resistance R-BB R-BB is equal to R-B1 plus R-B2. Suppose a voltage VBB is applied across the UJT between B2 and B1 so that B2 is biased positive relative to B1. With 0 emitter input applied the voltage developed across R-B1 of the resistive voltage divider can be calculated as apply the voltage divider rule across the resistive divider network. Here eta VBB is the drop across R-B1. By using voltage divider network VR-B1 is equal to R-B1 upon R-B1 plus R-B2 into VBB supply voltage. VR-B1 is equal to R-B1 upon R-BB into VBB where R-BB is equal to R-B1 plus R-B2 VR-B1 is equal to eta into VBB. For a UJT the resistive ratio of R-B1 to R-BB is shown above is called the intrinsic standard ratio and is given the Greek symbol eta. Typical standard value of eta range from 0.5 to 0.8 for most common UJT operation. As long as VEB1 is less than VBN plus eta VBB there is no emitter current because diode PN junction is reverse biased thus offering a very high impedance and the device does not conduct. The UJT is switched off and zero current flows when VEB1 crosses VBN plus eta VBB the PN junction becomes forward biased and emitter current begins. The value of emitter voltage that causes the diode to start conducting is called as peak point voltage and the current is called as a peak point current here. This voltage is called as a peak point voltage VB is equal to VBN plus eta into VBB VB is equal to VD plus point VD plus eta into VBB or VB is equal to 0.7 plus eta into VBB here. From 0 to VB this region is the cut off region this region is the negative resistance region and this region is the saturation region. The voltage corresponds to this point is called as a value point voltage and this current is called as a value point current. The region from VE is equal to 0 to VE is equal to VB is called as cut off region because no emitter current flows except a leakage current. This region is the cut off region once VE exceeds the peak point voltage. The heavily doped P region injects the large number of charge carriers into the lightly doped N region because of these additional charge carriers the internal dynamic resistance R dash V1 decreases rapidly the effect of this emitter current increases and at the same time the VE decreases. In this region IE increases and VE decreases up to certain point called as a value point voltage and the current corresponds to that value point current. This is called as negative resistance region beyond this IE increases with VE this is saturation region. The value current is holding current is in between 1 to 10 milliampere. What is meant by negative resistance region of UZT here? In this region the charge carriers are injected by the P type region into the N type region because of additional charge carriers the internal dynamic resistance R dash P1 decreases. The effect of this current increases rapidly and voltage is decreases the region it is the region between the peak point voltage and value point voltage this region corresponds to the negative resistance region UZT relaxation oscillator. Initially assume that the UZT is in cut off region when input is applied capacity starts charges through the supply R3C and capacitor starts charges exponentially these are the charging and discharging waveform of capacitor. As long as voltage across capacitor is less than peak point voltage the UZT remains off and the drop across R1 remains zero. Once the capacitor voltage crosses the peak point voltage the UZT conducts and once the UZT conducts the current increases rapidly and capacitor starts discharges and the discharging path of C is C UZT R1 and the pulse is produced across R1. As long as C is charges the UZT remains in conducting state once the voltage across capacitor becomes less than value point voltage the UZT turns off and again capacitor C starts charges through Vs R3C and this process is continuous. How to trigger SCR using UZT this is the triggering circuit of SCR using UZT when input is applied the capacitor starts charges through the VBBREC and UZT remains off so no pulse is produced across R. When the capacitor voltage crosses the peak point voltage the UZT conducts and once the UZT conducts capacitor C starts discharges through the UZT RC and current increases rapidly and pulse is produced across R. This pulse acts as a triggering pulse of SCR and SCR fires these are references thank you.