ujt (industrial electronic)

Chapter 2Chapter 2Unijunction Transistor

Basic OperationBasic Operation

UJT is an electronic semiconductor device that only has only one junction UJT is a digital device because its only have two state that is on and off Made by combining three layers of semiconductor material as shown in figure (b) above. Current flows in two path through the UJT. One is through Base 2 to Base 1 and the other path is through Emitter to the Base 1. In normal state, current does not flow through either path until the voltage applied to the emitter is about 10V higher than the voltage applied to the Base 1. when it turn ON, the current will continue flow until the voltage applied to the emitter drops to a point that is about 3V higher than voltage applied to Base 1. UJT will off in this condition and will turn ON again when voltage at emitter higher 10V that the voltage applied at the Base 1. EAD 3043 (Industrial Electronic)

Nor Aida Idayu Binti Abdullah. 2012

(a) symbol (c) Equivalent circuit(b) construction

Characteristic of UJTCharacteristic of UJT

The static emitter characteristic (a curve showing the relation between emitter voltage VE and emitter current IE) of a UJT at a given inter base voltage VBB is shown in figure. From figure it is noted that for emitter potentials to the left of peak point, emitter current IE never exceeds IEo . The current IEo corresponds very closely to the reverse leakage current ICo of the conventional BJT. This region, as shown in the figure, is called the cut-off region. Once con duction is established at VE = VP the emitter po tential VE starts decreasing with the increase in emitter current IE. This Corresponds exactly with the decrease in resistance RB for increasing cur rent IE. This device, therefore, has a negative resistance region which is stable enough to be used with a great deal of reliability in the areas of applications listed earlier. Eventually, the valley point reaches, and any further increase in emitter current IE places the device in the saturation region, as shown in the figure. EAD 3043 (Industrial Electronic)

Nor Aida Idayu Binti Abdullah. 2012

Characteristic (cont)Characteristic (cont)

Three other important parameters for the UJT are IP, VV and IV and are defined below:Peak-Point Emitter Current. Ip. It is the emitter current at the peak point. It repre sents the rnimrnum current that is required to trigger the device (UJT). It is inversely proportional to the interbase voltage VBB.Valley Point Voltage VV The valley point voltage is the emitter voltage at the valley point. The valley voltage increases with the increase in interbase voltage VBB.Valley Point Current IV The valley point current is the emitter current at the valley point. It increases with the increase in inter-base voltage VBB.

EAD 3043 (Industrial Electronic)Nor Aida Idayu Binti Abdullah. 2012

Operation of UJTOperation of UJT


The variable resistor is control the capacitor’s rate of charge time. When the capacitor is been charged to about 10V, the UJT turns ON and discharges the capacitor through Emitter and Base 1. When the capacitor is been discharges to about 3V, the UJT will turn off and permits the capacitor to begin charging again. By varying the resistance connected in series with the capacitor, the amount of time needed for charging the capacitor can be changed, thereby controlling the pulse rate of the UJT (T =RC)

The amount of capacitance that can be connected to the UJT is limited

Most UJT should not connected to the capacitor larger than 10µF because the UJT may not be able to handle the current spike produced by the larger capacitor, and UJT could become damaged

important

Advantages of UJTAdvantages of UJT


It is a Low cost device It has excellent characteristics It is a low-power absorbing device under normal operating conditionsA stable triggering voltage (VP)— a fixed fraction of applied inter base voltage VBB.A very low value of triggering current.A high pulse current capability.A negative resistance characteristic.

Model for UJTModel for UJT



Model for VpModel for Vp

Operation Operation


The relaxation oscillator in Figure below is an application of the unijunction oscillator. RE charges CE until the peak point. The unijunction emitter terminal has no effect on the capacitor until this point is reached. Once the capacitor voltage, VE reaches the peak voltage point VP, the lowered emitter-base 1 resistance quickly discharges the capacitor. Once the capacitor discharges below the valley point VV, the E-RB1 resistance revert back to high resistance, and the capacitor is free to charge again.

Example of calculation of Example of calculation of 2n2647 2n2647


ExampleExample


ExerciseExercise


QuizQuiz


AssignmentAssignment


ujt (industrial electronic)

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capacitor voltage

valley point current

peakpoint emitter current

valley point voltage

peak voltage point vp

ujt islimitedmost ujt

current iv

current ieo