UJT As Relaxation Oscillator

• The UJT is often used in the timing and triggering circuits. 
• Figure A shows the circuit diagram for the UJT relaxation oscillator.
• When the switch S is kept closed, the capacitor C is charged through resistance R towards voltage V_DC. 
• The voltage across capacitor in a given time depends upon circuit time constant RC. 
• When the voltage across capacitor becomes equal to peak point voltage, the emitter diode is forward biased and UJT conducts. 
• The peak voltage is given by

         V_P = [ ( R₁ + R_B1 ) /  ( R₁ + R₂ + R_B1 + R_B2 ) ] V_dc + V_D

         Where V_dc = DC supply voltage and

                    V_D = Diode thresh hold voltage

• As soon as the UJT starts to conduct, the capacitor discharges through interbase resistance R_B1 and the resistance R₁. 
• The discharging time constant of the capacitor is very small as compared to charging time constant. 
• When the voltage across capacitor becomes less than the valley point voltage during discharging of the capacitor, the UJT comes in to non conducting state. ( It means that the UJT remains in conducting state until the emitter current drops below valley current ).
• The capacitor charging and discharging waveform is shown in the Figure A in which time t₁ denotes for charging time and time t₂ denotes for discharging time of the capacitor.
• As the discharging time of the capacitor is very small as compared to charging time, the time required to complete one cycle is considered only for charging time of the capacitor.

         T = RC Log_e ( V_dc / V_dc – V_p )

         As V_p = ηV_dc + V_D

         Where V_p = Peak point voltage

                      η = Intrinsic stand off ratio

                   V_D = Threshold voltage of the diode

        T = RC Log_e ( V_dc / V_dc – ηV_dc – V_D )

        As V_D is small, it is neglected.

        T = RC Log_e ( 1 / 1 – η )

• The frequency of the oscillation is given by f = 1 / T. 
• If the output is taken from base B₁, it is train of pulses of high magnitude during discharge of the capacitor. 
• These positive pulses are used for the triggering of the SCR. 
• If the output is taken from base B₂, it is waveform of negative pulse obtained during discharging of the capacitor.

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