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 VDC
  • 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
         VP = [ ( R1 + RB1 ) /  ( R1 + R2 + RB1 + RB2 ) ] Vdc + VD
         Where Vdc = DC supply voltage and
                    VD = Diode thresh hold voltage

ujt relaxation oscillator circuit
  • As soon as the UJT starts to conduct, the capacitor discharges through interbase resistance RB1 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 Loge ( Vdc / Vdc – Vp )
         As Vp = ηVdc + VD
         Where Vp = Peak point voltage
                      η = Intrinsic stand off ratio
                   VD = Threshold voltage of the diode
        T = RC Loge ( Vdc / Vdc – ηVdc – VD )
        As VD is small, it is neglected.
        T = RC Loge ( 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.
ujt relaxation oscillator waveforms

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