## 05/10/2017

### 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

• 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.

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