Electrical Revolution: 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.

Electrical Revolution