UJT Relaxation Oscillator – Circuit Diagram and its Workings:

UJT Relaxation Oscillator – The relaxation oscillator shown in Fig. 26.94 (a) consists of UJT and a capacitor C which is charged through resistor R_E when interbase voltage V_BB is switched on. During the charging pe­riod, the voltage across the capacitor increases exponentially until it attains the peak point voltage V_P. When the capacitor voltage attains voltage V_P, the UJT switches on and the capacitor C rapidly discharges via B₁. The resulting current through the external resistor R develops a voltage spike, as illustrated in Fig. 26.94 (b) and the capacitor voltage drops to the value V_V. The device then cuts off and the capacitor commences charging again.

The cycle is repeated continually generating a sawtooth waveform across capacitor C. The resulting waveforms of capacitor voltage V_C and the voltage across resistor R(V_R) are shown in Fig. 26.94 (b). The frequency of the out­put sawtooth wave can be varied by varying the value of resistor R_E as it controls the time constant (T = R_EC) of the capacitor charging circuit. The discharge time t₂ is difficult to cal­culate because the UJT is in its nega­tive resistance region and its resist­ance is continually changing. How­ever, t₂ is normally very much less than t₁ and can be neglected for approximation.

For satisfactory operation of the above UJT Relaxation Oscillator, the fol­lowing two conditions for the turn-on and turn-off of the UJT must be met, as derived in previous article (UJT Triggering of SCR Working Principle) (Eqs. (26.37) and (26.38)).

i.e., the range of resistor R_E should be as given below

The time period and, therefore, frequency of oscillation can be derived as below.

During charging of capacitor, the voltage across the capaci­tor is given as

where R_EC is the time constant of the capacitor charging circuit and t is the time from the commencement of the charging.

The discharge of the capacitor commences at the end of charg­ing period t₁, when the voltage across the capacitor v_C becomes equal to V_P i.e., (η V_BB + V_B)

Neglecting V_B in comparison to η V_BB we have

So charging time period,

Since discharging time duration t₂ is negligibly small as com­pared to charging time duration t₁, so taking time period of the wave, T = t₁

Time period of the sawtooth wave,

and frequency of oscillation

By including a small resistor in each base circuit, three useful outputs (sawtooth waves, positive triggers, and negative triggers), as shown in Fig. 26.95, can be obtained.

When the UJT fires, the sudden surge of current through B₁ causes a voltage drop across R₁ and produces the positive going spikes. Also, at the UJT firing time, the fall of V_EB1 causes I_B2 to rise rapidly and generate the negative going spikes across R₂, as shown in Fig. 26.95. R₁ and R₂ should be much smaller than R_BB to avoid altering the firing voltage of the UJT. A wide range of oscillation frequencies can be achieved by making R_E adjustable and including a switch to select different values of capacitance, as illustrated.

As already mentioned in previous article (UJT Triggering of SCR Working Principle), there is upper and lower limits to the signal source resistance R_E for the satisfactory operation of the UJT [refer to Eqs. (26.37) and (26.38)].