**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 B_{1}. 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_{E}C) of the capacitor charging circuit. The discharge time t_{2 }is difficult to calculate because the UJT is in its negative resistance region and its resistance is continually changing. However, t_{2} is normally very much less than t_{1} 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_{E}C 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 t_{1}, 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 t_{2 }is negligibly small as compared to charging time duration t_{1}, so taking time period of the wave, T = t_{1}

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 B_{1} causes a voltage drop across R_{1} 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 R_{2}, as shown in Fig. 26.95. R_{1} and R_{2} 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)].