UJT Relaxation Oscillator

UJT Relaxation Oscillator

The relaxation oscillator shown in figure consists of UJT and a capacitor C which is charged through resistor R_E when inter base 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 through B_1. The resulting current through the external resistor R develops a voltage spike, as illustrated in figure 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 saw-tooth waveform across capacitor C. The resulting waveforms of capacitor voltage V_C and the voltage across resistor R (V_R) are shown in figure. The frequency of the input saw- tooth 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 oscillator the following two conditions for the turn-on and turn-off of the UJT must be met.

R_E < V_BB – V_P / I_P and R_E > V_BB – V_V / I_V

That is the range of resistor R_E should be as given below

V_BB – V_P / I_P > R_E >  V_BB – V_V / I_V

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

V_c = V_BB(l-e^-/ReC)

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, that is, (ȠV_BB + V_B)

V_P = ȠV_BB + V_B = V_BB(l-e^-/ReC) Neglecting V_B in comparison to ȠV_BB we have

ȠV_BB = V_BB(l-e^-1/ReC)

Or e^-t1/ReC = 1 – Ƞ

So charging time period, t₁ = 2.3 R_E C log₁₀ 1/1- Ƞ

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 saw-tooth wave, T = 2.3 R_E C log₁₀ 1/1- Ƞ and frequency of oscillation f = 1/T = 1/2.3R_EClog₁₀ (1-Ƞ)

By including a small resistor in each base circuit, three useful outputs (saw- tooth waves, positive triggers, and negative triggers), as shown in figure, can be obtained. When the UJT fires, the surge of current through B_t causes a voltage drop across R₁ and produces the positive going spikes.

Also  at  the  UJT  firing         time,  the  fall of  V_EB causes I_B to  rise  rapidly  and generate the negative-going spikes across R₂, as shown in figure. 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 blog post there is upper and lower limits to the signal source resistance R_E for the satisfactory operation of the UJT.