UJT Oscillator Triggering and UJT Relaxation Oscillator

• The UJT (Uni Junction Transistor) is a semiconductor device that has only one junction, and it is widely used in relaxation oscillators. The UJT relaxation oscillator utilizes the characteristics of the UJT, including its negative resistance region, to generate a timing interval.
• The unijunction transistor (UJT) can function as a highly efficient switch and a relaxation oscillator.
• The external resistances R₁, R₂ are small in comparison with the internal resistances R_B1,R_B2 of UJT bases. 
• The charging resistance R should be such that its load line intersects the device characteristics only in the negative resistance region.
• In the oscillator mode, when a source voltage V_BB is applied, the capacitor C charges through a resistor R towards V_BB.
• During this charging, emitter circuit of UJT is an open circuit. the capacitor voltage V_c equal to emitter voltage V_e is given by V_c = V_e = V_BB(1-e^-t/RC).
• The time constant of the charge circuit is τ₁ = RC.
• When the emitter voltage V_e (or V_c) reaches the peak-point voltage V_p (= ηV_BB +V_D),the UJT turns on, and the capacitor C rapidly discharges through a low resistance R₁ with a time constant τ₂ = R₁C.
•  Here τ₂ is much smaller than τ₁. 
• When the emitter voltage decays to the valley-point voltage V_v,UJT turns off.
• The time T required for the capacitor C to charge from the initial voltage V_v to the peak-point voltage V_p through the resistance R can be obtained using the equation T = 1/f = RC In [1/(1 - η)].
• The firing angle α₁, which represents the time delay before the UJT turns on, can be calculated as α₁ = ωT = ωRC In [1/(1-η)].where ω is the angular frequency of UJT oscillator.
• The amplitude of the pulse voltage is obtained by drawing a load line Bb for R on the UJT characteristic curve.
• The value of R₁ should be sufficiently small to ensure that the leakage current drop across R₁ when the UJT is off does not trigger the SCR.
• The value of R₂ can be calculated from the relation R₂ = 10⁴ / (ηV_BB) to compensate for the thermal effects on the UJT's forward characteristics.
• The width of the triggering pulse can be taken as R₁C.
• The circuit cannot function as an oscillator if the load line for R intersects the UJT characteristics in the region CQ.
• The maximum and minimum values of the charging resistor R and the oscillator output frequency are determined by the peak-point and valley-point values of the UJT's voltage and current.
• The maximum value of R is given by R_Max = [V_BB - (ηV_BB + V_D)] / I_p.
• The minimum value of R is given by R_Min = (V_BB - V_v) / I_v.
• These characteristics and modes of operation make the UJT an excellent switch for applications requiring fast switching times, typically in the nanosecond range. The UJT relaxation oscillator takes advantage of these properties to create a timing interval using a charging capacitor and the rapid discharge of the same capacitor.

Unijunction transistor oscillator circuit diagram and waveform

Applications of relaxation oscillator

Relaxation oscillators are used in function generators, electronic beepers, SMPS (Switched-Mode Power Supplies), inverters, blinkers, and voltage controlled oscillators.