SOLUTION

UJT Relaxation Oscillator

A UJT relaxation oscillator consists of a capacitor C and a resistor R connected to the emitter of the UJT, as shown in Fig. Base B₂ of the UJT is connected to the power-supply positive through resistor R₂, and base B₁ is connected to the ground through resistor R, Usually, R₁ and R₂ are low-value resistors, a typical value being 47 ohms each. These are used to limit the current through the UJT and protect it. We can take three outputs from the UJT relaxation oscillator. These are Vo₁, V_o2, and V_o3, respectively, as shown in Fig.

An extensive application of UJT as a relaxation oscillator is SCR triggering. The basic circuit for gate triggering of an SCR is shown in Figure. It consists of R and C timing elements connected with the UJT to form an oscillator circuit. Also, voltage +V is applied to the circuit. UJT produces output pulses at B₁ which are applied at the gate (G) terminal of SCR. As the SCR gets triggered, the subsequent pulses at the gate have no effect.

The working of the oscillator can be explained as follows:

• Initially, the UJT is in the OFF-state, and the capacitor C is uncharged. C now gets charged through R towards V_BB, 
• The UJT gets fired and C gets discharged through the now turned-on UJT. C discharges to the valley voltage V_v, at which the UJT turns off and C stop discharging.
• Once the UJT stops conducting and C discharges to Vv, we find that C again starts conducting, and the whole operation repeats. It can be seen that the charging and the discharging of the capacitor produce across C a type of sweep, or sawtooth waveform, as shown in Fig. 
• This figure shows C charging towards VBB in time T₁, and discharging towards V_v in time T₂, producing a sawtooth waveform, with a period  T = T₁ + T₂.
• Usually, the UJT discharges with a very low ON-resistance, so that T₂ << T₁, and hence can be neglected. Thus, the period of oscillation is determined mainly by T = T₁.
• For producing oscillations, UJT operates between the peak point and the valley point. Since they are the stable operating points of the UJT, we say that the UJT operates in its relaxed modes of operation. So, we call the UJT oscillator a relaxation oscillator.