Operation of Unijunction Transistor Relaxation Oscillator

The unijunction transistor (UJT) relaxation oscillator is a simple electronic circuit that generates a periodic waveform without the need for external input signals. It consists of a UJT, a capacitor, resistors, and a power supply.

Working Principle:
The UJT relaxation oscillator operates on the principle of capacitor charging and discharging through the UJT. The UJT is a three-layer, two-junction semiconductor device that behaves as a negative resistance device. It has a unique characteristic of having a minimum point in its voltage-current characteristics.

Components of Unijunction Transistor Relaxation Oscillator:
1. Unijunction Transistor (UJT): The UJT is the heart of the relaxation oscillator circuit. It is a three-terminal device with an emitter terminal (E) and two base terminals (B1 and B2).

2. Capacitor (C): The capacitor is connected in parallel with the emitter terminal of the UJT. It charges and discharges through the UJT, creating the oscillating waveform.

3. Resistors (R1 and R2): The resistors are connected in series with the UJT and the power supply. They control the charging and discharging time constants of the capacitor, thus affecting the frequency of the generated waveform.

4. Power Supply: The power supply provides the necessary voltage for the circuit to operate.

Operation:
1. Charging Phase: Initially, the capacitor is discharged, and the UJT is in a non-conducting state. The power supply voltage charges the capacitor through resistor R1. As the capacitor voltage increases, it reaches a point where it exceeds the UJT's intrinsic standoff voltage (Vp).

2. Peak Point and Triggering: When the capacitor voltage reaches the peak point voltage (Vp), the UJT enters the conduction region. This triggers the UJT to turn on, and it starts conducting current from the emitter to the base terminal B1.

3. Discharging Phase: As the UJT starts conducting, the capacitor begins to discharge through the UJT. The discharge current flows through resistor R2 and the UJT, causing the voltage across the capacitor to decrease.

4. Valley Point and Reset: When the capacitor voltage reaches the valley point voltage (Vv), the UJT turns off, and the current flow stops. This resets the UJT, and the capacitor starts charging again, repeating the cycle.

Frequency Control:
The frequency of the relaxation oscillator can be controlled by adjusting the values of resistors R1, R2, and the capacitor C. Higher resistor values and larger capacitor values result in longer charging and discharging times, leading to a lower frequency. Lower resistor values and smaller capacitor values, on the other hand, result in shorter times and higher frequency.

Advantages of UJT Relaxation Oscillator:
- Simple circuit design and easy to construct.
- Low component count and cost-effective.
- Generates a stable and continuous waveform without external input signals.

Applications:
- Timing circuits
- Signal generation
- Frequency modulation
- Pulse generation
- Test and measurement equipment