In power electronics, the Unijunction Transistor (UJT) Relaxation Oscillator is a classic circuit used to generate sawtooth waveforms and trigger pulses for SCRs and TRIACs. Its beauty lies in its simplicity, requiring only a handful of components to create a repetitive switching cycle.
Table of Contents
How it Works
The circuit operates by exploiting the negative resistance characteristic of the UJT. The process follows a continuous loop:
- Charging: A capacitor \(C\) charges through a resistor \(R\) from a supply voltage \(V_{BB}\). During this phase, the UJT is in a non-conducting (OFF) state.
- Firing: Once the capacitor voltage reaches the Peak Point Voltage (\(V_p\)), the UJT’s emitter-base junction breaks down and becomes highly conductive.
- Discharging: The capacitor rapidly discharges through the UJT into a small load resistor at Base 1.
- Relaxing: When the voltage drops to the Valley Point (\(V_v\)), the UJT turns OFF, and the cycle repeats.
Key Design Formulas
To ensure the circuit oscillates correctly, the charging resistor \(R\) must stay within specific bounds. If \(R\) is too high, the circuit won’t fire; if it is too low, the circuit won’t turn off.
\[R_{max} = \frac{V_{BB} – V_p}{I_p}\]Minimum Resistance (to ensure turn-off):
\[R_{min} = \frac{V_{BB} – V_v}{I_v}\]
The Peak Point Voltage is determined by the Intrinsic Standoff Ratio (\(\eta\)) and the diode barrier potential (usually \(0.7V\)):
Frequency of Oscillation
The time period \(T\) of the oscillation depends on the RC time constant. It is approximately calculated as:
\[f = \frac{1}{T} \approx \frac{1}{RC \ln\left(\frac{1}{1 – \eta}\right)}\]
Summary Table
| Parameter | Definition | Role in Oscillator |
|---|---|---|
| \(V_{BB}\) | Supply Voltage | Provides energy to the circuit. |
| \(\eta\) | Intrinsic Standoff Ratio | Determines the firing threshold. |
| \(R\) | Charging Resistor | Controls the charging rate and frequency. |
| \(C\) | Timing Capacitor | Stores charge to create the sawtooth wave. |
Conclusion
The UJT Relaxation Oscillator remains a fundamental building block for timing circuits. By selecting the right \(R\) and \(C\) values, engineers can precisely control the frequency of triggering pulses for high-power applications.