Relaxation oscillators in electronics Which device is most commonly used to build a simple, stable relaxation oscillator for timing and triggering applications?

Introduction / Context:
Relaxation oscillators generate repetitive waveforms (typically sawtooth or pulse) using a charge–discharge process and a threshold device. They are widely used for timing, triggering SCRs, and generating clock pulses in low-to-medium frequency ranges.

Given Data / Assumptions:

• Devices considered: UJT, tunnel diode, PIN diode.
• Goal: a simple, economical, and predictable RC-based oscillator.

Concept / Approach:
The UJT features a distinct negative-resistance region and a well-defined intrinsic stand-off ratio, making it ideal for RC charge–discharge oscillators. When the capacitor voltage reaches the peak point, the UJT conducts rapidly, discharging the capacitor to the valley point, then turns off, and the cycle repeats—creating a relaxation oscillation.

Step-by-Step Solution:
Use RC network to charge a capacitor toward a supply.When capacitor voltage hits UJT peak voltage (≈ ηV_BB + V_D), the UJT turns on.Capacitor discharges through the UJT until the valley point, where the UJT turns off.Capacitor starts charging again—completing the oscillation cycle.

Verification / Alternative check:
Classic SCR trigger circuits and many textbooks demonstrate UJT-based relaxation oscillators due to their simplicity and reliable frequency control via RC.

Why Other Options Are Wrong:
Tunnel diode: Can form relaxation oscillators, but is less common in basic power electronics labs and triggers compared to UJT.

Common Pitfalls:
Assuming any diode with nonlinearity will suffice. UJT’s characteristics and three-terminal control make it the go-to device.

Final Answer:
UJT (uni-junction transistor)