Device used in relaxation oscillators for triggering and timing Which device is commonly used to build a relaxation oscillator because of its negative-resistance switching characteristic?

Introduction / Context:
Relaxation oscillators generate sawtooth or pulse waveforms using a capacitor that charges through a resistor and discharges abruptly through a device with a negative-resistance region.

Given Data / Assumptions:

• We seek the device typically used for simple, low-cost relaxation oscillators.
• Focus on negative-resistance triggering behavior.

Concept / Approach:
The unijunction transistor (UJT) has a distinct peak and valley characteristic. When the capacitor voltage reaches the peak point, the UJT conducts suddenly, discharges the capacitor to the valley point, and then turns off, producing repetitive oscillations.

Step-by-Step Solution:
Charge capacitor through R until V_C rises to ηV_BB + V_D.At peak point, UJT turns on → rapid discharge through emitter path.After reaching valley point, UJT turns off → cycle repeats.

Verification / Alternative check:
Classic lamp dimmers and SCR triggering circuits use UJT relaxation oscillators to generate timing pulses.

Why Other Options Are Wrong:
SCR/DIAC/TRIAC: Can participate in triggering but are not the primary negative-resistance device used alone for RC relaxation oscillators.
BJT: Requires additional biasing; not inherently a relaxation device.

Common Pitfalls:
Confusing UJT-based oscillators with DIAC-triggered TRIAC dimmers; the UJT directly creates the relaxation action.

Final Answer:
UJT