Design condition for a UJT relaxation oscillator When using a UJT as a relaxation oscillator, what should the source/emitter current condition be for stable oscillation?

Introduction / Context:
A UJT relaxation oscillator charges a capacitor through a resistor until the UJT reaches its peak-point condition, then rapidly discharges until the current falls below the valley current. Proper resistor choice ensures reliable turn-on and turn-off each cycle. This question asks for the correct current condition relative to the peak current Ip and the valley current Iv.

Given Data / Assumptions:

• UJT parameters: peak current Ip (turn-on threshold) and valley current Iv (turn-off threshold).
• Capacitor charges via a resistor from a DC supply.
• Oscillation requires periodic triggering and quenching.

Concept / Approach:

To trigger, the emitter current at the instant the capacitor reaches the peak-point voltage must exceed Ip so the UJT switches on and discharges the capacitor. For oscillation to stop and allow the next cycle, the current through the UJT path after discharge must fall below Iv so the device turns off. Therefore, the design aims for source/emitter current capability that is greater than Ip at firing but less than Iv once the capacitor has discharged and emitter voltage collapses.

Step-by-Step Solution:

Verification / Alternative check:

Application notes specify R selection so that VBB/R < Iv (turn-off ensured) and timing ensures Ip is surpassed at V = Vp (turn-on ensured).

Why Other Options Are Wrong:

“Less than Ip” prevents turn-on; “more than Iv” risks latch-on preventing oscillation; negative current is non-physical in the standard biasing.

Common Pitfalls:

Confusing instantaneous current at firing with steady bias current; ignoring Iv requirement.

Final Answer:

more than Ip but less than Iv