Triac conduction — operating quadrants under AC excitation: “All triacs operate in __________.” Choose the pair of I–V plane quadrants that correspond to normal current/voltage conduction regions of a triac device (not the gate-trigger quadrants).

Introduction / Context:
A triac is a bidirectional thyristor that conducts current in both polarities of an AC cycle once triggered. Understanding its operation on the voltage–current (V–I) plane prevents confusion between conduction quadrants and gate-triggering quadrants used in datasheets.

Given Data / Assumptions:

• Standard quadrant definition: voltage across the device on the horizontal axis, current through the device on the vertical axis.
• Positive voltage/current → quadrant I; negative voltage/current → quadrant III for symmetric conduction.
• Gate triggering can occur in I, II, III, or IV, but conduction regions are of interest here.

Concept / Approach:
Once turned on, a triac behaves like two antiparallel SCRs, allowing current to flow for both half-cycles of AC. Thus, it conducts in quadrant I (positive voltage and current) and quadrant III (negative voltage and current). Triggering sensitivity varies by quadrant, but the steady conduction regions remain I and III for power flow. This makes triacs especially suitable for AC control where bidirectional conduction is required without two separate devices.

Step-by-Step Solution:

Verification / Alternative check:
Manufacturer application notes depict conduction lobes in quadrants I and III, and list gate-trigger modes by quadrants I–IV for sensitivity, reinforcing the distinction between where it can be triggered and where it primarily conducts.

Why Other Options Are Wrong:

• Quadrants 2/4: Would imply voltage and current of opposite signs, not the standard conduction states.
• Quadrant 1 only or 3/4 only: A triac is bidirectional by design.

Common Pitfalls:
Confusing “trigger quadrant” (gate polarity combinations) with “conduction quadrant”; mixing triac behavior with SCRs which conduct in one polarity.

Final Answer:
quadrant 1 and quadrant 3.