DIAC fundamentals — conduction direction and control:\nConsider the DIAC (diode for alternating current). The statement claims: “The DIAC conducts in only one direction when gated on.” Evaluate this claim about its conduction characteristics and triggering.

Introduction / Context:
The DIAC (diode for alternating current) is widely used as a symmetrical trigger device in AC phase-control circuits, commonly paired with TRIACs for lamp dimmers and speed controllers. This question probes whether a DIAC conducts in only one direction and whether it needs a gate like an SCR or TRIAC.

Given Data / Assumptions:

• Device under discussion: DIAC.
• Operating context: AC applications with alternating polarity.
• Idealized understanding of its I–V characteristics, ignoring minor asymmetry between polarities.

Concept / Approach:
A DIAC is a bidirectional, two-terminal trigger device. It exhibits a breakover voltage of roughly the same magnitude in both polarities. Below breakover, only leakage flows; once the applied voltage exceeds its threshold, the device switches to a low-impedance state and conducts. Because it is two-terminal and symmetrical, it does not have a gate terminal. Therefore, it does not conduct “in only one direction,” nor is it “gated on.”

Step-by-Step Solution:

Verification / Alternative check:
Standard DIAC datasheets plot nearly symmetrical V–I curves in the first and third quadrants, confirming conduction in both polarities after threshold is exceeded. Application notes show DIACs triggering TRIAC gates during both half-cycles of mains.

Why Other Options Are Wrong:

• Correct: Not correct—contradicted by the device’s bidirectional nature.
• Applies only in DC circuits: DIACs are intended for AC; DC operation is atypical and misses the point.
• True for negative half-cycles only: Incorrect; both polarities conduct after breakover.
• Depends on the gate resistor value: DIACs have no gate; a gate resistor applies to TRIACs.

Common Pitfalls:
Confusing DIACs with diodes or TRIACs; assuming the presence of a gate; forgetting that DIACs are symmetric trigger devices, not rectifiers.

Final Answer:
Incorrect