For a TRIAC used in AC phase control, which statement about gate triggering polarities is correct?

Introduction / Context:
TRIACs are bilateral thyristors that conduct in both directions and can be triggered in multiple quadrants. Understanding permitted gate polarities is crucial for reliable dimmer and motor-control designs.

Given Data / Assumptions:

• TRIAC terminals: MT1, MT2, and Gate (G).
• Device supports multiple triggering quadrants depending on the polarities of MT2 and gate relative to MT1.
• Typical circuits use DIACs or direct resistive/capacitive networks for triggering.

Concept / Approach:
TRIACs can be triggered with either positive or negative gate current relative to MT1, enabling conduction in both half-cycles. Trigger current thresholds differ slightly by quadrant, but both polarities are valid and commonly used.

Step-by-Step Solution:
Recognize bilateral conduction: TRIAC conducts for either MT2 polarity after triggering.Gate triggering: Provide sufficient I_G in the appropriate quadrant (I–IV) as per datasheet.Result: A TRIAC may be triggered by positive or negative gate drive.Therefore, option stating “either positive or negative” is correct.

Verification / Alternative check:
Triggering curves in datasheets show different I_GT for quadrants I–IV; all are feasible with proper design, confirming bidirectional gate triggering capability.

Why Other Options Are Wrong:

• a) and b): Restrict to one polarity; not generally true.
• c): Contradicts the basic TRIAC operation which depends on gate triggering.
• e): Limiting to one quadrant is incorrect and overly restrictive.

Common Pitfalls:

• Ignoring quadrant-dependent gate current requirements, leading to unreliable firing.
• Failing to provide proper dv/dt snubbing, causing unintended triggering.

Final Answer:
can be triggered by either a positive or a negative gate voltage