Choosing a safe resistor rating: For a carbon-composition resistor expected to dissipate up to 1.2 W continuously, which nominal power rating should be selected?

Introduction / Context:
Proper power derating is critical for reliability. Resistors should be chosen with a power rating above the expected dissipation to handle tolerances, ambient temperature, and intermittent surges without overheating.

Given Data / Assumptions:

• Maximum expected dissipation ≈ 1.2 W.
• Standard resistor power ratings are considered (e.g., 0.5 W, 1 W, 2 W, 5 W).
• Continuous operation and typical ambient conditions.

Concept / Approach:
Select the next higher standard rating above the calculated worst-case power. This provides margin for tolerance, temperature rise, and component aging. A common practice is to operate resistors at 50–60% of their rated power for longevity.

Step-by-Step Reasoning:

Verification / Alternative check:
Applying a derating guideline (e.g., use ≤ 60% of rating): 1.2 W / 0.6 = 2.0 W minimum rating. This corroborates the 2 W choice.

Why Other Options Are Wrong:

• 1 W and 0.5 W: Below the required dissipation, likely to overheat.
• 5 W: Electrically safe but unnecessarily large and costly for a 1.2 W need.

Common Pitfalls:

• Selecting a rating equal to calculated power with no margin.
• Ignoring ambient temperature and ventilation, which further justify using a higher rating.

Final Answer:
2 W