Design
What the domain tests
Design questions assess a structured process: clarify the need, write specifications, generate and select concepts, calculate values, build a prototype, test against criteria, and document. The exam rewards justified choices, tolerance awareness and safe, economical engineering.
Core rules
- A specification should be measurable: voltage, current, power, size, cost, reliability, environmental range and safety requirements.
- Select components with margin. Resistor power rating should exceed calculated dissipation; capacitor voltage rating should exceed expected peak voltage.
- Account for tolerance. A 5% resistor can be 5% high or low, so check worst-case current or voltage.
- Design is iterative. Test results may require changes to values, layout or specification.
Worked example
Design an LED indicator from a 5 V supply. LED forward voltage 2.0 V, desired current 20 mA. Series resistor R = (5 − 2)/0.02 = 150 Ω. Resistor power P = I²R = 0.02² × 150 = 0.06 W, so a 0.25 W resistor gives margin. With a 5% tolerance, R ranges 142.5–157.5 Ω; current ranges about 21.1–19.0 mA, acceptable for the LED. If the supply rises to 5.25 V, current becomes (5.25 − 2)/150 = 21.7 mA, still reasonable.
Traps that cost marks
Ignoring power rating causes overheating. Selecting a resistor from calculation alone without checking standard values and tolerance is weak design. Do not omit test criteria or safety constraints; a design that cannot be verified is incomplete.