Questions: Circuit Design Basics

Question 1

An LED requires 20 mA of current and has a 2 V forward voltage drop. If powered by a 9 V battery, what value resistor is needed in series with the LED?

• A. 100 Ω
• B. 350 Ω
• C. 450 Ω
• D. 9 Ω

Answer: (B)

The resistor must drop the remaining voltage: 9 V - 2 V = 7 V. Using Ohm's Law: R = V/I = 7 V / 0.020 A = 350 Ω. This current-limiting resistor protects the LED from excessive current.

Question 2

A circuit schematic uses the actual physical appearance of components in its drawings.

• T. True
• F. False

Answer: False

Schematics use standardized symbols (zigzag lines for resistors, parallel lines for capacitors, etc.) rather than realistic drawings. This makes schematics universal -- engineers worldwide can read them regardless of the specific physical components used.

Question 3

Why must an engineer check the power rating of a resistor, not just its resistance value?

Model answer: A resistor dissipates power as heat (P = I²R). If the actual power exceeds the resistor's rating, it will overheat, potentially burning out, catching fire, or damaging nearby components. The resistance value determines current flow, but the power rating determines how much heat the resistor can safely handle.

A 100 Ω resistor rated for 0.25 W is fine carrying 50 mA (P = 0.05² × 100 = 0.25 W), but the same resistor at 100 mA would dissipate 1 W -- four times its rating. The engineer must either use a larger resistor or redesign the circuit to reduce current.