Unit 2: Electrotechnology and Control Systems
Core concepts
Unit 2 introduces electrical and electronic systems alongside control concepts. Candidates must understand basic electrical quantities - voltage (V), current (I), resistance (R), power (P) - and apply Ohm's law (V = IR) and power formulas (P = VI = I²R = V²/R). You should understand series and parallel circuits, calculating equivalent resistance and current distribution. Knowledge of components (resistors, capacitors, diodes, LEDs, transistors as switches, relays) and circuit symbols is essential, along with safety (fuses, earthing, insulation).
Control systems content includes open-loop vs closed-loop systems, sensors, actuators, and feedback. Students must interpret block diagrams and describe how a controller responds to input signals.
Traps in questions
Frequent traps include mixing series and parallel rules: in series, R_total = R1 + R2 + … and current is the same; in parallel, 1/R_total = 1/R1 + 1/R2 + … and voltage is the same across branches. Another trap is confusing power with energy (energy = power × time, in joules or kWh). Students also misidentify open-loop systems as having feedback when they do not. Be careful with LED polarity and diode direction - current flows from anode to cathode. Also watch for unit prefixes (mA, kΩ, MΩ) causing calculation errors.
Worked example / rule of thumb
Two resistors, 6 Ω and 3 Ω, in parallel: 1/R = 1/6 + 1/3 = 1/2, so R = 2 Ω. With a 12 V supply, total current I = V/R = 12/2 = 6 A. Rule of thumb: in parallel, equivalent resistance is always less than the smallest resistor. Always convert prefixes before calculating and check whether the circuit is series, parallel, or mixed.
Study guidance
Practice drawing and interpreting circuit diagrams, calculating values in mixed circuits, and describing feedback in control systems. Link sensors to real applications.