Practical / Laboratory Skills and Circuit Construction
What the exam tests
Practical / Laboratory Skills and Circuit Construction tests safe bench work, component identification, breadboarding, soldering, PCB handling and fault-finding. The examiner looks for correct polarity, current-limiting, isolation and a logical test sequence.
Core rules
- Before powering a circuit, check supply polarity, rail continuity and for solder bridges. Use a current-limited supply or a series bulb for first power-up.
- Breadboard: power rails run vertically; the centre rows are connected horizontally in groups of five. An IC straddles the centre channel.
- LED series resistor: R=(Vsupply-Vf)/I. Choose the next standard value upward, then recalculate the actual current.
- Soldering: clean tip, 300-350 °C, heat the joint and feed solder-not the iron. A good joint is shiny and concave; a cold joint is dull and grainy.
- Electrolytic capacitors and diodes are polarised. Reversed electrolytics can vent; reverse-biased LEDs do not light.
- Use an isolation transformer when servicing mains-powered equipment, and never work alone on live circuits.
Worked example
A red LED has Vf=2.0 V and a target current of 15 mA from a 5 V rail. R=(5.0-2.0)/0.015=200 Ω. The nearest standard value is 220 Ω, giving I=(5.0-2.0)/220=13.6 mA, which is safe and still visible. The resistor dissipates I²R=(0.0136)²×220=0.041 W, so a 0.25 W part is more than adequate.
Traps
Connecting an LED directly across the supply; putting an electrolytic capacitor in backwards; and trusting a breadboard's rail continuity without checking it. When a circuit fails, measure supply voltages first, then signal levels, rather than replacing components at random.