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VCE Systems Engineering (Year 11) · Systems Engineering Skills · 12m read

Components, Multimeters and Electrical Safety

Before building a system you need to know your components and how to test them safely. These practical skills make your production work reliable.

Learning objective: Identify common electronic components, measure circuits with a multimeter and work safely with extra-low voltage.

By the end of this lesson you can:

  • I can describe the function of common components.
  • I can calculate a current-limiting resistor for an LED.
  • I can measure voltage and current correctly with a multimeter.

Knowing your toolbox. A carpenter knows what each tool does before starting a job. A systems engineer knows what each component does before building a circuit.

Components and measurement

COMPONENTS
• Resistor — limits current (value shown by colour bands).
• LED — light-emitting diode; needs a series resistor; has polarity (long leg = anode +).
• Diode — allows current one way only (protects circuits).
• Switches — push-button, toggle, microswitch, reed switch.
• Capacitor — stores charge; used in timing and smoothing.
• Transistor — electronic switch or amplifier; lets a small signal control a larger current (e.g. a motor).
• Relay — electrically operated switch for higher-power loads.

MULTIMETER
• Voltage: connect in PARALLEL across the component.
• Current: connect in SERIES (break the circuit).
• Resistance and continuity: test with the power OFF.

SAFETY — school projects use extra-low voltage (ELV), defined in Australian wiring rules as not more than 50 V AC or 120 V ripple-free DC. Never work on mains (230 V). Watch for short circuits and hot components; use eye protection when soldering, ventilation for flux fumes, and a soldering iron stand.

Worked example (Medium)

An LED (forward voltage 2 V, current 20 mA) runs from a 9 V battery. Calculate the series resistor and choose a standard value.

  1. Voltage across resistor. Supply minus LED forward voltage. V_R = 9 - 2 = 7\ \text{V}
  2. Resistance. Ohm’s law with 20 mA = 0.02 A. R = \dfrac{7}{0.02} = 350\ \Omega
  3. Standard value. Round UP to the next standard (E12) value so current stays below the maximum. 390\ \Omega

Answer: Calculated 350 Ω; use a 390 Ω resistor.

Common mistakes

  • Measuring current with the meter in parallel This creates a short circuit and can blow the meter’s fuse. Fix: Put the ammeter in series.
  • Rounding an LED resistor down More current flows and can damage the LED. Fix: Round up to the next standard value.

Practice questions

  1. To measure voltage across a resistor, the multimeter is connected:

    1. In series
    2. In parallel
    3. With power off
    4. Across the battery only

    (1 marks)

  2. Which component allows a microcontroller pin to switch a motor that needs more current?

    1. Capacitor
    2. Transistor
    3. LED
    4. Fuse

    (1 marks)

  3. An LED needs 2 V at 15 mA from a 5 V supply. Calculate the series resistor.

    (2 marks)

Summary

  • Know resistors, LEDs, diodes, switches, capacitors, transistors and relays.
  • Voltmeter in parallel; ammeter in series.
  • LED resistor: (supply − forward voltage) ÷ current, round up.
  • Work only with extra-low voltage and follow safety rules.

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