Electronic Components, Schematics and Measurement (Review and Extension)
How do I read a circuit diagram and prove with a meter what the circuit is really doing?
- Distinguish active from passive components.
- Identify resistors, capacitors, diodes, transistors and ICs and state their function.
- Interpret standard schematic symbols and trace a circuit onto a physical board.
- Measure voltage and resistance and perform continuity tests with a digital multimeter.
Overview
Term 1 covered basic electronic components and their specifications in depth. Here we revise the identification quickly and spend the time on two technician skills: reading a schematic onto a real board, and using a multimeter to gather evidence instead of guessing. Read the Term 1 chapters 'Basic Electronic Components' and 'Data Books & Component Specifications' alongside this one.
Passive and active components
Passive components cannot amplify or control current gain; active ones can.
Identify and state the function:
- Resistor (passive): Limits current and divides voltage. Value read from the colour bands or printed code.
- Capacitor (passive): Stores charge; smooths supplies and blocks DC while passing AC.
- Inductor (passive): Stores energy in a magnetic field; used in filters and supplies.
- Diode (active): Allows current in one direction only; used in rectifiers and protection. An LED emits light.
- Transistor (active): Switch or amplifier — the building block of all digital logic.
- Integrated circuit (active): A complete circuit on one chip, such as a voltage regulator, op-amp or microcontroller.
Cross-link: see Term 1 · 'Basic Electronic Components' for colour codes and full specifications.
From schematic to board
A schematic shows the electrical connections, not the physical layout. Learn the standard symbols, then find the same components on the printed circuit board by their reference designators: R for resistors, C for capacitors, D for diodes, Q for transistors, U or IC for integrated circuits. Trace a net by following the copper track or by using the multimeter's continuity function between two points.
Using a digital multimeter safely
The meter is your evidence-gathering tool. Set it correctly or you will damage it.
Three measurements you must be able to make:
- Voltage (DC): Measured in parallel, across the component, with the circuit powered. Check PSU rails: +3.3 V, +5 V, +12 V.
- Resistance: Measured with the power OFF and the component ideally out of circuit. Confirms a resistor's value or a burnt-out track.
- Continuity: Power off; the meter beeps when the path is complete. Finds broken cables, blown fuses and cracked joints.
Never measure resistance on a live circuit, and never place the meter in current mode across a voltage source.
Practical — Measure and diagnose
- Identify ten components on a scrap circuit board and record the reference designator, type and likely function of each.
- Read the value of five resistors from their colour bands, then measure each one and record the difference.
- Perform a continuity test on three cables, one of which your teacher has damaged, and identify the faulty one.
- Measure the +5 V and +12 V rails of a bench power supply and state whether they are within ±5% tolerance.
- Give one difference between a passive and an active component.
Reveal answer
Active components can amplify or control current gain; passive components cannot.
- Why must resistance be measured with the power off?
Reveal answer
A live circuit will give a false reading and can damage the meter.
- What does a continuity beep tell you?
Reveal answer
That there is an unbroken electrical path between the two probe points.
- What does the designator C7 refer to on a board?
Reveal answer
The seventh capacitor on that board.
- Which PSU rails would you check on a desktop?
Reveal answer
+3.3 V, +5 V and +12 V.
Draw the schematic for a simple LED-and-resistor circuit powered by 5 V, and calculate the resistor value needed for a 20 mA LED with a 2 V forward voltage.