Circuit Symbols, Voltage and Current Rules
Why Symbols and Rules Matter
Before any circuit can be built or analysed, it must be drawn correctly. Circuits must use the correct standard symbols — a battery drawn as a single line scores no marks even if the rest of the circuit is perfect. Equally, getting the voltage and current rules wrong can cascade errors through every subsequent calculation. This lesson gives you the foundations everything else builds on.
Quantities, Units and Symbols
Three quantities appear in almost every electronics calculation. Confusing their units is the most common error in exams.
Quantity — Symbol — Unit — Unit symbol:
Voltage — V — volt — V
Current — I — ampere — A
Resistance — R — ohm — Ω
Always include the unit symbol in your answer. Writing “5” when the answer is “5 V” will cost you a mark. Voltage is measured in volts (V), current in amperes (A) and resistance in ohms (Ω). These three are linked by Ohm’s law, covered in the next lesson.
Standard Circuit Symbols
Circuit diagrams use internationally agreed symbols. You are expected to know and draw the symbols for these components: cell, battery (multiple cells), resistor (rectangle), variable resistor (rectangle with an arrow), lamp, LED (arrow pointing away from the triangle), diode (triangle with a bar), capacitor (two parallel lines), switch, relay, fuse, voltmeter (circle with V), ammeter (circle with A), LDR (resistor with arrows pointing inward), thermistor (resistor with a T or slanted line), npn transistor, MOSFET, operational amplifier (triangle with + and − inputs), AND gate, OR gate, NOT gate, NAND gate and NOR gate.
Use only these standard symbols in exam circuit drawings. A rough sketch of a real component — or any invented symbol — scores zero. The specification appendix shows the exact forms required.
Series Circuits
In a series circuit, all components sit on a single loop. Two key rules apply:
- Current is the same everywhere. If 20 mA flows through one resistor in a series chain, 20 mA flows through every other component in that chain.
- Voltages add up. The sum of the voltages across each component equals the supply voltage: \(V_S = V_1 + V_2 + V_3\). This is a consequence of Kirchhoff’s Voltage Law.
Series circuit rules: current is the same at every point; voltages across each component add up to the supply voltage: \(V_S = V_1 + V_2 + \ldots\)
Parallel Circuits
In a parallel circuit, each branch connects directly between the same two points. Two rules apply:
- Voltage is the same across every branch. A 9 V supply means 9 V appears across every parallel branch.
- Currents add up at junctions. The total current from the supply equals the sum of the currents in all parallel branches: \(I_T = I_1 + I_2 + I_3\). This is Kirchhoff’s Current Law.
Series example: A 9 V supply drives two resistors in series. Voltmeter across R1 reads 4 V. What is the voltage across R2?
\(V_S = V_1 + V_2\) ⇒ \(9 = 4 + V_2\) ⇒ \(V_2 = 5 \text{ V}\)
Parallel example: Two branches carry 30 mA and 20 mA. What current does the supply deliver?
\(I_T = I_1 + I_2 = 30 + 20 = 50 \text{ mA}\)
Test Equipment
You need to know how to connect and use four instruments:
- Voltmeter — always connected in parallel with the component being measured. Has very high input resistance so it draws negligible current.
- Ammeter — always connected in series in the branch being measured. Has very low resistance so it introduces minimal voltage drop.
- Logic probe — detects HIGH or LOW logic levels in digital circuits. It lights a different LED for logic 1 and logic 0, useful for testing gate outputs quickly.
- Oscilloscope — displays a graph of voltage against time. The horizontal axis is time (controlled by the time-base setting, in ms/div or μs/div); the vertical axis is voltage (controlled by the Y-gain, in V/div or mV/div). Used to measure waveform amplitude, period and frequency.
A multimeter can measure voltage, current or resistance. Selecting the wrong mode — for example, connecting it in parallel on the current setting — can damage the instrument because it presents almost zero resistance across the supply.
A signal generator produces test waveforms. Three types appear in the specification:
Sine wave — a smoothly varying AC signal. The oscilloscope shows a regular S-curve. Amplitude is the peak height above zero, measured in volts from the centre line to the peak (NOT peak-to-peak).
Square wave — switches instantly between HIGH and LOW. Used to test digital circuits. Each half-period at HIGH and LOW should be equal for a 50% duty cycle.
Triangular wave — rises and falls linearly at a constant rate. Used for testing linearity of circuits.
Reading an oscilloscope: count the number of divisions for one complete cycle horizontally and multiply by the time-base value to get the period T. Frequency \(f = \frac{1}{T}\). For amplitude, count divisions from the centre to the peak and multiply by the Y-gain.