1.4 Resistor Colour Code and Component Symbol Identification

ACMA Amateur Radio Standard Recognition Certificate - Theory Exam

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1 Chapter 1: Electrical Principles and Components

1.1 Mathematics, Units and Metric Prefixes 1.2 DC Circuits, Voltage, Current, and Ohm's Law 1.3 Power, Energy, and Resistance Calculations 1.4 Resistor Colour Code and Component Symbol Identification 1.5 Capacitors: Construction, Ratings and Safety 1.6 Inductors: Construction, Factors Affecting Inductance and Combinations 1.7 Capacitor and Inductor Combination Calculations 1.8 AC Circuits, Frequency, Wavelength, and RMS Values 1.9 Reactance and Impedance in AC Circuits 1.10 Resonance, Q Factor and Tuned Circuits 1.11 Transformers: Turns Ratio, Impedance Matching and Eddy Currents 1.12 Rectifiers, PIV and Power Supply Smoothing 1.13 Active Components: Semiconductors, Diodes, and Transistors 1.14 Transistor and FET Symbols, Configurations and External Behaviour

2 Chapter 2: Transmitters and Receivers

2.1 Superheterodyne Receiver Architecture and Block Diagrams 2.2 Frequency Converters, IF Amplifiers and Transceiver Sharing 2.3 Receiver Performance: Sensitivity, Selectivity, and AGC 2.4 Signal-to-Noise Ratio and Receiver Noise Performance 2.5 Transmitter Architecture, Oscillators and Frequency Synthesisers 2.6 Transmission Quality: Frequency Drift, Parasitic Oscillation and Over-Modulation 2.7 Modulation Techniques: AM, FM, and SSB 2.8 Amplifier Classes and Power Amplifiers (Class A, B, C, and AB) 2.9 Modulation, Duty Cycle and Rated Transmitter Power 2.10 Transceiver Control: ALC, Squelch, and Digital Modes 2.11 Digital Transmission Modes and Bandwidth

3 Chapter 3: Transmission Lines and Antennas

3.1 Transmission Lines: Coaxial, Twin-Lead, and Characteristic Impedance 3.2 Standing Wave Ratio (SWR) and Reflection Loss 3.3 Impedance Matching, Baluns, and Antenna Tuning Units (ATUs) 3.4 Resonant Antennas: Dipoles, Verticals, and End-Fed Antennas 3.5 Angle of Radiation, Antenna Height and Current Distribution 3.6 Directional Antennas: Yagis, Gain and Radiation Patterns 3.7 Effective Radiated Power: System Gains and Losses

4 Chapter 4: Radio Wave Propagation

4.1 Electromagnetic Waves: Velocity, Wavelength and Polarisation 4.2 Ground Wave, Direct Wave and Line-of-Sight Propagation 4.3 Ionospheric Layers (D, E, F1, F2) and Skywave Propagation 4.4 Critical Frequency, Maximum Usable Frequency (MUF), and Fading 4.5 Solar Activity, Sunspot Cycles, and VHF Anomalous Propagation 4.6 Seasonal and Diurnal Effects on Band Selection

5 Chapter 5: Electrical and RF Measurements

5.1 Multimeter Operation: Voltage, Current, and Resistance Measurements 5.2 RF Power and SWR Measurement (PEP vs. Average Power) 5.3 Frequency Counters, Dummy Loads, and Test Equipment 5.4 Frequency Standards, Crystal Calibrators and Standard-Frequency Transmissions

6 Chapter 6: Interference and Electromagnetic Compatibility (EMC)

6.1 Harmonic Emissions, Spurious Signals, and Transmitter Filtering 6.2 How Interference Enters Television and Broadcast Receivers 6.3 Masthead Amplifier Overload and Audio-Stage Demodulation 6.4 Electromagnetic Interference (EMI), Breakthrough, Ferrites, and EMC 6.5 Filter Types, Response Curves and EMC Filter Selection 6.6 EMC Good Practice: Minimum Power, Balanced Antennas and Feedline Routing 6.7 Vehicle EMC and Resolving Community Interference Complaints

7 Chapter 7: Safety and Environmental Hazards

7.1 Mains Voltage Safety, Earthing, and High-Voltage Capacitors 7.2 Equipment Approval, Mains Wiring and Fuse Practice 7.3 Station Layout, Cabling and Emergency Shut-Off 7.4 Lightning Protection and Station Security 7.5 Electric Shock: Prevention, Rescue and Emergency Response 7.6 Battery, Chemical and Hearing Safety in the Amateur Station 7.7 RF Exposure: EMR Hazards and Safe Distance 7.8 Antenna Erection, Siting and Power Line Safety

1.4 Resistor Colour Code and Component Symbol Identification

Key Takeaways Explain This Topic AI

In the four-band resistor colour code the first two bands are digits, the third band is the multiplier (the number of zeros) and the fourth band is the tolerance.

The colour-to-digit sequence runs black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9.

As tolerance bands, gold means plus or minus 5%, silver plus or minus 10%, brown plus or minus 1% and red plus or minus 2%.

Brown-black-red-gold decodes as digits 1 and 0 followed by two zeros, giving 1,000 ohms, or 1 kilohm with a 5% tolerance.

In both the NPN and PNP transistor symbols the arrow sits on the emitter lead: it points away from the base for NPN and towards the base for PNP.

Last updated: July 2026

1.4 Resistor Colour Code and Component Symbol Identification

ACMA Exam Focus: Syllabus item 4.6 requires you to identify the value of a resistor using the resistor colour code, and item 4.30 requires you to identify the symbols of NPN and PNP transistors and the field effect transistor (FET). The syllabus defines "identify" as picking the correct object, diagram or matter from a supplied set, and notes that candidates are supplied with reference materials that may include look-up tables, diagrams and photographs. Learn the code anyway — reading it fluently is faster than hunting through a supplied table with the clock running.

1. Why resistors are colour coded

A quarter-watt resistor is a few millimetres long. There is no room for printed digits that would still be legible after the part is soldered into a crowded board, and printed text can end up face-down. Coloured bands are readable from any angle, so the value travels with the component. The bands encode the resistance in ohms and the manufacturing tolerance . They do not encode the power rating — that is signalled only by the physical size of the body.

2. The colour code table

• Colour Digit Multiplier Tolerance
• Black 0 × 1 \times 1 × 1 —
• Brown 1 × 10 \times 10 × 10 ± 1 % \pm 1\% ± 1%
• Red 2 × 100 \times 100 × 100 ± 2 % \pm 2\% ± 2%
• Orange 3 × 1 000 \times 1\,000 × 1 000 —
• Yellow 4 × 10 000 \times 10\,000 × 10 000 —
• Green 5 × 100 000 \times 100\,000 × 100 000 ± 0.5 % \pm 0.5\% ± 0.5%
• Blue 6 × 1 000 000 \times 1\,000\,000 × 1 000 000 ± 0.25 % \pm 0.25\% ± 0.25%
• Violet 7 × 10 000 000 \times 10\,000\,000 × 10 000 000 ± 0.1 % \pm 0.1\% ± 0.1%
• Grey 8 — —
• White 9 — —
• Gold — × 0.1 \times 0.1 × 0.1 ± 5 % \pm 5\% ± 5%
• Silver — × 0.01 \times 0.01 × 0.01 ± 10 % \pm 10\% ± 10%
• (no band) — — ± 20 % \pm 20\% ± 20%

A traditional mnemonic for the digit order is " B ig B rown R abbits O ften Y ield G reat B ig V ocal G roans W hen G ingerly S lapped" — black, brown, red, orange, yellow, green, blue, violet, grey, white, gold, silver.

Notice that the multiplier column is simply the digit column expressed as a power of ten. Orange is digit 3 and multiplier 10 3 10^{3} 1 0 3 ; yellow is digit 4 and multiplier 10 4 10^{4} 1 0 4 . Once you see that, you only have to remember one list.

3. Reading direction, and four bands versus five

Which end do you start from? Two clues resolve it:

The tolerance band is gold or silver on almost every general-purpose resistor, and gold and silver never appear as a first digit. Put the gold or silver band on the right .

The bands are printed grouped towards one end of the body, with a wider gap before the tolerance band. Start from the crowded end.

Four-band (general purpose): band 1 = first digit, band 2 = second digit, band 3 = multiplier, band 4 = tolerance.

Five-band (precision, typically 1% metal film): band 1 = first digit, band 2 = second digit, band 3 = third digit , band 4 = multiplier, band 5 = tolerance. Five-band parts usually carry a brown ( ± 1 % \pm 1\% ± 1% ) or red ( ± 2 % \pm 2\% ± 2% ) tolerance band, which is your cue that you are counting three digits rather than two.

• 4. Worked decodes
• Worked Example 1 — brown, black, red, gold

Brown = 1 , black = 0 , so the digits are 10 .

Red as a multiplier = × 100 \times 100 × 100 , i.e. add two zeros.

10 × 100 = 1 000 Ω 10 \times 100 = 1\,000\;\Omega 10 × 100 = 1 000 Ω , written 1 k Ω \Omega Ω .

Gold = ± 5 % \pm 5\% ± 5% , so the true value lies between 950 Ω \Omega Ω and 1 050 Ω \Omega Ω .

Worked Example 2 — yellow, violet, orange, gold

Yellow = 4 , violet = 7 , so the digits are 47 .

Orange as a multiplier = × 1 000 \times 1\,000 × 1 000 , i.e. add three zeros.

47 × 1 000 = 47 000 Ω 47 \times 1\,000 = 47\,000\;\Omega 47 × 1 000 = 47 000 Ω , written 47 k Ω \Omega Ω .

Gold = ± 5 % \pm 5\% ± 5% , so the true value lies between 44.65 k Ω \Omega Ω and 49.35 k Ω \Omega Ω .

Worked Example 3 — red, red, brown, gold

Red = 2 , red = 2 , so the digits are 22 .

Brown as a multiplier = × 10 \times 10 × 10 , i.e. add one zero.

22 × 10 = 220 Ω 22 \times 10 = 220\;\Omega 22 × 10 = 220 Ω , written 220 Ω \Omega Ω .

Gold = ± 5 % \pm 5\% ± 5% , so the true value lies between 209 Ω \Omega Ω and 231 Ω \Omega Ω .

Worked Example 4 — a five-band part: brown, black, black, brown, brown

Digits 1 , 0 , 0 give 100; brown as the fourth band multiplies by 10.

100 × 10 = 1 000 Ω = 1 k Ω 100 \times 10 = 1\,000\;\Omega = 1\;\text{k}\Omega 100 × 10 = 1 000 Ω = 1 k Ω , and the final brown band means ± 1 % \pm 1\% ± 1% .

The same nominal value as Worked Example 1, but ten times tighter.

5. E12 and E24 preferred values

Resistors are not made in every conceivable value. They are made in preferred value series chosen so that consecutive values just overlap within their tolerance bands, giving full coverage with the fewest part numbers.

E12 (for ± 10 % \pm 10\% ± 10% parts), 12 values per decade: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82.

E24 (for ± 5 % \pm 5\% ± 5% parts) adds the intermediate values: 11, 13, 16, 20, 24, 30, 36, 43, 51, 62, 75, 91.

This is why 47 k Ω \Omega Ω and 22 Ω \Omega Ω exist in every catalogue while 45 k Ω \Omega Ω does not, and it is a useful sanity check in the examination: if your decode produces a value that is not near an E12 or E24 number, re-read the bands.

6. Power rating and physical size

The colour bands say nothing about how much heat the part can shed. That is set by surface area, so physical size is the only visual guide to power rating . As a rough ordering for carbon-film and metal-film axial parts, 0.25 W bodies are roughly 6 mm long, 0.5 W around 9 mm, 1 W around 12 mm and 2 W noticeably larger again. High-power wirewound and ceramic-bodied resistors are usually not colour coded at all — the value, power rating and tolerance are printed on the body. Good practice is to choose a resistor rated at roughly twice the calculated dissipation.

7. Component symbols you must be able to identify

The examination supplies the diagrams; your job is to recognise them. Because this guide is text only, each symbol is described in words. Sketch each one as you read it.

Component How the symbol is drawn

Fixed resistor A plain rectangle with a lead from each end (IEC style), or an angular zig-zag between two leads (older American style).

Variable resistor / potentiometer The resistor body with a third lead ending in an arrowhead touching the body — the wiper. A rheostat connects only the wiper and one end.

Preset / trimmer resistor The resistor body with a short line ending in a T-bar or small square across it, indicating screwdriver adjustment.

Capacitor (non-polarised) Two equal, straight, parallel plates with a gap between them, one lead to each plate.

Polarised (electrolytic) capacitor One straight plate and one curved plate. The curved plate is the negative terminal; a + sign is often printed beside the straight plate.

Variable capacitor Two parallel plates with a diagonal arrow drawn through them.

Inductor / coil A row of semicircular humps (or a rectangle in some IEC drawings) between two leads.

Iron-cored inductor The same coil with two solid parallel lines drawn alongside it; a dashed line indicates a ferrite or dust core.

Transformer Two coils drawn facing each other , with the core lines between them — two solid lines for an iron core, dashed for ferrite, nothing for air.

Signal diode A solid triangle pointing at a straight bar . The triangle is the anode, the bar is the cathode, and conventional current flows in the direction the triangle points.

Zener diode The same triangle and bar, but the bar has short flags bent back at each end , forming a shape like a Z or a bracket.

LED A diode symbol with two small arrows pointing away from it, indicating emitted light.

Fuse A small rectangle with a line running through it end to end, or a narrow S-shaped link between two terminals.

Switch (SPST) Two terminals with a hinged line lifted away from one of them, showing the open contact.

Earth / ground A vertical lead ending in three horizontal lines of decreasing length . A hatched rectangle indicates a chassis connection rather than a true earth.

Aerial / antenna A vertical line ending in a Y shape, or in a short horizontal bar for the whip and dipole variants respectively.

Meter A circle containing a letter — A for ammeter, V for voltmeter, W for wattmeter — or a diagonal pointer.

The three transistor symbols (syllabus 4.30)

All three are drawn as a circle (sometimes omitted) containing a straight base bar with three leads.

NPN bipolar transistor. Base lead on the left striking a vertical bar; collector and emitter leads angle away to the right. The arrow is on the emitter lead and points AWAY from the base , out of the device. Memory aid: N ot P ointing i N .

PNP bipolar transistor. Identical drawing, except the arrow on the emitter points IN, towards the base . Memory aid: P ointing i N P roudly.

Field effect transistor (FET). The terminals are named gate, drain and source instead of base, collector and emitter. A junction FET is drawn with the gate lead striking a vertical channel bar; for an N-channel device the gate arrow points in towards the channel , and for a P-channel device it points out. A MOSFET is distinguished by a visible gap between the gate lead and the channel , representing the insulating oxide layer.

The single most examinable point here is that in both bipolar symbols the arrow lives on the emitter , never the collector and never the base. Which way it points is the only difference between NPN and PNP.