1.4 Resistor Colour Code and Component Symbol Identification

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.

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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.

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2. The colour code table

A traditional mnemonic for the digit order is "Big Brown Rabbits Often Yield Great Big Vocal Groans When Gingerly Slapped" — 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 $10^{3}$; yellow is digit 4 and multiplier $10^{4}$. Once you see that, you only have to remember one list.

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3. Reading direction, and four bands versus five

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

1. 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.
2. 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 ($\pm 1%$) or red ($\pm 2%$) tolerance band, which is your cue that you are counting three digits rather than two.

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4. Worked decodes

Worked Example 1 — brown, black, red, gold

• Brown = 1, black = 0, so the digits are 10.
• Red as a multiplier = $\times 100$, i.e. add two zeros.
• $10 \times 100 = 1,000;\Omega$, written 1 k$\Omega$.
• Gold = $\pm 5%$, so the true value lies between 950 $\Omega$ and 1 050 $\Omega$.

Worked Example 2 — yellow, violet, orange, gold

• Yellow = 4, violet = 7, so the digits are 47.
• Orange as a multiplier = $\times 1,000$, i.e. add three zeros.
• $47 \times 1,000 = 47,000;\Omega$, written 47 k$\Omega$.
• Gold = $\pm 5%$, so the true value lies between 44.65 k$\Omega$ and 49.35 k$\Omega$.

Worked Example 3 — red, red, brown, gold

• Red = 2, red = 2, so the digits are 22.
• Brown as a multiplier = $\times 10$, i.e. add one zero.
• $22 \times 10 = 220;\Omega$, written 220 $\Omega$.
• Gold = $\pm 5%$, so the true value lies between 209 $\Omega$ and 231 $\Omega$.

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.
• $100 \times 10 = 1,000;\Omega = 1;\text{k}\Omega$, and the final brown band means $\pm 1%$.
• The same nominal value as Worked Example 1, but ten times tighter.

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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 $\pm 10%$ parts), 12 values per decade: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82.
• E24 (for $\pm 5%$ parts) adds the intermediate values: 11, 13, 16, 20, 24, 30, 36, 43, 51, 62, 75, 91.

This is why 47 k$\Omega$ and 22 $\Omega$ exist in every catalogue while 45 k$\Omega$ 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.

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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.

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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.

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: Not Pointing iN.
• PNP bipolar transistor. Identical drawing, except the arrow on the emitter points IN, towards the base. Memory aid: Pointing iN Proudly.
• 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.