How to Read Resistor Colour Codes When Building RC Circuits

The process of building resistor-capacitor (RC) circuits brings with it a need to select the correct resistance for establishing accurate time constants and filtering frequencies.

However, these components are often too small to have readable text printed on them. This helps to explain why resistor colour codes exist. The standardised system of coloured bands is widely trusted as a means of denoting each resistor’s value.

These colour bands indicate such crucial properties as the resistor’s electrical resistance (ohms), tolerance (accuracy), and sometimes temperature coefficient.

Why Do Resistor Colour Codes Matter So Much in RC Circuits?

The importance of understanding the colour codes on resistors is especially great for RC circuit projects. After all, this circuit type combines resistors (R) and capacitors (C) to create time-dependent behaviours, such as filters, timers, or signal delays.

In an RC circuit, the value of the resistor directly impacts on the time constant (τ = R x C), which determines the speed at which the capacitor charges or discharges.

The misreading of a given resistor’s colour code might mean that a 10-kiloohm (kΩ) resistor is used instead of a 1kΩ one, for example. Such errors can dramatically alter a circuit’s performance, potentially leading to it malfunctioning, and perhaps even damage being caused to the components.

The Standard Resistor Colour Code System

To read a resistor, hold it so that the bands are grouped towards the left. The final band, which is often spaced further apart or a metallic hue such as gold or silver, should be on your right.

Here is a quick reference table setting out the colours and meanings of resistor colour codes:

How To Read 4-Band Resistors

Most basic resistors in RC circuits use four bands. Those are as follows:

• Band 1: the first digit of the resistance value.
• Band 2: the second digit of the resistance value.
• Band 3: the multiplier, telling you how many zeroes to add. For example, the third band being red means you should multiply by 100.
• Band 4: the tolerance, typically gold (+5%) or silver (+10%). This band being gold, then, signals that the actual value could vary by 5%.

Let’s picture, for instance, a resistor with the bands brown, black, red, and gold. The resistance value digits would be 1 and 0, while the multiplier would be 100. When 10 is multiplied by 100, it produces 1,000Ω, or 1kΩ. Meanwhile, the gold band denotes a 5% resistance.

How To Read Five-Band and Six-Band Resistors

These resistors with more colour bands are typically used to ensure higher precision in advanced RC circuits, such as in precision timers or filters. They can be interpreted as follows:

• Band 1: the first digit of the resistance value.
• Band 2: the second digit of the resistance value.
• Band 3: the third digit of the resistance value, for greater accuracy.
• Band 4: the multiplier.
• Band 5: the tolerance, usually brown (+1%) or red (+2%) in precision circuits.
• Band 6: the temperature coefficient, indicating how the resistance changes in line with the heat or cold in ppm/°C (parts per million per degree Celsius).

So, for example, if a five-band resistor’s colour bands are (from left to right) blue, grey, black, orange, and brown, this indicates a 680kΩ resistor with a tolerance of +1%. This is because the first three bands signal digits of 6, 8, and 0, and when 680 is multiplied by 1,000, it brings the aforementioned result.

Bookmark A Resistor Colour Code Calculator for Even Greater Convenience

It’s good to practise the manual reading and interpretation of colour bands on resistors, as doing so will help you build skill and confidence across your RC circuit projects.

This doesn’t mean, however, that you should ignore the relevance and usefulness of digital tools, given the contribution they can make to saving you time and boosting accuracy.

The resistor colour code calculator on the RS website, for example, is free and easy to use. In the case of any given resistor, you simply input the colours of each band into the tool and click “calculate” to get a resistor value.

It could, then, be well worth bookmarking a tool like this for future use. We wish you all the best in your efforts to build the RC circuits that serve your performance and reliability needs!