Describe the function and types of resistor, read the four-band resistor colour code, and choose a resistor value

What this dot point is asking

SEAB wants you to describe what a resistor does, to know the main types, and to read the four-band colour code to find a resistor's value and tolerance. The central insight is that a resistor sets the size of a current or shares out a voltage, and that the coloured bands are a compact code: the first two are digits, the third is a multiplier, and the fourth is the tolerance.

The answer

What a resistor does

A resistor opposes the flow of current. In a circuit it is used to limit the current to a safe value (for example, in series with an LED), to share out a voltage (in a potential divider), and to set the operating conditions of transistors and other components. By Ohm's law, a chosen resistance fixes the current for a given voltage, which is why resistors are the most common component of all.

Types of resistor

• A fixed resistor has one value, set when it is manufactured and marked by its colour code. Carbon-film and metal-film resistors are typical.
• A variable resistor can have its resistance changed by moving a slider or turning a shaft. As a potentiometer it has three terminals and taps off an adjustable voltage; as a rheostat it is used with two terminals to adjust current.
• Special resistors respond to the surroundings: the light-dependent resistor changes with light and the thermistor changes with temperature. These are covered in the sensors topic.

The four-band colour code

A common resistor carries four coloured bands read from the end with the bands grouped together:

• Band 1 = first digit.
• Band 2 = second digit.
• Band 3 = multiplier (the power of ten to multiply by).
• Band 4 = tolerance (how far the true value may stray from the marked value).

The digit colours are: black 0, brown 1, red 2, orange 3, yellow 4, green 5, blue 6, violet 7, grey 8, white 9. The tolerance band is usually gold ($\pm 5\%$) or silver ($\pm 10\%$).

Tolerance and preferred values

No resistor is exactly its marked value. The tolerance tells you the allowed spread: a $\pm 5\%$ resistor of $1\ \text{k}\Omega$ may be anywhere from $950\ \Omega$ to $1050\ \Omega$. Because of tolerance, resistors are made in a set of preferred values rather than every possible number, so you choose the nearest preferred value to your calculated one.

Examples in context

Example 1. Picking a series resistor for an LED. Suppose Ohm's law gives a calculated value of $315\ \Omega$ to set the LED current. Because resistors come in preferred values, you choose the nearest available, often $330\ \Omega$, and the $\pm 5\%$ tolerance means the actual current is close enough to the target. The colour code lets you confirm you have picked the right part from a drawer.

Example 2. A light dimmer. A variable resistor wired as a rheostat in series with a lamp lets you change the current and so the brightness. Turning the shaft increases the resistance, reduces the current by $I = V/R$, and dims the lamp. The same idea, used as a potentiometer, becomes a volume control or a sensor calibration adjustment.

Try this

• Cue. Decode the bands yellow, violet, brown, silver. Yellow $4$, violet $7$, brown multiplier $\times 10$, so $47 \times 10 = 470\ \Omega$ at $\pm 10\%$.

• Cue. State two functions of a resistor in a circuit. To limit the current to a safe value, and to share out a voltage as in a potential divider.

• Cue. A $\pm 5\%$ resistor is marked $2.0\ \text{k}\Omega$. Find its allowed range. Five percent of $2000$ is $100\ \Omega$, so it lies between $1900\ \Omega$ and $2100\ \Omega$.