What is a resistor?

Resistors are the most common components in electronics. A typical one is a small cylinder with two wire legs and coloured stripes, but phones and computers use tiny surface-mount resistors smaller than a grain of rice, thousands of them on one board.

What does a resistor do?

• Limits current: stops too much current flowing, which could damage a component such as an LED.
• Sets voltages: two resistors in series split a voltage into smaller parts, called a voltage (or potential) divider.
• Holds inputs steady: pull-up and pull-down resistors keep a microcontroller's input at a definite HIGH or LOW when a button isn't pressed.
• Sets timing: with a capacitor, a resistor sets how fast it charges, used in timers and flashing lights.
• Turns electrical energy into heat: on purpose in heaters, toasters and kettles, which use wires with high resistance.

Resistance and Ohm's law

Resistance is how much a component opposes the current through it. The unit is the ohm, symbol Ω: a resistance of 1 ohm lets 1 amp flow when 1 volt is across it. The three quantities are linked by Ohm's law:

Rearranged, I = V ÷ R and R = V ÷ I. For a fixed voltage, a bigger resistance means a smaller current. That is how a resistor controls current. AQA's GCSE Physics specification states that the current through an ohmic conductor, such as a resistor at constant temperature, is directly proportional to the potential difference across it, so its resistance stays the same.

Why do materials have resistance?

In a metal, current is a flow of electrons. As they drift through the wire, they keep colliding with the vibrating ions of the metal, which transfers energy and slows their flow. That opposition is resistance, and the energy ends up as heat. A longer wire means more collisions, so more resistance. A thicker wire gives more room to flow, so less resistance. Heating a metal makes its ions vibrate more, so its resistance rises.

Resistor symbol

In UK and international circuit diagrams, a resistor is drawn as a small rectangle. In US diagrams, it is a zigzag line. A variable resistor adds a diagonal arrow through the symbol. See our guide to variable resistors for potentiometers and rheostats.

Resistor colour code

Through-hole resistors are too small to print numbers on, so their value is shown by coloured bands (color code in the US). On a common four-band resistor, the first two bands are digits, the third is a multiplier (the number of zeros), and the fourth, set apart slightly, is the tolerance.

A gold tolerance band means ±5% and silver means ±10%. A common memory aid for the order is the colours of the rainbow, with black and brown before and grey and white after.

Example: reading the bands

• Yellow, violet, red, gold: 4, 7, then × 100 = 4,700 Ω, or 4.7 kΩ, ±5%.
• Red, red, brown, gold: 2, 2, then × 10 = 220 Ω, ±5%.
• Brown, black, orange, gold: 1, 0, then × 1,000 = 10,000 Ω, or 10 kΩ, ±5%.

Resistors come in standard values rather than every possible number. The common E12 series runs 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68 and 82, repeated in every power of ten: 220 Ω, 4.7 kΩ and 10 kΩ are all in it.

Worked examples

Example 1: current through a resistor

A 450 Ω resistor is connected to a 9 V battery. The current is I = V ÷ R = 9 ÷ 450 = 0.02 A, which is 20 mA.

Example 2: choosing a resistor for an LED

An LED needs about 2 V across it and should have about 10 mA through it. It is powered from a 9 V battery. What resistor goes in series with it?

1. Voltage left for the resistor: 9 − 2 = 7 V.
2. Resistance needed: R = V ÷ I = 7 ÷ 0.010 = 700 Ω.
3. 700 Ω isn't a standard value, so pick the next one up from the E12 series: 820 Ω. Going up keeps the current safely below the target.
4. Check the current: 7 ÷ 820 = 0.0085 A, about 8.5 mA.

Example 3: is the resistor powerful enough?

Resistors have a power rating: the most power they can turn into heat without overheating. Many small through-hole resistors are rated at 0.25 W. The power in Example 2 is P = I²R = 0.0085² × 820 = 0.06 W, comfortably under 0.25 W.

Example 4: resistance from a graph

A current-voltage (I-V) graph for a resistor is a straight line through the origin. If it shows 0.3 A at 6 V, the resistance is R = V ÷ I = 6 ÷ 0.3 = 20 Ω. A steeper line on a graph of current against voltage means a lower resistance.

Try reading a resistance from a graph yourself, then press Mark to check it in our Electricity course.

Types of resistors

Resistors in circuits

In series, resistances add up. In parallel, the total resistance is less than the smallest resistor, because each branch is an extra path for the current. Our guide to resistors in series and parallel works through both, and our guides to voltage and potential difference explain the voltage side of Ohm's law. You can wire up resistors and watch the current change in the Circuit Builder.

Common mistakes

• Saying a resistor uses up current. The current is the same on both sides of a resistor in series. It transfers energy, not current.
• Forgetting the LED resistor. Without one, an LED can draw too much current and burn out.
• Reading the colour bands from the wrong end. Start from the end away from the gold or silver tolerance band.
• Ignoring the power rating. A resistor with too much power through it overheats.
• Assuming every component is ohmic. A filament lamp's resistance rises as it heats up, so its I-V graph curves.

Read next

Physics · Electricity and magnetismVariable resistors: how they work, symbol and usesWhat a variable resistor does, how potentiometers and rheostats work, their circuit symbol and uses, with GCSE examples and the I-V practical.