How to Wire a Resistor

Resistors turn up in almost every circuit you'll build, and they're one of the easiest parts to get right. The best news for beginners: a resistor has no polarity, so unlike an LED or an electrolytic capacitor, it doesn't matter which way round it goes. Here's what a resistor does and how to wire one.

Resistors have no + or - side

You can fit a resistor either way round, and flip it end for end, and it behaves exactly the same. There's nothing to line up and no way to install it backwards. That makes resistors far more forgiving than LEDs, diodes and electrolytic capacitors, which only work one way.

What a resistor does

A resistor limits the flow of current and drops some voltage across itself. Its value is measured in ohms (Ω): the higher the value, the more it restricts current. Two everyday jobs cover most of what a beginner needs:

• Limiting current so a delicate part, like an LED, isn't damaged.
• Setting a voltage level at a pin, such as a pull-up or pull-down on a button.

The classic job: protecting an LED

Connect an LED straight to power and it draws as much current as it can, then burns out almost instantly. A resistor placed in series (in line with the LED) holds the current to a safe level.

1. Run the LED's long leg (+) toward your positive supply.
2. Put the resistor in the line between the supply and the LED. Before or after the LED is fine, and either way round.
3. Take the LED's short leg (-) back to ground, then power up.

To choose a value, use Ohm's law: R = (supply voltage - LED voltage) / current. For a typical red LED on 5V, where the LED drops about 2V and wants roughly 20mA:

R = (5V - 2V) / 0.02A = 150Ω. Round up to the nearest common value, so a 220Ω resistor is a safe, popular choice.

Rather skip the maths entirely? Our pre-wired LEDs have the correct resistor built into the lead, so they run straight from 5V, 12V or 3-24V with nothing extra to add.

Series and parallel

Need a value you don't have on hand? Combine resistors:

• In series (end to end), the values add up: 220Ω + 100Ω = 320Ω.
• In parallel (side by side), the total is always less than the smallest one. Two equal resistors in parallel give half their value.

Pull-up and pull-down resistors

When a microcontroller pin reads a button, it needs a resistor to hold it at a known level while the button isn't pressed, otherwise the pin "floats" and reads randomly. A pull-up resistor (commonly 10kΩ) ties the pin to +, a pull-down ties it to ground. Pressing the button then switches the pin to the opposite level.

Mind the power rating

Every resistor has a power rating: how much heat it can shed before it cooks. Most through-hole resistors are 1/4W (0.25W), which is plenty for typical low-voltage electronics. You can check the load with P = V² / R.

Reading the value

Resistors are marked with coloured bands. A common metal-film resistor has five bands: three digits, a multiplier, then a tolerance band (brown = ±1%). Hold it with the group of four bands on the left and read left to right. If in doubt, a multimeter set to ohms reads it in seconds.

Quick recap

• No polarity: fit a resistor either way round.
• Put it in series to limit current, for example protecting an LED.
• Pick the value with Ohm's law; 220-470Ω suits most 3-5V LEDs.
• Series values add, parallel values reduce.
• Stay within the power rating; 1/4W is fine for low-voltage projects.