LEDs and Resistors
Why an LED only works one way round, why it needs a resistor, and how to pick the right one.
The question
Why does an LED need a resistor, and why does it only work one way round?
What you’ll discover: how to tell an LED’s + leg from its − leg, why it needs a resistor, and how to work out which resistor to use.
In simple terms
BeginnerA one-way light that needs a bodyguard
An LED (light-emitting diode) is a tiny light that only lets current flow one way. Put it in backwards and it simply stays dark.
- The longer leg is the anode. It goes toward the battery’s + side.
- The shorter leg is the cathode. It goes toward the − side. The rim of the LED is usually flat on this side too.
LEDs are delicate in one way: once they start glowing, they’ll try to take as much current as they can get, and too much burns them out in a flash. A resistor in the circuit acts like a bodyguard, keeping the current at a safe level.
Reading resistor colour bands
Resistors are too small to print numbers on, so they use coloured bands. On a common four-band resistor, the first two bands are digits, the third is how many zeros to add, and the fourth (often gold) is the tolerance.
| Colour | Digit | Colour | Digit |
|---|---|---|---|
| Black | 0 | Green | 5 |
| Brown | 1 | Blue | 6 |
| Red | 2 | Violet | 7 |
| Orange | 3 | Grey | 8 |
| Yellow | 4 | White | 9 |
Examples: red, red, brown = 2, 2, one zero = 220 Ω. Brown, black, red = 1, 0, two zeros = 1,000 Ω (1 kΩ). Brown, black, orange = 10 kΩ. A gold fourth band means the real value is within 5% of the marked value.
Five bands? Many kits now include precise 1% resistors with five bands. These use three digit bands, then the zeros band, then a brown tolerance band: 220 Ω = red, red, black, black, brown. 10 kΩ = brown, black, black, red, brown. If in doubt, check with a multimeter (Lesson 6).
Go deeper
IntermediateChoosing the right resistor
Each LED colour needs a certain voltage across it before it lights, called its forward voltage. Typical values for small 5 mm LEDs (check your LED’s datasheet, as they vary):
| LED colour | Typical forward voltage |
|---|---|
| Red | about 2.0 V |
| Yellow | about 2.1 V |
| Blue or white | about 3.0 V |
Most small indicator LEDs are rated for about 20 mA maximum. Aiming for 10 to 15 mA gives plenty of light and a long life.
The resistor gets whatever voltage is left over, so: R = (battery voltage − LED forward voltage) ÷ current.
Worked example: a red LED on a 4.5 V pack, aiming for 15 mA: R = (4.5 − 2.0) ÷ 0.015 = 167 Ω. Resistors come in standard values, so round up to the next one: 180 Ω, or the very common 220 Ω. With 220 Ω the current is (4.5 − 2.0) ÷ 220 ≈ 11 mA. Bright and safe.
Notice a blue LED (3.0 V) on a 3 V pack (2 × AA) will barely glow, if at all: there’s almost no voltage left over to push current through it.
AdvancedWhy LEDs behave this way
An LED is a PN junction made of semiconductor materials. Below its forward voltage, almost no current flows. Above it, current rises roughly exponentially with voltage, so a tiny increase in voltage causes a big jump in current. Without a resistor, the LED’s current is limited only by the battery’s internal resistance, which is far too little protection.
The colour depends on the semiconductor’s band gap: electrons dropping across a larger gap release higher-energy photons, which is why blue LEDs need a higher forward voltage than red ones.
Reverse voltage: most small LEDs are rated for only about 5 V in reverse. On a 9 V battery, a backwards LED can be damaged, so always check the polarity.
Resistor power: P = I² × R. At 11 mA through 220 Ω, P ≈ 0.027 W, far below a ¼-watt (0.25 W) resistor’s rating.
Try it
Build it for real with My First LED Circuit, or test different resistors and LED colours on the Virtual Workbench.
Quick check
Which LED leg goes toward the battery’s + side?
The longer leg, called the anode.
What colours are the bands on a 330 Ω resistor?
Orange (3), orange (3), brown (one zero).
What resistor would you use for a red LED on 4.5 V, aiming for about 10 mA?
R = (4.5 − 2.0) ÷ 0.010 = 250 Ω. Round up to a standard value: 270 Ω, or 330 Ω for a little less current.
Words to know
- LED
- Light-emitting diode: a light that lets current flow only one way.
- Anode / cathode
- The + leg (longer) and − leg (shorter) of an LED.
- Forward voltage
- The voltage an LED needs across it before it lights.
- Tolerance
- How close a resistor’s real value is to its marked value.
Learn more
- Science World (Vancouver): Current Electricity: eight hands-on circuit activities
- Next: Schematics, breadboards and multimeters
Electronics School is part of the Discovery Zone. Every activity here runs on small batteries or a USB cable: never wall outlets. The one plug-in tool is the optional soldering iron, set up by an adult.