The short answer

An LED needs a current-limiting path. Subtract an assumed LED forward drop from the supply, then calculate and check the resistor.

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Resistor and LED in seriesA positive low-voltage source feeds a resistor, then the LED anode. The LED cathode returns to the negative terminal. The resistor limits current.+−ResistorLED: anode → cathodeSeries path · choose a safe current limit
Resistor and LED in series

An LED is not a little fixed resistor

A light-emitting diode conducts predominantly in one direction, and its forward voltage changes with operating current and temperature. A small change in voltage can produce a large change in current, so placing a bare LED across a supply is not a reliable way to set brightness and can destroy it. A series resistor takes up the remaining voltage and limits current in a simple low-voltage DC circuit. Consult the specific LED’s data sheet for current limits, polarity marks, and forward-voltage range rather than assuming all colors and packages behave alike.

Build a complete, correctly oriented path

Connect source positive through the resistor to the LED anode, then from the LED cathode back to source negative. The resistor may be on either side of the LED so long as it is in series and not bypassed. Many through-hole LEDs use a longer lead for the anode and a flat on the package near the cathode, but physical markings can vary; check the part. Reverse orientation usually prevents the expected light in this simple circuit, and a sufficiently high reverse voltage can damage an LED. Use a suitable isolated low-voltage source.

Worked example: 5 V, assumed 2 V LED drop

Assume a regulated 5 V DC supply, one LED with approximately 2 V forward drop at the chosen operating point, and a target of 10 mA. The resistor must drop about 5 V − 2 V = 3 V. Calculate 3 V / 0.010 A = 300 Ω. A 330 Ω standard resistor would give approximately 3 V / 330 Ω = 9.1 mA under these assumptions. Its power is approximately 3 V × 0.0091 A = 27 mW. The 2 V drop is an assumption for this example, not a promise for every LED.

R ≈ (Vsupply − Vf) / I; 330 Ω → I ≈ 9.1 mA, Presistor ≈ 27 mW

Run your own numbers with the LED resistor calculator.

Check the extremes, not just one answer

If the LED’s forward voltage falls, more supply voltage appears across the resistor and current rises. If the supply rises, current rises too. Use the LED’s published forward-voltage range, the supply tolerance, and resistor tolerance to estimate a worst-case current; compare that with the LED’s continuous-current rating and thermal conditions. A resistor with a nominal power rating above the calculation still needs margin and derating where specified. If Vsupply is at or below the LED’s forward voltage, this subtraction no longer predicts a controlled target current; choose a different supply or driver.

Series and parallel are not interchangeable

Two LEDs in series share one current; subtract both forward drops when sizing their common resistor, but only if the supply has enough headroom. Two LEDs in parallel may not share current equally, even if they look identical. Give each parallel LED branch its own suitable current-limiting resistor rather than relying on a single shared resistor. Recheck voltage and power for each branch. Microcontroller pins and small supplies also have their own output-current limits: a safe LED current on paper is not automatically safe for a particular pin.

If it is dark or unexpectedly bright

Power off and inspect the LED orientation, resistor marking, and full return path. A resistor accidentally bypassed by a breadboard strip cannot limit the LED current. With a suitable meter in voltage mode across the resistor under safe low-voltage power, estimate branch current as Vresistor / R. If the resistor reads nearly zero volts, determine whether the branch is open or whether its terminals share a node. If the LED appears too bright, shut off power and recheck supply voltage, resistance, and data-sheet limits before continuing.

Try changing one assumption

Repeat the 5 V example with a 2.2 V assumed LED drop and the same 330 Ω resistor: the estimate becomes 2.8 V / 330 Ω ≈ 8.5 mA. That difference illustrates why forward voltage matters. NPNLab’s browser Lab can help explore supported LED/resistor circuits, but measurements and data-sheet limits should decide any physical build. Record your source voltage and actual resistor value, and never remove current limiting just to make an LED brighter.

Sources and further reading

Diagrams are simplified teaching illustrations, not measured results or construction plans. See how these guides are prepared.