Circuit Diagram of an LED

Circuit Diagram of an LED — interactive diagram. Open it in the editor to customise components and wiring.

This is a free printable circuit diagram of led: download the diagram as SVG or open it and print to paper or PDF.

A complete reference for LED circuit diagrams showing series current-limiting resistor calculation, polarity identification, and correct wiring for single and multiple LED configurations.

A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it in the forward direction. Unlike an incandescent lamp, an LED does not self-limit current—without a series current-limiting resistor, the LED will draw excessive current, overheat, and fail within seconds.

The fundamental LED circuit consists of a power supply, a series resistor, and the LED itself. The resistor value is calculated using Ohm's law: R = (V_supply – V_f) / I_f, where V_supply is the supply voltage, V_f is the LED forward voltage, and I_f is the desired forward current.

Forward voltage (V_f) depends on the LED semiconductor material: red and yellow LEDs typically exhibit V_f of 1.8–2.2 V; green LEDs approximately 2.0–3.0 V; blue and white LEDs 3.0–3.6 V. Typical operating current for standard 5 mm indicator LEDs is 10–20 mA (0.010–0.020 A), with a test or minimum current of around 1–2 mA for illumination.

Example calculation: a red LED (V_f = 2.0 V, I_f = 20 mA) powered from a 5 V supply requires R = (5.0 – 2.0) / 0.020 = 150 Ω. The nearest standard resistor value is 150 Ω. Always select the next higher standard value if an exact value is unavailable, as this reduces current slightly and extends LED life.

LEDs are polarity-sensitive. The anode (positive lead, longer leg on through-hole types, or marked with a triangle symbol on circuit diagrams) must connect toward the positive supply. The cathode (negative lead, shorter leg, often with a flat on the LED body) connects toward negative. Reversing polarity does not illuminate the LED and may damage it if reverse voltage exceeds the LED's rated reverse breakdown voltage, which is typically only 5–10 V for many types.

Multiple LEDs can be wired in series to share supply voltage, or in parallel (each with its own resistor) for equal brightness. Series wiring requires a supply voltage greater than the sum of all forward voltages plus the voltage drop across the resistor.

A basic LED circuit requires just a power source, a current-limiting resistor, and the LED itself — but a practical LED bulb circuit is more involved, incorporating a bridge rectifier, filter capacitor, and multiple LEDs wired in series-parallel strings. Understanding the circuit diagram of an LED bulb helps with fault-finding, driver design, and safe replacement of components. You can build and annotate any LED circuit diagram free in the browser at circuitdiagrammaker.com without installing any software.

How to wire circuit diagram of led

Determine supply voltage and LED parameters Note the supply voltage (V_supply). From the LED datasheet or colour type, identify the forward voltage (V_f) and maximum continuous forward current (I_f max). Choose an operating current of approximately 70–80% of the maximum for longer LED life.
Calculate the series resistor value Apply R = (V_supply – V_f) / I_f. For example: 9 V supply, white LED with V_f = 3.2 V, I_f = 15 mA: R = (9 – 3.2) / 0.015 = 386.7 Ω. Round up to the nearest standard value: 390 Ω.
Calculate resistor power rating Calculate power dissipation: P = (V_supply – V_f) × I_f = (9 – 3.2) × 0.015 = 87 mW. A ¼ W (250 mW) resistor is adequate. For safety margin, select a resistor rated at least double the calculated dissipation.
Identify LED polarity Identify the anode (longer lead or flat-side marking on body indicates cathode). On the circuit diagram, connect the anode to the resistor-supply junction and the cathode to the negative rail (0 V / GND).
Assemble the circuit Connect the resistor in series between the positive supply and the LED anode. Connect the LED cathode to the negative supply or ground. Double-check polarity before applying power.
Apply power and measure current Apply supply voltage. The LED should illuminate. Measure the actual current through the circuit using a series multimeter set to DC milliamps. The measured current should be close to the calculated I_f. If significantly higher, the resistor value is too low; if the LED is very dim, check polarity or increase I_f or reduce resistor value.

Specifications

Typical forward voltage, red LED	1.8–2.2 V
Typical forward voltage, green LED	2.0–3.0 V
Typical forward voltage, blue / white LED	3.0–3.6 V
Typical operating forward current (5 mm indicator LED)	10–20 mA
Maximum forward current (5 mm through-hole LED, typical)	20–30 mA (continuous)
Typical reverse breakdown voltage	5–10 V (check specific datasheet)
Resistor formula	R = (V_supply – V_f) / I_f

Safety warnings

Never connect an LED directly to a power supply without a series current-limiting resistor. Without current limiting, the LED will fail immediately and may produce heat or glass fragments.
Verify polarity before applying power. Reverse connection does not illuminate the LED and, if the reverse voltage exceeds the LED's reverse breakdown voltage (typically 5–10 V), can permanently damage the device.
When working with high-power LEDs (1 W and above), the forward current can reach 350 mA to several amps. These devices require proper heatsinking and current-regulated drivers, not simple resistors—excessive junction temperature will cause rapid failure.
High-brightness LEDs produce intense light that can cause eye damage if viewed directly, particularly blue and white LEDs. Do not stare directly into operating high-brightness LEDs.

Tools needed

Digital multimeter (DC voltage and DC milliamp functions)
Breadboard (for prototyping)
Wire cutters and strippers
Regulated DC power supply or battery holder
Resistor colour code chart or calculator

Common mistakes

Inserting the LED in reverse polarity on a breadboard, resulting in no illumination and confusion about whether the LED or resistor is faulty.
Using an incorrect resistor value—either too low (LED runs too hot and has shortened life) or too high (LED is too dim for the application).
Connecting multiple LEDs in parallel with a single shared resistor, causing current imbalance and likely failure of the LED with the lowest forward voltage.
Ignoring the power rating of the resistor, using a 1/8 W resistor in a circuit where the calculation shows 150–200 mW dissipation, leading to overheating and resistor failure.
Assuming all LEDs of the same colour have the same forward voltage—V_f varies between manufacturers and even production batches, so always use the datasheet value or measure with a multimeter on the diode-test range.

Troubleshooting

LED does not illuminate when power is applied: Cause: LED is inserted with reversed polarity, the series resistor value is too high for the supply voltage, or the power supply is not delivering the expected voltage. Fix: Measure supply voltage at the circuit input with a multimeter. Check LED orientation—swap anode and cathode if needed. Measure voltage across the LED (should equal V_f if conducting). If voltage across LED is near zero, current is not flowing; verify resistor and wiring continuity.
LED is very dim: Cause: Series resistor value is too large, limiting current below the LED's intended operating current, or supply voltage is lower than expected. Fix: Measure current through the circuit (series milliamp reading). If significantly below the design I_f, reduce the resistor to the next lower standard value. Verify supply voltage is at its nominal value under load.
LED fails (stops illuminating) after a short time: Cause: Forward current is too high due to an undersized series resistor, or the LED has been operated above its maximum junction temperature. Fix: Recalculate the series resistor. Replace the failed LED and install the correct resistor value. Verify the supply voltage matches the design value—an unregulated supply may be higher than nominal under light load.

Frequently asked questions

Why does an LED need a series resistor?

An LED has a very low dynamic resistance once its forward voltage threshold is exceeded. Without a series resistor to limit current, the LED draws destructively high current, overheats, and fails within seconds. The series resistor drops the difference between supply voltage and LED forward voltage across itself, controlling the current through the LED.

How do I calculate the correct resistor value for an LED circuit?

Use the formula R = (V_supply – V_f) / I_f. Determine your supply voltage (V_supply), look up the LED's forward voltage (V_f, typically 1.8–3.6 V depending on colour) and desired forward current (I_f, typically 10–20 mA for indicator LEDs). Divide the voltage difference by the current to get resistance in ohms, then select the next higher standard resistor value.

How do I identify the anode and cathode of an LED?

On a standard 5 mm through-hole LED, the longer lead is the anode (positive) and the shorter lead is the cathode (negative). The LED body also typically has a flat on the cathode side. On circuit diagram symbols, the anode is the tip of the triangle pointing in the direction of current flow, and the cathode is the bar across the flat side.

Can LEDs be connected in parallel?

LEDs can be connected in parallel, but each LED must have its own individual series resistor to account for slight differences in forward voltage between LEDs. Connecting LEDs in parallel with a single shared resistor causes current imbalance—the LED with the lowest forward voltage hogs current and may fail, while others are dim.

What resistor power rating should I use in an LED circuit?

Power dissipated in the resistor equals P = I² × R, or equivalently P = (V_supply – V_f) × I_f. For typical 5 V / 20 mA LED circuits, resistor power dissipation is low—around 40–60 mW—so a standard ¼ W (250 mW) resistor has ample margin. At higher supply voltages or currents, recalculate and use an appropriately rated resistor.

What is the circuit diagram of an LED bulb?

An LED bulb circuit diagram typically starts with an AC mains input feeding a bridge rectifier (four diodes in a bridge configuration) that converts AC to pulsing DC. A smoothing electrolytic capacitor filters the ripple, and a current-limiting resistor or constant-current driver IC controls the current through the LED string. The LEDs are arranged in series-parallel combinations to match the available voltage and desired wattage. Some designs also include a small transformer or capacitive dropper to reduce voltage before rectification.