Simple LED Circuit Diagram: Series Resistor, Polarity, and Voltage Calculation

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An LED requires a series resistor to limit current — without it, the LED will burn out in seconds. This guide explains the calculation, polarity identification, and correct wiring for simple LED circuits at common voltages.

A Light-Emitting Diode (LED) is a semiconductor device that emits light when current flows through it in the forward direction. Unlike an incandescent bulb, an LED does not have an inherent resistance that increases with temperature — it is a current-controlled device with a relatively fixed forward voltage drop (Vf). This means that if you connect an LED directly to a voltage source higher than its forward voltage, the junction offers very little resistance, current rises uncontrollably, and the LED overheats and fails within seconds.

The solution is a series current-limiting resistor. The resistor takes up the difference between the supply voltage and the LED's forward voltage drop, and limits the current to the LED's rated operating current.

The calculation is derived from Ohm's Law:

R = (Vs – Vf) / If

Where: - Vs = supply voltage (e.g. 5V, 12V) - Vf = LED forward voltage (typically 1.8–2.2V for red/yellow/orange; 3.0–3.5V for blue, white, green) - If = desired forward current (typically 10–20mA for standard 5mm LEDs)

Example: Red LED on a 12V supply: R = (12 – 2.0) / 0.020 = 10 / 0.020 = 500 ohms Nearest standard E24 value: 510 ohms Power dissipated: P = If² × R = (0.020)² × 510 = 0.204W — use a 0.25W or 0.5W resistor

Example: White LED on a 5V supply: R = (5 – 3.3) / 0.020 = 1.7 / 0.020 = 85 ohms Nearest standard: 82 ohms or 100 ohms

Polarity is essential. LEDs are diodes — they only conduct in one direction. The anode (+) is connected toward the positive supply; the cathode (–) is connected toward the ground. On a standard 5mm through-hole LED, the longer lead is the anode and the shorter lead is the cathode. The cathode side of the LED body also has a flat cut on the rim of the plastic lens.

For multiple LEDs from a single supply, the preferred topology is individual series resistors for each LED rather than a single shared resistor. Shared resistors cause unequal current distribution because LEDs have manufacturing variation in their Vf — one LED with a slightly lower Vf will hog current at the expense of others.

Power dissipation of the resistor must be verified. Most standard resistors are 0.25W. A resistor carrying 20mA in a 12V circuit dissipates up to 0.2W — marginal for a 0.25W part. Using a 0.5W resistor provides adequate margin and improved reliability.

How to wire simple led circuit diagram

Determine supply voltage and LED specifications Note your supply voltage (Vs). From the LED datasheet or product listing, obtain the forward voltage (Vf) and maximum forward current (If_max). If no datasheet is available, use typical values: red/yellow/orange Vf ≈ 2.0V, green Vf ≈ 2.1V, blue/white Vf ≈ 3.3V. Target If = 10–20mA for standard 5mm LEDs; most operate efficiently at 10mA with less heat.
Calculate the series resistor value Apply the formula: R = (Vs – Vf) / If. Example: 5V supply, red LED (Vf = 2.0V), If = 15mA: R = (5 – 2.0) / 0.015 = 200 ohms. Select the nearest higher standard E24 or E12 resistor value. If your calculation lands exactly on a standard value, still consider rounding up slightly to ensure the LED is not over-driven due to supply voltage tolerance.
Calculate and verify resistor power rating Calculate power dissipation: P = (Vs – Vf) × If. Using the example above: P = (5 – 2.0) × 0.015 = 0.045W. A standard 0.25W resistor is more than adequate. For 12V circuits with 20mA, dissipation can approach 0.2W — use a 0.5W resistor. Always select a resistor with a power rating at least 50% above the calculated dissipation for long-term reliability.
Identify LED anode and cathode For a 5mm through-hole LED: the longer lead is the anode (+), the shorter lead is the cathode (–). There is also a flat cut on the cathode side of the lens rim. Confirm with a multimeter in diode mode if there is any doubt. Never connect an LED without knowing which terminal is which — reversed connection simply prevents the LED from illuminating, but repeated reverse voltage above the LED's reverse breakdown voltage can damage it.
Assemble the circuit on a breadboard or PCB Connect the series resistor between the positive supply and the LED anode. Connect the LED cathode to the supply negative (ground). The resistor position (before or after the LED in the series path) does not matter electrically — current is the same throughout a series circuit — but convention places the resistor between supply and LED anode. Verify correct orientation before applying power.
Power on and verify operation Apply the supply voltage. The LED should illuminate at a steady, appropriate brightness. If the LED does not illuminate, check polarity (swap leads if necessary), check that the resistor is correctly valued and fully inserted into the circuit, and verify supply voltage with a multimeter. If the LED appears very dim, check that the resistor value is not too high for the desired brightness and current.

Specifications

Forward voltage (Vf) — red, orange, yellow	1.8V to 2.2V typical
Forward voltage (Vf) — green (standard)	2.0V to 2.4V typical
Forward voltage (Vf) — blue, white, green (high-efficiency)	3.0V to 3.5V typical
Typical forward current (standard 5mm LED)	10–20mA; maximum 20mA — operate at 10–15mA for longer life
Resistor formula	R = (Vs – Vf) / If (ohms); power P = (Vs – Vf) × If (watts)
Maximum reverse voltage (typical)	5V — do not apply reverse voltage above this value
Standard resistor wattage ratings	0.125W (1/8W), 0.25W (1/4W), 0.5W (1/2W), 1W — select above calculated dissipation
LED indicator wavelength bands	Red: ~620–750nm; Yellow: ~570–590nm; Green: ~495–570nm; Blue: ~450–495nm; White: broadband

Safety warnings

Even low-voltage LED circuits use semiconductor components that can be permanently damaged by over-voltage, reverse voltage exceeding the LED's reverse breakdown rating, or excess current. Calculate and install the correct series resistor before applying power.
Do not stare directly into an illuminated high-brightness LED, particularly blue or white LEDs. High-intensity LEDs can cause retinal discomfort or damage with prolonged direct viewing. This applies especially to high-power LED modules (above 0.5W).
For mains-powered LED circuits (such as LED driver boards, strip light power supplies, or retrofitted LED luminaires), all mains-side work must be carried out by a licensed electrician in accordance with applicable wiring codes. Low-voltage DC output from a compliant LED driver is safe to handle; the mains input side is not.
Electrostatic discharge (ESD) can damage LED junctions before they are ever powered. Handle LEDs by the body, not the leads, and use ESD precautions (anti-static mat and wrist strap) when working in ESD-sensitive environments.

Tools needed

Digital multimeter (DC voltage and diode test functions)
Breadboard or soldering station and PCB
Wire cutters and lead-bending tool (for through-hole component lead forming)
Resistor colour code chart or multimeter resistance function (to verify resistor value)
Regulated DC power supply or appropriate battery with connector

Common mistakes

Connecting the LED directly to the supply without a series resistor: the LED will fail within seconds from thermal runaway. This is the single most common LED circuit mistake.
Reversing LED polarity: the LED will not illuminate but the circuit appears complete. Reverse the LED leads — do not increase voltage trying to force it to light.
Using a resistor that is too high in value: the LED will light dimly or not at all. Recalculate using the correct If target. A resistor calculated for 1mA instead of 20mA will result in barely visible light.
Ignoring resistor power rating: a 0.25W resistor in a circuit that dissipates 0.22W will run near its rating continuously. Under sustained load it will overheat and fail open — switching off the LED. Use the next higher wattage rating.
Assuming all LEDs of the same colour have identical Vf: manufacturing variation can be ±0.2V or more. For precision applications, measure Vf with a multimeter in diode test mode before calculating the exact resistor value.

Troubleshooting

LED does not illuminate when power is applied: Cause: Reversed polarity, incorrect resistor value (too high), open circuit, or no supply voltage Fix: Verify supply voltage with a multimeter. Check LED orientation (longer lead to positive). Test resistor value with multimeter resistance function. Test LED directly with a 3V supply and no resistor for 1-2 seconds to confirm the LED itself is functional (if it lights briefly, LED is fine; fault is in the circuit).
LED illuminates briefly then stops working: Cause: No series resistor installed, or resistor value is too low — LED driven into thermal destruction by excess current Fix: The LED is permanently destroyed and must be replaced. Install a correctly calculated series resistor before powering the replacement LED. If a resistor was already in circuit, verify its value with a multimeter — resistors can be mis-read by colour code.
LED is much dimmer than expected: Cause: Series resistor value is too high, supply voltage is lower than expected, or LED forward current is much lower than intended Fix: Measure actual supply voltage. Recalculate the correct resistor value. Replace the resistor with the correctly calculated value. Also verify the LED itself is not partially failed — a degraded junction can produce reduced light output at normal current.

Frequently asked questions

What happens if I connect an LED without a series resistor?

Without a series resistor, only the LED's junction resistance limits current — which is very low. Current rises extremely rapidly as the junction heats, causing a thermal runaway condition. The LED will overheat and its junction will be destroyed, typically within one to a few seconds. The LED will cease to illuminate and cannot be repaired. Always use a series resistor.

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

On standard 5mm through-hole LEDs: the longer lead is the anode (+) and the shorter lead is the cathode (–). The LED body also has a flat cut on the rim of the lens on the cathode side. On SMD LEDs, the cathode is usually marked with a line, a triangle, or a dot on the package. When in doubt, test with a multimeter in diode mode — the LED will forward bias (and may emit faint light) when the red probe is on the anode.

Can I use the same series resistor for different coloured LEDs?

Only if their forward voltages (Vf) are similar. Red, yellow, and orange LEDs typically have Vf around 1.8–2.2V. Blue, white, and green LEDs typically have Vf around 3.0–3.5V. Using a resistor calculated for a red LED with a blue LED will under-drive the blue LED (less than intended brightness). Recalculate the resistor value for each LED colour or type.

How many LEDs can I run from a 5V supply?

In series: the forward voltages add up — for white LEDs at 3.3V each, only one fits in a 5V supply with a usable resistor voltage (5 – 3.3 = 1.7V for the resistor). Two would require 6.6V minimum. In parallel (each with its own resistor): you can run as many as the supply current capacity allows. At 20mA per LED, ten LEDs draw 200mA total — a common USB supply can handle this.

What resistor wattage should I use for a 12V LED circuit?

Calculate power dissipation: P = (Vs – Vf) × If. For a red LED at 20mA on 12V: P = (12 – 2.0) × 0.020 = 0.2W. A standard 0.25W resistor is at its limit — use a 0.5W resistor for adequate thermal margin. For higher current LEDs or higher voltage supplies, calculate carefully and select the next standard wattage rating above your calculated dissipation.

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