LED series-resistor sizing calculator

LED resistor from assumed forward drop and current

circuits-led-resistor.napkin / editable

SCENARIO COMPARISON

Compare the numbers.

This source describes an analytical model. Its quantities and checks are the view.

Scenario	resistorohm	resistorPowerW
	150	0.06
	350	0.14

All calculated quantities are listed with the inputs. Switch to Source to inspect the equations.

SCENARIOS / CALCULATED VALUESSAME SOURCE

Change an input to see the consequences.

resistor minimum150.00 ohm✓ passes

BASELINE INPUTS

supply5 V312forward2 Vcurrent20 mA540

CALCULATED

resistor150 ohm

resistorPower0.06 W

Edits change the baseline source.

THE SOURCE

Edits below update the same model.

# LED resistor from assumed forward drop and current
supply = 5 V in 3..12
forward = 2 V "Assumed LED forward voltage"
current = 20 mA in 5..40
resistor := (supply - forward) / current [ohm]
resistorPower := (supply - forward) * current [W]
check resistor minimum: resistor >= 100 ohm
scenario higher supply: supply = 9 V

Calculated from your source · not saved

Baseline · Baseline

resistor minimum150.00 ohm ✓ passes

higher supply · higher supply

resistor minimum350.00 ohm ✓ passes

Illustrative first-order example. Checks are editable requirements, not design standards. The broader topic may require additional equations, data, and validation.

Formula and units

This example uses the following equations. Input names and units match the editable source above.

resistor: resistor = (supply - forward) / current [ohm]
resistor power: resistorPower = (supply - forward) * current [W]

Original worked example

These are the baseline values in the downloadable source. Edits above let you test your own values.

Given

supply: 5 V
forward: 2 V
current: 20 mA

Calculated

resistor: 150 ohm
resistor power: 0.06 W

Assumptions and limits

LED resistor from assumed forward drop and current

The requirement checks use the limits written in this example. Passing a check does not establish that the model covers every load, material behavior, or operating condition in a real design.

Download this .napkin example ↓

Take it further with your agent

Napkin keeps the inputs, equations, checks and assumptions together in one study. Your agent sets it up; you change the numbers.

Try with Claude ↗

codex 'Use Napkin (https://napkin.engineering/mcp, or the local Napkin plugin) to build a study: LED series-resistor sizing. Inputs: supply and forward-voltage bounds; target current; resistor tolerance. Outputs: resistance range; current bounds; resistor power. Model scope: Ideal or parameterized lumped components. Read napkin_capabilities and the current snapshot. Ask me for the values that matter, label assumptions with units, author .napkin source, validate it and apply it with napkin_source using the current revision, then open the study. Add checks for the limits I supply. Keep unsupported analyses explicit.'

Claude needs the Napkin connector; Codex needs the Napkin plugin. Set up your agent ↗

The broader study

The topic brief below describes the broader scope. The worked example above states the particular calculation it implements.

Inputs: supply and forward-voltage bounds, target current, resistor tolerance
Outputs: resistance range, current bounds, resistor power
Modes: calculate, compare, sweep
Drawing: Schematic and waveform

EXECUTABLE WORKED EXAMPLE · FIRST-ORDER MODEL · CHECK THE SCOPE & ASSUMPTIONS