# Ohmforge LED Resistor Calculator

> Series resistor for LEDs, rounded to a standard value and checked with a real diode model. Calculates the series resistor for one or more LEDs in series: R = (Vs − n·Vf) / I, rounds up to the next E6/E12/E24 value, and then verifies the actual current, LED voltage and resistor dissipation by simulating the circuit with Ohmforge's diode model. Suggests a power rating with 2× margin.

Part of usefulweb.site (https://usefulweb.site/). Results are deterministic calculations from the inputs and the listed assumptions; the API returns exactly what the web page shows.

## Use it
- Human interface: https://ohmforgeledresistor.usefulweb.site/ (interactive tool at https://ohmforgeledresistor.usefulweb.site/tool)
- JSON API: POST https://ohmforgeledresistor.usefulweb.site/api/v1/calculate with a JSON object of inputs (schema: https://ohmforgeledresistor.usefulweb.site/api/v1/tool)
- OpenAPI: https://ohmforgeledresistor.usefulweb.site/openapi.json
- MCP (Streamable HTTP): https://ohmforgeledresistor.usefulweb.site/mcp - tool `ohmforge_led_resistor`

## Inputs
- supply (number, required): Supply voltage
- colour (select): LED colour
- vf (number): Forward voltage (custom) - Only used for "Custom".
- current (number, required): LED current
- count (integer): LEDs in series
- series (select): Resistor series

## Method
- Exact value R = (Vs − n·Vf) / I with the typical forward voltage of the chosen colour at 20 mA.
- Rounded UP to the next preferred value of the chosen E-series so the current never exceeds the target.
- The circuit (supply, resistor, n LEDs) is then simulated with Ohmforge's LED diode model (n = 2, Rs = 4 Ω, fitted to Vf at 20 mA) to report the real current, which is slightly different because Vf depends on current.
- Resistor rating: next of 0.125/0.25/0.5/1/2/5 W at or above twice the dissipation.

## Limits
- Forward voltages vary by LED type and batch (±0.2 V is common); check the datasheet of your LED. Mains-powered LED circuits need proper isolation – do not build them from this calculation.