# Ohmforge Resistor Network Calculator

> Series/parallel resistor networks: total resistance, power per part, best E-series combinations. Two jobs in one: find the combination of up to three standard resistors (E6 to E192) that comes closest to a resistance you cannot buy, or evaluate a series/parallel expression such as 100 + (220 || 330) with the voltage, current and power of every resistor at a given voltage.

Part of usefulweb.site (https://usefulweb.site/). Part of [Ohmforge](https://ohmforge.usefulweb.site/) (ownerApp: ohmforge). 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://ohmforgeresistornetwork.usefulweb.site/ (interactive tool at https://ohmforgeresistornetwork.usefulweb.site/tool)
- JSON API: POST https://ohmforgeresistornetwork.usefulweb.site/api/v1/calculate with a JSON object of inputs (schema: https://ohmforgeresistornetwork.usefulweb.site/api/v1/tool)
- OpenAPI: https://ohmforgeresistornetwork.usefulweb.site/openapi.json
- MCP (Streamable HTTP): https://ohmforgeresistornetwork.usefulweb.site/mcp - tool `ohmforge_resistor_network`

## Inputs
- mode (select): Task
- target (text): Target resistance - For "find": engineering notation, e.g. 1234, 2k15, 12.5.
- series (select): Standard resistor series
- maxParts (select): Resistors at most
- expression (text): Network expression - For "evaluate": values joined with + (series) and || or // (parallel); parentheses group; || binds tighter than +.
- voltage (number): Voltage across the network - For "evaluate": used for the current and power of each resistor.

## Method
- Evaluate: series resistances add, parallel conductances add (1/R = Σ 1/Ri). The applied voltage is divided along series branches in proportion to their resistance and appears in full across each parallel branch; P = V² / R per resistor.
- Find: one part = the nearest series value. Two parts: every series value a below (series) or above (parallel) the target is tried with the two series neighbours of the exact partner (T − a, or 1/(1/T − 1/a)). Three parts: every pair a ≤ b with the exact third value for a + b + c, a ∥ b ∥ c, (a + b) ∥ c and (a ∥ b) + c, again rounded down and up. Values span target/100 … target·100 (target/10 … target·10 for the first two parts of three-part E48 … E192 combinations).
- Ranking: smallest |error|, then fewer parts. The best result for 1, 2 and 3 parts is listed separately so you can see what an extra resistor buys.

## Limits
- Nominal values only: the tolerance of each part (5 % for E24, 1 % for E96) usually exceeds the combination error, so measure or use tighter-tolerance parts where accuracy matters. Check the power rating of every resistor.