Documentation sections

Inrush Current Limiter (NTC) Calculator

POSThttps://rftools.io/api/py/v1/calculate

Slug inrush-current-limiter. Calculate NTC thermistor requirements for inrush current limiting, including cold resistance, peak inrush current, time constant, and energy absorbed

Inputs

Send every one of these in inputs, in the units shown. Most are required: a call without one is refused (400) and costs nothing; one the calculator can do without takes the default shown. An input outside its range is still computed and flagged in the provenance.

Inputs of inrush-current-limiter
Name	Unit	Range	Default	Description
supplyVoltagenumber	V	at least 0.1	12	Supply voltage applied at power-on
filterCapacitancenumber	μF	at least 0.1	1000	Total input filter capacitance that must be charged
targetInrushnumber	A	at least 0.001	5	Maximum allowable inrush current at power-on
ntcResistanceHotnumber	Ω	at least 0.001	0.5	NTC thermistor resistance when fully heated (steady-state)
ntcR25number	Ω	0 … 1000	0	The R25 of the thermistor you fit; for the worst-case peak to meet the target it must be at least the required resistance over (1 − tolerance). 0 fits exactly the required resistance
ntcTolerancenumber	%	0 … 50	20	The thermistor's R25 tolerance, ±; TDK's S238 inrush limiters are ±20 %. The low end sets the worst-case peak

Outputs

Returned in values, by key. A value that is unbounded or undefined is null.

Outputs of inrush-current-limiter
Field	Unit	Description
ntcResistanceColdheadline	Ω	Required NTC Cold Resistance
peakInrush	A	Peak Inrush Current
timeConstant	ms	Time Constant
energyAbsorbed	mJ	Energy Absorbed
limitingResistor	Ω	Limiting Resistance (cold)
hotRestartInrush	A	Peak current if power is reapplied before the thermistor has cooled: V_s / R_hot
peakInrushMax	A	The cold-start peak with the fitted thermistor at 25 °C and the low end of its R25 tolerance: V_s / (R25 × (1 − δ_R)). A warmer thermistor lets more through

Calling it

Calculate: one call, the request and response in full, and its errors.
Batch: this calculator with others, or a sweep, in one request.
Solve for a target: the value of one input that brings an output to a target.

curl -X POST https://rftools.io/api/py/v1/calculate \
  -H "X-API-Key: $RFTOOLS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "slug": "inrush-current-limiter",
    "inputs": {
      "supplyVoltage": 12,
      "filterCapacitance": 1000,
      "targetInrush": 5,
      "ntcResistanceHot": 0.5,
      "ntcR25": 0,
      "ntcTolerance": 20
    }
  }'

import rftools

client = rftools.Client()  # reads RFTOOLS_API_KEY, or the key `rftools login` saved

result = client.calculate("inrush-current-limiter", {
    "supplyVoltage": 12,
    "filterCapacitance": 1000,
    "targetInrush": 5,
    "ntcResistanceHot": 0.5,
    "ntcR25": 0,
    "ntcTolerance": 20,
})
print(result.values)
print(result.warnings, result.provenance.formula_ref)

const response = await fetch("https://rftools.io/api/py/v1/calculate", {
  method: "POST",
  headers: {
    "X-API-Key": process.env.RFTOOLS_API_KEY,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "slug": "inrush-current-limiter",
    "inputs": {
      "supplyVoltage": 12,
      "filterCapacitance": 1000,
      "targetInrush": 5,
      "ntcResistanceHot": 0.5,
      "ntcR25": 0,
      "ntcTolerance": 20
    }
  }),
});
console.log(response.status, await response.json());

package main

import (
	"bytes"
	"fmt"
	"io"
	"net/http"
	"os"
)

func main() {
	body := []byte(`{
	  "slug": "inrush-current-limiter",
	  "inputs": {
	    "supplyVoltage": 12,
	    "filterCapacitance": 1000,
	    "targetInrush": 5,
	    "ntcResistanceHot": 0.5,
	    "ntcR25": 0,
	    "ntcTolerance": 20
	  }
	}`)
	req, err := http.NewRequest("POST", "https://rftools.io/api/py/v1/calculate", bytes.NewReader(body))
	if err != nil {
		panic(err)
	}
	req.Header.Set("X-API-Key", os.Getenv("RFTOOLS_API_KEY"))
	req.Header.Set("Content-Type", "application/json")
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	fmt.Println(res.Status, string(out))
}

application/json

{
  "slug": "inrush-current-limiter",
  "values": {
    "ntcResistanceCold": 2.4,
    "peakInrush": 5,
    "timeConstant": 2.4,
    "energyAbsorbed": 72.00000000000001,
    "limitingResistor": 2.4,
    "hotRestartInrush": 24,
    "peakInrushMax": 6.25
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:inrush-current-limiter",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "TDK/EPCOS, NTC thermistors for inrush current limiting, application notes; Ametherm, How to select an inrush current limiter",
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "supplyVoltage": {
          "min": 0.1,
          "max": null,
          "unit": "V"
        },
        "filterCapacitance": {
          "min": 0.1,
          "max": null,
          "unit": "μF"
        },
        "targetInrush": {
          "min": 0.001,
          "max": null,
          "unit": "A"
        },
        "ntcResistanceHot": {
          "min": 0.001,
          "max": null,
          "unit": "Ω"
        },
        "ntcR25": {
          "min": 0,
          "max": 1000,
          "unit": "Ω"
        },
        "ntcTolerance": {
          "min": 0,
          "max": 50,
          "unit": "%"
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "supplyVoltage": 12,
      "filterCapacitance": 1000,
      "targetInrush": 5,
      "ntcResistanceHot": 0.5,
      "ntcR25": 0,
      "ntcTolerance": 20
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "supplyVoltage"
      ],
      "msg": "Input should be a valid number, unable to parse string as a number",
      "input": "abc"
    }
  ]
}

An input sent as text. Nothing was counted.

{
  "detail": "More than 120 requests in 60 seconds from this address.",
  "errorKind": "rate_limited",
  "limit": 120,
  "windowSeconds": 60,
  "upgradeUrl": "https://rftools.io/pricing/"
}

Too many requests in a minute, with Retry-After.

Same inputs on the calculator pageInrush Current Limiter (NTC) Calculator, at these values