Documentation sections

BJT Transistor Switch Calculator

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

Slug transistor-switch. Calculate BJT transistor switch parameters including load current, required base current, base resistor value, saturation check, and power dissipation

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 transistor-switch
Name	Unit	Range	Default	Description
vccnumber	V	at least 0.1	5	Supply Voltage
loadResistornumber	Ω	at least 0.1	100	Load Resistance
inputVoltagenumber	V	at least 0	3.3	Logic-high voltage driving the transistor base through R_B
betanumber	none	at least 1	100	Minimum DC current gain (hFE) from datasheet
vbenumber	V	0.1 … 0.9	0.7	Base-Emitter Voltage
vce_satnumber	V	at least 0.01	0.2	Collector-emitter saturation voltage from datasheet
overdriveFactornumber	×	1 … 20	5	Factor by which base current exceeds minimum required (typically 5-10×)
eSeriesnumber	none	0 … 96	0	Rounds every part to the nearest value in the series; the response and the export are the rounded design's.One of: 0 Ideal values · 12 E12 (10%) · 24 E24 (5%) · 48 E48 (2%) · 96 E96 (1%)

Outputs

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

Outputs of transistor-switch
Field	Unit	Description
loadCurrent	mA	Load Current
ib_required	μA	Required Base Current
rbheadline	kΩ	The base resistor for the overdrive chosen; with a part series, the nearest value of the series
ib_actual	μA	Actual Base Current
powerDissipation	mW	Transistor Power Dissipation
switchingState	none	Saturated? (1=Yes, 0=No)

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": "transistor-switch",
    "inputs": {
      "vcc": 5,
      "loadResistor": 100,
      "inputVoltage": 3.3,
      "beta": 100,
      "vbe": 0.7,
      "vce_sat": 0.2,
      "overdriveFactor": 5,
      "eSeries": 0
    }
  }'

import rftools

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

result = client.calculate("transistor-switch", {
    "vcc": 5,
    "loadResistor": 100,
    "inputVoltage": 3.3,
    "beta": 100,
    "vbe": 0.7,
    "vce_sat": 0.2,
    "overdriveFactor": 5,
    "eSeries": 0,
})
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": "transistor-switch",
    "inputs": {
      "vcc": 5,
      "loadResistor": 100,
      "inputVoltage": 3.3,
      "beta": 100,
      "vbe": 0.7,
      "vce_sat": 0.2,
      "overdriveFactor": 5,
      "eSeries": 0
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "transistor-switch",
	  "inputs": {
	    "vcc": 5,
	    "loadResistor": 100,
	    "inputVoltage": 3.3,
	    "beta": 100,
	    "vbe": 0.7,
	    "vce_sat": 0.2,
	    "overdriveFactor": 5,
	    "eSeries": 0
	  }
	}`)
	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": "transistor-switch",
  "values": {
    "loadCurrent": 48,
    "ib_required": 480,
    "rb": 1.0833333333333333,
    "powerDissipation": 9.600000000000001,
    "switchingState": 1,
    "ib_actual": 2400
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:transistor-switch",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "vcc": {
          "min": 0.1,
          "max": null,
          "unit": "V"
        },
        "loadResistor": {
          "min": 0.1,
          "max": null,
          "unit": "Ω"
        },
        "inputVoltage": {
          "min": 0,
          "max": null,
          "unit": "V"
        },
        "beta": {
          "min": 1,
          "max": null,
          "unit": ""
        },
        "vbe": {
          "min": 0.1,
          "max": 0.9,
          "unit": "V"
        },
        "vce_sat": {
          "min": 0.01,
          "max": null,
          "unit": "V"
        },
        "overdriveFactor": {
          "min": 1,
          "max": 20,
          "unit": "×"
        },
        "eSeries": {
          "min": 0,
          "max": 96,
          "unit": ""
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "vcc": 5,
      "loadResistor": 100,
      "inputVoltage": 3.3,
      "beta": 100,
      "vbe": 0.7,
      "vce_sat": 0.2,
      "overdriveFactor": 5,
      "eSeries": 0
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "vcc"
      ],
      "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 pageBJT Transistor Switch Calculator, at these values