Documentation sections

Zener Diode Voltage Regulator Calculator

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

Slug zener-diode. Calculate series resistor, power dissipation, and load current for zener diode voltage regulators

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 zener-diode
Name	Unit	Range	Default	Description
inputVoltagenumber	V	at least 0	12	Input Voltage
zenerVoltagenumber	V	at least 0.1	5.1	Zener Voltage
loadCurrentnumber	mA	at least 0	20	Load Current
zenerCurrentMinnumber	mA	at least 0.1	5	Minimum zener current for regulation (from datasheet)
zenerPowerMaxnumber	mW	at least 10	500	Maximum zener power rating
eSeriesnumber	none	0 … 96	24	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 zener-diode
Field	Unit	Description
seriesResistorheadline	Ω	Series Resistor
zenerCurrent	mA	Zener Current
zenerPower	mW	Zener Power Dissipation
totalCurrent	mA	Total Current
e24Resistor	Ω	The series resistor rounded to the nearest value of the part series (E24 unless another is chosen); the exact value with ideal values
zenerCurrentBuilt	mA	The zener current at full load through the series resistor fitted: (V_in − V_Z)/R − I_L. Below zero the zener is off and the output falls below V_Z
zenerPowerBuilt	mW	The zener dissipation at full load through the series resistor fitted, V_Z × I_Z (zero when the zener is off)

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": "zener-diode",
    "inputs": {
      "inputVoltage": 12,
      "zenerVoltage": 5.1,
      "loadCurrent": 20,
      "zenerCurrentMin": 5,
      "zenerPowerMax": 500,
      "eSeries": 24
    }
  }'

import rftools

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

result = client.calculate("zener-diode", {
    "inputVoltage": 12,
    "zenerVoltage": 5.1,
    "loadCurrent": 20,
    "zenerCurrentMin": 5,
    "zenerPowerMax": 500,
    "eSeries": 24,
})
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": "zener-diode",
    "inputs": {
      "inputVoltage": 12,
      "zenerVoltage": 5.1,
      "loadCurrent": 20,
      "zenerCurrentMin": 5,
      "zenerPowerMax": 500,
      "eSeries": 24
    }
  }),
});
console.log(response.status, await response.json());

package main

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

func main() {
	body := []byte(`{
	  "slug": "zener-diode",
	  "inputs": {
	    "inputVoltage": 12,
	    "zenerVoltage": 5.1,
	    "loadCurrent": 20,
	    "zenerCurrentMin": 5,
	    "zenerPowerMax": 500,
	    "eSeries": 24
	  }
	}`)
	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": "zener-diode",
  "values": {
    "seriesResistor": 276,
    "zenerCurrent": 5,
    "zenerPower": 25.5,
    "totalCurrent": 25,
    "e24Resistor": 270,
    "zenerCurrentBuilt": 5.555555555555557,
    "zenerPowerBuilt": 28.33333333333334
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:zener-diode",
    "version": "api@3f2c1a9b04de",
    "formulaRef": null,
    "assumptions": [],
    "validRange": {
      "status": "inside",
      "bounds": {
        "inputVoltage": {
          "min": 0,
          "max": null,
          "unit": "V"
        },
        "zenerVoltage": {
          "min": 0.1,
          "max": null,
          "unit": "V"
        },
        "loadCurrent": {
          "min": 0,
          "max": null,
          "unit": "mA"
        },
        "zenerCurrentMin": {
          "min": 0.1,
          "max": null,
          "unit": "mA"
        },
        "zenerPowerMax": {
          "min": 10,
          "max": null,
          "unit": "mW"
        },
        "eSeries": {
          "min": 0,
          "max": 96,
          "unit": ""
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "inputVoltage": 12,
      "zenerVoltage": 5.1,
      "loadCurrent": 20,
      "zenerCurrentMin": 5,
      "zenerPowerMax": 500,
      "eSeries": 24
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "inputVoltage"
      ],
      "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 pageZener Diode Voltage Regulator Calculator, at these values