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Javascript Resistor Calculation

October 7th, 2009 8:53 pm / Posted in Javascript by Karl

I recently wrote the following javascript class which can be used to calculate information about electronic resistors. There are various online apps which already do this sort of thing, but I thought I would have a go at creating something as it meant I would learn a bit about resistors in the process, and it may even help someone who is interested in looking at how these calculations work as well as getting the results.

The idea is, you create a Resistor object and provide some known information such as the colour bands, or the value, or the SMT code. The object will then calculate any missing information based on what you have provided. For instance, if you only have the resistor in front of you, you can set the object’s colour bands and it will calculate the resistor’s value, tolerance, temperature coefficient and SMT code.

Here is the class:

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function Resistor() {
   
    this.smtCode = null;
    this.value = 0;
    this.scaledUnit = null;
    this.scaledValue = 0;
    this.tolerance = 0;
    this.tempCoefficient = 0;
    this.bands = new Array();
   
    var that = this;
    var colorMap = {
        black:  {color: "black",  value: 0,    multiplier: 1,      tolerance: null, tempCoefficient: null},
        brown:  {color: "brown",  value: 1,    multiplier: 1e1,    tolerance: 1,    tempCoefficient: 100},
        red:    {color: "red",    value: 2,    multiplier: 1e2,    tolerance: 2,    tempCoefficient: 50},
        orange: {color: "orange", value: 3,    multiplier: 1e3,    tolerance: null, tempCoefficient: 15},
        yellow: {color: "yellow", value: 4,    multiplier: 1e4,    tolerance: null, tempCoefficient: 25},
        green:  {color: "green",  value: 5,    multiplier: 1e5,    tolerance: 0.5,  tempCoefficient: null},
        blue:   {color: "blue",   value: 6,    multiplier: 1e6,    tolerance: 0.25, tempCoefficient: null},
        violet: {color: "violet", value: 7,    multiplier: 1e7,    tolerance: 0.1,  tempCoefficient: null},
        gray:   {color: "gray",   value: 8,    multiplier: 1e8,    tolerance: 0.05, tempCoefficient: null},
        white:  {color: "white",  value: 9,    multiplier: 1e9,    tolerance: null, tempCoefficient: null},
        gold:   {color: "gold",   value: null, multiplier: 1e-1,   tolerance: 5,    tempCoefficient: null},
        silver: {color: "silver", value: null, multiplier: 1e-2,   tolerance: 10,   tempCoefficient: null},
        none:   {color: "none",   value: null, multiplier: 1,      tolerance: 20,   tempCoefficient: null}
    };
    var unitMap = new Array("pΩ", "nΩ", "µΩ", "mΩ", "Ω", "kΩ", "MΩ", "GΩ", "TΩ");

    this.setColorBands = function(colors) {
        for (index in colors) {
            if (typeof colorMap[colors[index]] != "undefined") {
                this.bands.push(colorMap[colors[index]]);
            }
        }
    }
   
    this.setSmtCode = function(code) {
        this.smtCode = String(code);
    }
   
    this.setValue = function(value) {
        this.value = Number(value);
        this.scaledUnit = calculateScaledUnit(this.value);
        this.scaledValue = calculateScaledValue(this.value);
    }
   
    this.calculate = function() {
        if (!this.value > 0) {
            this.setValue(calculateValue());
        }
        if (this.smtCode == null) {
            this.setSmtCode(calculateSmtCode(this.value));
        }
        if (!this.bands.length > 0) {
            this.setColorBands(calculateBands(this.value));
        }
        if (this.bands.length > 4) {
            this.tolerance = this.bands[4].tolerance;
            if (this.bands.length == 6) {
                this.tempCoefficient = this.bands[5].tempCoefficient;
            }
        }
    }
   
    this.toString = function() {
        var string = "Value: " + this.scaledValue + this.scaledUnit + " - ";
        string += "Colours: ";
        for (i in this.bands) {
            string += this.bands[i].color + ", ";
        }
        string = string.slice(0, -2) + " - ";
        string += "SMT Code: " + this.smtCode + " - ";
        string += "Tolerance: " + (this.tolerance == 0 ? "Unknown" : this.tolerance + "%") + " - ";
        string += "Temperature Coefficient: " + (this.tempCoefficient == 0 ? "Unknown" : this.tempCoefficient + "ppm");
        return string;
    }
   
    var calculateValue = function() {
        if (that.bands.length > 3) {
            return calculateValueViaBands(that.bands);
        }
        if (that.smtCode != null) {
            return calculateValueViaSmtCode(that.smtCode);
        }
        return false;
    }
   
    var calculateValueViaBands = function(bands) {
        var value = String(bands[0].value) + String(bands[1].value);
        var multiplier = bands[2].multiplier;
        if (bands.length > 4) {
            value += String(bands[2].value);
            multiplier = bands[3].multiplier;
        }
        return Number(value) * multiplier;
    }
   
    var calculateValueViaSmtCode = function(smtCode) {
        var value;
        if (smtCode.indexOf("R") > -1) {
            value = smtCode.replace(/R/i, ".");
        } else {
            var multiplier = 1;
            switch (smtCode.length) {
                case 2:
                    value = smtCode;
                    break;
                case 3:
                    value = smtCode.substring(0, 2);
                    multiplier = smtCode.substring(2);
                    break;
                case 4:
                    value = smtCode.substring(0, 3);
                    multiplier = smtCode.substring(3);
                    break
                default:
                    return false;
            }
            value = Number(value) * Math.pow(10, Number(multiplier));
        }
        return value;
    }
     
    var calculateBands = function(value) {
        var multiplier = Math.pow(10, String(value).length - 2);
        var values = String(value / multiplier).split("");
        var result = new Array();
        for (i in colorMap) {
            if (colorMap[i].multiplier == multiplier) {
                multiplier = i;
                break;
            }
        }
        for (i in values) {
            for (j in colorMap) {
                if (colorMap[j].value == Number(values[i])) {
                    result.push(j);
                }
            }
        }
        result.push(multiplier);
        return result;
    }
   
    var calculateSmtCode = function(value) {
        var indeces = calculateBands(value);
        var multiplier = indeces.pop();
        var lastDigit = (Math.log(colorMap[multiplier].multiplier) / Math.LN10);
        if (lastDigit > 9) {
            return false;
        }
        var code = "";
        for (i in indeces) {
            code += colorMap[indeces[i]].value;
        }
        return code + lastDigit;
    }
   
    var calculateScaledValue = function(value) {
        return value/Math.pow(1000, getExponent(value));
    }
   
    var calculateScaledUnit = function(value) {
        if (value == 0) {
            return "Ω";
        }
        var unit = unitMap[getExponent(value) + 4];
        if (typeof unit == "undefined") {
            return false;
        }
        return unit;
    }
   
    var getExponent = function(value) {
        return Math.floor(Math.log(value) / Math.log(1000));
    }
   
}

It should be pretty self explanatory, but here are a few simple very examples of usage (these are really just tests and will only work in firefox due to the console.log and String::toString methods):

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var resistor = new Resistor();
resistor.setColorBands(["brown", "black", "black", "black", "brown", "yellow"]);
resistor.calculate();
console.log(resistor.toString());
// Value: 100Ω - Colours: brown, black, black, black, brown, yellow - SMT Code: 101 - Tolerance: 1% - Temperature Coefficient: 25ppm
       
var smtResistor = new Resistor();
smtResistor.setSmtCode("101");
smtResistor.calculate();
console.log(smtResistor.toString());
// Value: 100Ω - Colours: brown, black, brown - SMT Code: 101 - Tolerance: Unknown - Temperature Coefficient: Unknown
     
var knownResistor = new Resistor();
knownResistor.setValue(220);
knownResistor.calculate();
console.log(knownResistor.toString());
// Value: 220Ω - Colours: red, red, brown - SMT Code: 221 - Tolerance: Unknown - Temperature Coefficient: Unknown

This entry was posted on Wednesday, October 7th, 2009 at 8:53 pm and is filed under Javascript. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

One Response to “Javascript Resistor Calculation”

  1. Boomg says:
    October 27, 2009 at 2:13 am

    Its pretty good. Well done!

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