RC Filter Calculator

RC Low-Pass Filter - 3 Button

Enter any two of parameters R, C or fc. Then hit the button of the desired parameter.

Design Example:
Suppose you have R=100 and a desired cutoff of fc=1e6. Find the capacitance needed by entering the two parameters above and hit "Get C".

Schematic

rc filter

Enter Components

Download / Basics

To run or modify on your PC, download a simple version of this file with image (*.zip).
For a quick tour of some JavaScript / HTML basics, check out Inside the Code.

JavaScript

This code features 3 separate function calls, one per "Get_x" button.

//////////////////////////////////////////////
// calc R
//////////////////////////////////////////////
	function getR_RC_Fil() {
		// get values directly from form
		var fc=document.myForm.fc.value
		var C=document.myForm.C.value
		var R
 	    // calc 
 	    R=1/(fc*C*2*Math.PI);
 	    // place in text box
	    document.myForm.R.value = (R).toPrecision(4);
	}

//////////////////////////////////////////////
// calc C
//////////////////////////////////////////////
	function getC_RC_Fil() {
		// get values directly from form
		var R=document.myForm.R.value
		var fc=document.myForm.fc.value
		var C
 	    // calc 
 	    C = 1/(R*fc*2*Math.PI);
 	    // place in text box
	    document.myForm.C.value = (C).toPrecision(4);
	}

//////////////////////////////////////////////
// calc fc
//////////////////////////////////////////////
	function getfc_RC_Fil() {
		// get values directly from form
		var R=document.myForm.R.value
		var C=document.myForm.C.value
		var fc
 	    // calc
 	    fc=1/(R*C*2*Math.PI);
 	    // place in text box
	    document.myForm.fc.value = (fc).toPrecision(4);
	}
///////////////////////////////////////////////

HTML

Each input button calls its own JavaScript function when clicked.

R Ohms			R = 1/(2pifc*C)
C Farads			C = 1/(2pifc*R)
fc (Hz)			fc = 1/(2piR*C)