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RC Filter - Step Response vs. Time

Given R, C and Vin, calculate the step response and error for N time constants.

R (Ohms)
C (F)
Vin (V) Input voltage.
N Number of Tau to calculate vo.

Tau (s) Tau = R*C
fc (Hz) fc = 1/(2*pi*R*C)

Step Response: vo = Vin * Exp(-t/Tau)

Download / Basics

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


Some fairly straightforward RC filter calculations appear in the code. The intersting bit lies in writing to the "textarea". The textbox is only written to once at the end!

First, the code adds a line of text to the variable "TextOut" after each calculation in the for loop. Finally, "TextOut" is written to "myTextArea" as defined by the form.

// calc 
function get_StepResponse() {

   // get values directly from form
   var R=document.myForm.R.value;
   var C=document.myForm.C.value;
   var N=document.myForm.N.value;

   var Tau, fc, t, vo, vo_err, vo_errp;
   var TextOut;
   // calc
   Tau = R*C;
   // place in text box
   document.myForm.fc.value = (fc).toPrecision(4);
   document.myForm.Tau.value = (Tau).toPrecision(4);
   // create text
   TextOut = "t(s)\t\tvo\t\tvo_error (%)\n";  
   TextOut += "---------------------------------------\n";
   // calc step response and place in text box
   for(var i = 0; i < N; i++) {
	vo = vin * (1-Math.exp(-t/Tau));
	vo_err = vin - vo;
	vo_errp = vo_err/vin * 100;
	TextOut +=  (t).toExponential(3) 
	      + "\t" + (vo).toExponential(3)+ "\t" 
	      + (vo_errp).toPrecision(3)+ "\n";
   // send to textArea
   document.myForm.myTextArea.value = TextOut  	    


The text box is easily createrd by the tag named "textarea". You can specify the Rows and Cols inline with the tag.

<textarea name="myTextArea" rows="15" cols="50"></textarea>