user25615: i'm trying to find out the capacitance value using the time constant of the RC ciruit. so i followed the technique on the below link.
http://www.gammon.com.au/forum/?id=12075
Now, it's working fine.. but still i need wider range so i want to know the full core logic of that program like how the capacitance formula has been derived out in those program?
thanks in advance...

user4143: The key concept is this line:

... we find the point where the voltage reaches 63.2% ...

The time constant τ is how long it takes for a capacitor to charge up to 63.2% of the supply voltage. That is governed by both the capacitance (C) and the resistance (R). τ=RC. That is one of the basic formulae you should know as an Electronic Engineer.
If you know R (which you do because you chose it at 10KΩ) and you know τ (which you do because you measured it) then you can calculate C (τ/R). 
Since τ, C and R are tied together like that it is possible to change the balance of C and R and keep τ the same. For instance, a 10KΩ resistor and a 47nF capacitor would give the same τ as a 1KΩ resistor and a 470nF capacitor. Also the same τ as a 100kΩ resistor and a 4.7nF capacitor.
You can easily change the range without having to change your time measurement system by changing the resistor to give a different scale.

25615: <p>i'm trying to find out the capacitance value using the time constant of the RC ciruit. so i followed the technique on the below link.</p>

<p><a href="http://www.gammon.com.au/forum/?id=12075" rel="nofollow">http://www.gammon.com.au/forum/?id=12075</a></p>

<p>Now, it's working fine.. but still i need wider range so i want to know the full core logic of that program like how the capacitance formula has been derived out in those program?</p>

<p>thanks in advance...</p>
 4143: <p>The key concept is this line:</p>

<blockquote>
  <p>... we find the point where the voltage reaches 63.2% ...</p>
</blockquote>

<p>The time constant τ is how long it takes for a capacitor to charge up to 63.2% of the supply voltage. That is governed by both the capacitance (C) and the resistance (R). τ=RC. That is one of the basic formulae you should know as an Electronic Engineer.</p>

<p>If you know R (which you do because you chose it at 10KΩ) and you know τ (which you do because you measured it) then you can calculate C (τ/R). </p>

<p>Since τ, C and R are tied together like that it is possible to change the balance of C and R and keep τ the same. For instance, a 10KΩ resistor and a 47nF capacitor would give the same τ as a 1KΩ resistor and a 470nF capacitor. Also the same τ as a 100kΩ resistor and a 4.7nF capacitor.</p>

<p>You can easily change the range without having to change your time measurement system by changing the resistor to give a different scale.</p>
