how to find find out capacitance using time constant? 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...
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.