# universal time constant formula

Discussion in 'Homework Help' started by ra1ph, Jan 8, 2010.

1. ### ra1ph Thread Starter Active Member

Jan 5, 2010
31
0
Hi i need your help , new to this forum.

I'm a first year electronics student with semester 1 examination tomorrow.
I'm trying to understand the universal time constant formula for capacitors. I understand the formula when given: time = >0 my problem is when time = Zero.
I have read and understand the example given on this site

Volume I - DC » RC AND L/R TIME CONSTANTS »

Could someone help me or point me to an example where t = 0?

Thanking You

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Look at it this way - voltage can only change as the time changes.

3. ### Boo Member

Oct 27, 2009
40
0
the expression for charging or discharging always e^-t/RC. Where Rc is the time constant. If 0 time has passed we get e^0 which is simply 1, in other words, nothing has changed.

4. ### ra1ph Thread Starter Active Member

Jan 5, 2010
31
0
Ok thanks, My problem was with the (final-start) voltage or current part of the formula. I get it now- nothing has changed!

Thanks

5. ### Boo Member

Oct 27, 2009
40
0
Remember that voltage does not change instantaneously in a capacitor!

6. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
And current doesn't change instantaneously in an inductor. Same principal, similar math.

7. ### ra1ph Thread Starter Active Member

Jan 5, 2010
31
0
Thanks!
so the following formula is valid for when t = 0

change=(final-Start)(1-1/e^(t/RC))

because change = 0 always when t = 0

It does not matter what the final and start voltages are. I expected to get what the actual voltage or current were are time = 0 when using the formula mentioned.

So what formula should I use to get the actual values when t = 0 or is just a understanding that you need?

8. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
If the problem starts at T = 0, then the initial states are as given.

9. ### hitmen Active Member

Sep 21, 2008
159
0
Just remember the magic formula

X(t) = x(∞) + x(0)e^-T/tau

This is what I remembered if I remembered it correctly