# How to find the time constant?

Discussion in 'Homework Help' started by Heidi, Sep 27, 2013.

1. ### steveBWell-Known MemberMost Helpful Member

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MrAl,

That's all fine.

A few comments I'd like to make about what you said though.

First, my argument is not just about what is more useful, but certainly that is important. I'm also concerned about standard accepted approaches and definitions, and what holds up as we generalize to more complicated circuits. This is particularly important in a student's thread, I feel.

" ... the paper is asking for a single time constant for example they want to limit us to a SINGLE one, while you yourself would be more comfortable with TWO ..."

Second, I don't want to consider TWO time constants for the example given by the OP. I already said it is ONE for that case. So again I feel that my message is lost in the confusion of this thread. The two time constants comes when Rs is not zero. Simply, put - number of time constants should equal the number of degrees of freedom (or order) of the circuit. Early on, I objected to Ratch saying that circuit had two TCs, when the reference given said Single Time Constant Circuits. I said this more than once, but now you attribute that error to me. Again, - message lost in the confusion.

Third, I don't have any problem with someone defining as many RC's as they want and calling them time constants. As long is someone is clear about the meaning of those time constants and why they are important, there will be no issue. (I even made this point in my previous post above) However, a problem comes up if we are not clear. If we say it's a time constant, many people will expect it to show up in the exponential decay for system variables.

Anyway, I think I've said all I can say about this that can useful information, and need to get back to my own work. If anyone finds what I said useful, - great. If not, - well at least I tried.

Last edited: Oct 1, 2013
2. ### MrAlWell-Known MemberMost Helpful Member

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Hello again Steve,

Oh no, i didnt mean that you were in error in any way. When i said you liked two time constants i was aware that you would put that forth only with Rs included, so i never doubted you knew what you were talking about . If i worded it wrong i certainly apologize for that.

I get the feeling we are all sort of on the same page now. We all seem to agree that there are at least two important time constants but for the linked question problem they wanted only one that describes the entire circuit operation, and that is only possible in theory with no Rs, or to relax slightly with very small Rs or with a ramped wavefront.

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