# Talk:RC time constant

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Time constant is also used in Control Systems for integral and derivitave action controllers. These controllers can be Pneumatic, they don't need to be electronic.

The second external link seems to be down. Perhaps whoever added can verify if it is down temporarily or ? 76.200.129.205 (talk) 09:31, 8 April 2008 (UTC)

## Merge from RC delay

The RC delay article is a struggling stub with a very narrow view of what RC delay is. Let's merge it here, as a separate article is not needed, and then explain that RC circuits, whether simple or distributed, cause delay to logic signals. Dicklyon (talk) 07:06, 4 September 2008 (UTC)

Done. Dicklyon (talk) 15:03, 9 October 2008 (UTC)

## Standard time constants table

I find this table unclear and perhaps superfluous. First, it does not explicitly mention that these are cutoffs for audio filters only. Second, I question the necessity of including this table, in the absence of additional tables, as filters (RC and other first order) are in a myriad of applications outside of audio (e.g. wireless communications or medical imaging, just off the top of my head). The inclusion of this table with only audio filter time constants is seems to suggest that filtering is only or predominantly relevant to audio. Finally, other than providing an example of a few ${\displaystyle \tau =RC={\frac {1}{2\pi f_{c}}}}$ calculations, I don't think such an expansive table is necessary. Nytewing07 (talk) 18:48, 26 April 2011 (UTC)

It's unsourced and also too uninformative and incomplete to even be meaningful. It should be removed. Dicklyon (talk) 18:54, 26 April 2011 (UTC)
Agreed, especially since it doesn't mention the relationship to R and C, the topics of this page. I have removed the table. In general this page could do with a basic drawing of an R-C circuit to which it refers, and a plot of capacitor charging voltage. Otherwise it's difficult for someone unfamiliar with the subject to understand what it refers to. Gwideman (talk) 00:44, 8 August 2016 (UTC)
I agree that a plot of Ec versus time would illustrate how the voltage rises (charging) or falls (discharging). In its present state, the article implies by omission that the rate of change is linear. Akld guy (talk) 23:14, 18 August 2016 (UTC)

## Problematical wording

The article states this as a definition of RC time constant:

• It is the time required to charge the capacitor, through the resistor, by ≈ 63.2 percent of the difference between the initial value and final value or discharge the capacitor to ≈36.8 percent.
• An ip very recently attempted to change the wording to define RC with an applied DC voltage, but he was reverted. I think he was right. I think that the wording is misleading in that it is presenting a definition on the basis of "initial value and final value" without explanation that a zero-time step change between values is intended. The reader is led to believe that the definition presented could be based on an applied AC waveform.
• Suppose that a capacitor and resistor, both of small value, are connected to a voltage that rises very slowly over several minutes. RC, being small, would have no difficulty in tracking such a slowly-rising source, so the definition probably leaves readers confused about where the 63.2% limit comes from. Even if the voltage were to rise more sharply, the time taken (lag) for the capacitor to reach 63.2% is not wholly dependent on the RC product, but also on the rate of rise of the waveform, which shouldn't even be a factor. Attempting a definition on the basis of "initial value and final value" without explanation that a zero-time step change is intended is downright misleading.
• Better to explain it with DC and then go into an explanation in the next section of how this time delay is the basis for RC cut-off filters. That's not even explained in that section. Akld guy (talk) 10:26, 6 December 2017 (UTC)