# Low pas filter time constant for a square input

I am trying to plan a low pass filter as in the picture: I use it to create an ON/OFF lag (Is this the correct term?) I have calculated the time constant τ of the system: My question is:

• How do I calculate the time constant for the same circuit, but with a square (not sine) input?
• The time constant refers to the system, not the signal.
– Chu
May 7, 2018 at 9:47
• The time constant, $\small \tau$, is defined by the circuit, not the signal(s). In this case, $\small\tau=RC$. Also 'delay' is the wrong word here; it should be 'lag'.
– Chu
May 7, 2018 at 10:03
• I would like to do that but there is no equation editor here In math forum for example I use  to insert an equation. Is there something similar here? May 7, 2018 at 18:26

the calculation seems a little problem

1. τ is a constant number which is only defined as τ=RC ,regardless of what input signals.
2. The phase lag @ f=fc is -45 degree calculate as follows: • Thanks! Is this the calculation for the square wave? Also, what is A(f) and, The graphs are from a simulator (what simulator if I may) Thanks again! May 7, 2018 at 15:03
• Matlab can do this simulation May 8, 2018 at 0:39
• And，if you don't have matlab ,there is also a website : May 8, 2018 at 0:40

Why people always take 4.7kΩ and not, say 5kΩ ?

Well, because there are some standard values (called E-values) for resistors they are manufactured. The reason behind this is easy: this way you can create any resistance value with the right combination of these.

How do I calculate the time constant for the same circuit, but with a square (not sine) input?

A square wave is a combination of infinite sinusoidals (see Fourier transformation) so you won't get an exact expression for any calculation regarding frequency, you can only calculate things for different sinusoidal components of the square wave

• Can I get an estimation? If yes is it what is done in the answer of Lukeluck? Thanks! May 7, 2018 at 15:04
• Estimation of what? The time constant you were talking about in the question is the property of the circuit and is independent from the source
– U.L.
May 9, 2018 at 10:44
• I thought so too but my adviser asked me to give a solution specifically for a square said what i gave was for sine only... I'm beginning to be tired of not understanding what people are taking about... anyways, thanks a lot for your effort and detailed calculation and graphs... May 9, 2018 at 17:33

And，if you don't have matlab ,there is also a website: Online simulator