Didn't understood that... I'll post it right a way!

Great.

When you post an image, take some time to do a bit of editing on it to make the size convenient. After you capture the image using whatever technique you like, open it in an image editing software (Paint works just fine) and resize the image so that it is around 400 pixels wide (more or less depending on level of detail needed) and then save it as a PNG file.

If it is a picture, like some hand-drawn stuff that is scanned or captured with your phone's camera, this is particularly important because modern imaging devices often produce images that are thousands of pixels wide with huge file sizes that most of us won't download. After resizing those images, save them as JPG instead of PNG as JPG is specifically designed to get superior compression on photographs, while PNG is superior (and lossless) for line art.

 

You use Uc for charging phase and Vc for the discharging phase. Was it on purpose??? Is there any difference?

There is no difference at all Vc = Uc.
I simply made a error, because in my country we do not have a V in our alphabet so we use letter "U" instead of "V".

What is exactly the formula you're using to calculate capacitor's voltage at a given time?

Is it:

For charging phase
\( V_c(t) = (V_{final} - V_{start})(1 - e^{\frac{-t}{RC}}) \)

And for discharge phase.
\( V_c(t) = V_{start}( e^{\frac{-t}{RC}}) \)

 

Ok, so when I change Vin pulse's frequency, will I still be able to see the same 220mS for the 1st time constant??? Is it valid for any frequency/period?

 

I certainly see your simulation's point of view... You used a very small frequency (0.33Hz->3S period) so that we can see the capacitor discharge phase. And also used a pretty high Von value of 8V.
The difference is not that much, I guess. It's only a matter choosing enough reasonable values to see the capacitor's charge and discharge phase. However our teacher said to us to use a Von peak value of 1V without negative values (from 0V to 1V).

So, this said, and knowing that a period based in seconds and not milliseconds, what would be the frequencies to use in a few additional simulations as teacher asks?

Hola Scorpie:

At first, use a period long enough to see the whole charge / discharge periods. Then, start reducing the period (increasing frequency) and you could still verify that the 63% point (which occurs at time tau) never changes. If you think of it is what you could do in real hardware with a function generator where you could change the frequency at will.

What Jony130 explained is obviously right but with your example, no easy for you to see at this stage of your understanding.

By the way, regarding voltage, 8V or 1V, would not change anything a long as the minimum is at 0V. If you feel comfortable with 1V, be my guest.

Actually, I used 8V thinking of showing a quite simple trick: if you have the voltage over 8 vertical divisions (in a scope you could do it fairly easily), the 63% point will be very close to the 5th division (8*0,63 = 5,04). Simple eh? I did not mention that because, LTSpice is not showing actually 5V so you need to use the cursors anyway.

Hope this helped.

In case of doubt: tau IS constant.

 

Ok, so when I change Vin pulse's frequency, will I still be able to see the same 220mS for the 1st time constant??? Is it valid for any frequency/period?

Yes, but keep in mind that it is the time it would take to reach 63% of the final step voltage change IF it were allowed sufficient time to get all the way to the final value. As your period reduces, eventually you will see that the peak value it gets to starts dropping, but not the value that it is at after one time constant. This makes sense because the circuit has no idea when the applied voltage will change or what the final voltage will actually be. As you reduce the period to less than two time constant, of course, you will not even be giving the circuit enough time to reach the voltage associated with one time constant, but the shape of the curve is still such that it would have reached that same value had it been allowed to.

 

Five times tau times two would be a fifty percent duty cycle square wave period. You would see the classic charge/discharge curves.

 

Ok, guys... Thanks to all... I think I got all the points now!

I'm already simulating and calculating the the cut-off frequency for high pass and low pass filters and also plotting the frequency response for both circuits. If I have any doubts about it, I'll upload the circuits files and also some prints...

 

How are you going about simulating the cut-off frequencies? There are a couple of ways to go about it, but there are many more wrong ways.

 

How are you going about simulating the cut-off frequencies? There are a couple of ways to go about it, but there are many more wrong ways.

I'm looking at this but I don't know how to do it in LTSpice...

A few tips will help...

 

TI in their site, in the area dedicated to LTSpice has some basics published.

Otherwise, google for Simon Bramble. Short and to the point.

When in LTSpice, one of the keywords: AC analysis. Buena suerte.

 

TI in their site, in the area dedicated to LTSpice has some basics published.

Otherwise, google for Simon Bramble. Short and to the point.

When in LTSpice, one of the keywords: AC analysis. Buena suerte.

Sorry, what is TI???

I was thinking about just plot the frequency response and look for the \(\displaystyle{\frac{1}{\sqrt{2}} \cdot V_{in}}\) and point it out with an arrow and comment!

 

TI is Texas Instruments.

Doing a frequency response (a.k.a., AC analysis) is one of the two basic ways and, in general, the better way. The other way is to do it the way you would do it in the real world if all you had was a meter or an o-scope, which is to adjust the frequency until you found the half-power frequency.

 

Well, I did it in LTSpice and placed a mark were Vout is around 0.7 of Vin. That matched the math I did!

 

Great! It's always reassuring when the practice matches the theory. Of course, simulation is one thing and building the circuit and actually measuring it is something else -- and another opportunity to feel that rush of excitement and achievement.

 

Ok, this one is finished and sent to the teacher.
Now I'm going to start preparing the next one in a new thread!

About this one, what is missing can only be done in school's labs. It's build those circuits in breadboards and take some measurements and note them in an excel sheet, write a few comments and send it to the teacher.

This said I can call it like Done or Solved!

Thanks to all that helped!

 

TI in their site, in the area dedicated to LTSpice has some basics published.

Do you have a link to that?

 

Do you have a link to that?

It would really be easier!

 

It would really be easier!

Twice on the same day I made the mistake confusing TI with LT (who owns LTSPice). My bad.

Linear Technology, sure you know that by now, has a lot on their site about LTSpice.

 

Twice on the same day I made the mistake confusing TI with LT (who owns LTSPice). My bad.

Linear Technology, sure you know that by now, has a lot on their site about LTSpice.

No problem at all.
It's human to make mistakes... Who have never done any, can throw a rock!