RC circuit and discharging a capacitor

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Homework Help Overview

The discussion revolves around an RC circuit involving a resistor and capacitor in series, specifically focusing on the discharging behavior of the capacitor when subjected to a square wave from a function generator. Participants are exploring how to graph the voltage across the capacitor over time.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the use of the equation V(t) = Vo * e^(-t/RC) and question the initial voltage Vo in the context of a square wave input. There are inquiries about the configuration of the circuit and the implications of the function generator's settings on the capacitor's charging and discharging behavior. Some participants suggest using Laplace transformations for analysis, while others seek numerical methods for graphing the voltage over time.

Discussion Status

The conversation is ongoing, with various interpretations of the problem being explored. Some participants have provided guidance on how to approach the graphing of Vcap versus time, while others are still clarifying concepts and seeking further information about the function generator's settings and the relationship between frequency and the RC time constant.

Contextual Notes

Participants are working within the constraints of the problem, including the specified values for resistance and capacitance, and the nature of the input signal from the function generator. There is a focus on understanding the implications of frequency settings and the time constant in relation to the behavior of the circuit.

-EquinoX-
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Homework Statement



RC circuits and discharging a capacitor

ok, say I have the following:

R = 4700 ohm
C = 39 micro farads
they are arranged in series and the battery is replaced using a function generator, which we set to produce a square wave (on/off). With any frequencies set.

How can I graph the Vcap vs time?

Homework Equations


The Attempt at a Solution



I know the equation V(t) = Vo * e^(-t/RC) can be used.. however in this case what is Vo (initial voltage) here?

if this is so then what is the purpose of the square wave function generator? Doesn't it matter on how big the frequencies are set?
 
Last edited:
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Are the R and C in series or parallel? :smile:
 
aniketp said:
Are the R and C in series or parallel? :smile:

oops forgot to mention that, it's in series
 
If you are familiar with Laplace transformation, this problem can simply be solved by making a laplace transformation of the circuit and calculating the necessary value.
 
no.. I am not familiar with Laplace transformation
 
anyone?
 
When you swith on the function generator, a peak voltage will be across RC circuit. During horizontal protion of the square wave, capacity will start charging. During vertically down ward portion and next horizontal portion of the square wave the capacity will discahrge.
 
so I want to get this numerically... is there a way to compute it
 
If you know the peak voltage and the period of the square wave, it is possible to find V(t).
 
  • #10
well the period of the square wave I know, but how do I get the peak voltage?

I am just using a normal function generator.. so maybe the peak voltage is known...
 
  • #11
You can select the peak voltage of the function generator, say 20 V peak-to-peak and period 500 ms. (greater then time constant RC). Find the voltage at every 50 ms. Plot the graph V(t) vs time.
 
  • #12
I don't want to do this experimentally.. I want to do this on paper actually... so if the peak voltage is 20V and period was 500 ms what is the Vo
 
  • #13
How can I graph the Vcap vs time?
To draw graph use V(t) = 20*e^-t/RC formula.
Put the values of t as 50ms. 100ms, ,,and so on. Substitute the values of RC. Find the values of V(t)
 
  • #14
ok.. gotcha.. is there actually a setting in the function generator to set the peak voltage... or is that hard coded in every function generator
 
  • #15
Yes. It is there. It can be varied, say from 0 to 30V.
 
  • #16
What does the amplitude take account for?
 
  • #17
-EquinoX- said:
What does the amplitude take account for?

Which amplitude? Amplitude of square wave it self is the Vo.
 
  • #18
What would be a good frequency to set on the function generator if the amplitude was around 2-5V and the RC is as I specified in my first post... I need some range of the frequency
 
  • #19
In the given circuit the time constant RC = 0.183 S. So to draw the graph, if possible set the function generator to 2 Hz.
 
  • #20
what is S there? 2 Hz.. isn't that really2 small? What should I set the time/div to be
 
  • #21
-EquinoX- said:
what is S there? 2 Hz.. isn't that really2 small? What should I set the time/div to be

Set to 500 ms.
 
  • #22
and what is S?
 
  • #23
RC is the time constant and it is measured in seconds.
 

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