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12th July 2012, 11:51 #1
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Question about a simple RC low pass filter
Hi,
A simple low pass filter consist of a series resistor,
and followed by a capacitor connected to ground.
Why can't a simple low pass filter be only a capacitor, without the resistor?
Thank you for your advise.
Siong Boon

12th July 2012, 12:40 #2
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Re: Question about a simple RC low pass filter
The Edge... there is no honest way to explain it, because the only people who really know where it is are the ones who have gone over.
 Hunter S. Thompson

12th July 2012, 12:40

12th July 2012, 14:01 #3
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Re: Question about a simple RC low pass filter
I don't think that single capacitors can act as a lowpass filter.
Explanation: A lowpass must have a pass band region and a second region with increasing attenuation (stop band). Both regions are separated by the corner frequency (in most cases: 3dB frequiency).
Otherwise, it is not a lowpass.
Case 1: voltagein and voltageout.
A first order RC lowpasss has a 3dB frequency of wo=1/RC.
If the resistor R is decreased towards zero the frequeny wo approaches infinity  there is no frequency dependence at all.
Case 2:: Currentin and voltageout
If in the parallel combination RC the resistor R approaches infinity, we have wo=0 and the circuit resembles a current integrator (without any passband).
_____________
That means: In both cases we have no lowpass filtering.
   Updated   
Remark: Of course, in some cases the necessary resistance is not a compact part but a wire or something else that has resistive properties to be exploited.

12th July 2012, 14:01

12th July 2012, 15:37 #4
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Re: Question about a simple RC low pass filter
I used to think like bigdogguru, and started to realise that a resistor may be required from other forum. A capacitor alone is not correct.
LvW is trying to explain the effect of the resistor in terms of the formula.
I can understand that, but still find it difficult to understand it.
How the power supply's capacitor managed to do low pass filtering without the resistor intentionally place in front/series of capacitor?
Can the RC filter be explained in a easy way without using the standard formula?
1/RC formula must representing one of the design.
I would like to understand it from the circuit and component's point of view.
Why must the resistor be there?
I suppose it has something to do with the impedance of the load/source.
I need more guidance on this, Thank you.

12th July 2012, 15:53 #5
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Re: Question about a simple RC low pass filter
From a purely theoretical standpoint, I fully agree with LvW, for the reasons he cited.
However, from a practical standpoint, as both of us noted:
It is the presence of this "resistance" which makes the implementation of a low pass filter with the application of a single capacitor possible and often utilized in the sense of a practical application.
As you did not specify the analysis of your query to be in theoretical or practical terms, you have receive both.
BigDogThe Edge... there is no honest way to explain it, because the only people who really know where it is are the ones who have gone over.
 Hunter S. Thompson

12th July 2012, 15:53

12th July 2012, 16:16 #6
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Re: Question about a simple RC low pass filter
It seems as though the equivalent resistance of the device is going to be the "R" in the lowpass filter equation. By putting a single capacitor in shunt with the device a filter is created. A single capacitor model just adds a phase shift of +/ 90 degrees to your voltage/current.

12th July 2012, 16:27 #7
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Re: Question about a simple RC low pass filter
OK, siongboon, I'll try another explanation without formulas.
At first, the main task of a power supply capacitor is not lowpass filtering in the classical sense. Rather, these capacitors shall serve as an ac short of the power line.
Regarding the RC lowpass: If you need a passive circuitry that delivers at its output a smaller voltage than at its input you need a voltage divider.
If the division ratio should be frequencydependent you need a frequency dependent voltage divider.
Low pass case: Large division ratio for the lower part of the frequency spectrum and smaller for increasing frequencies.
Thus, knowing the properties of a capacitor it is clear that we need the classical RC arrangement for low pass filtering.
Assuming zero source impedance (ideal voltage source) there is no voltage divider at all without this resistor R  and as the result we have input=output=const (no filtering) .
Of course, a real signal voltage source always has a source resistance Rin which can take over the task of the filter resistor  if the value of Rin is known (and constant).
Therefore, to be (nearly) independent on this unknown value Rin, the filter resistor should be selected much larger than the expected value for Rin.
Further questions?

12th July 2012, 18:42 #8
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Re: Question about a simple RC low pass filter
singboon,
Regarding your comment on power supply filters performing without the aid of a series resistor: The formula for cutoff frequency depending on a series resistor is based on linear circuit theory. A power supply filter has highly nonlinear circuit element(s), the diode(s). In this case, the capacitor is used strictly as an energy storage component.

12th July 2012, 20:52 #9
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Re: Question about a simple RC low pass filter
In a theoretic (non realistic) situation where you have an ideal voltage generator connected to a (ideal) capacitor with no series resistor, the voltage will be imposed by the generator and there is no way the capacitor can lower this voltage. Independently from the frequency.
If we instead suppose to have the voltage drop, this means that voltages across generator and capacitor will be different. But since the resistance in between is zero (ideal condition) means the current will be V/0 > infinite regardless from the level of V.
During the initial transient, if the capacitor is not charge, there will be an ideally infinite current pulse of duration > 0 that will charge istantaneously the capacitor.
this means single capacitor can be used as filter, only using the parasitics L and/or R of wiring and nonideal internal resistance of voltage sources that will dissipate the power generated by the flowing current and the voltage drop (depending from the frequency).

12th July 2012, 20:52

15th July 2012, 10:03 #10
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Re: Question about a simple RC low pass filter
Thank you for the various explanation.
I think I have got some of the insight after the explanation.

17th July 2012, 20:35 #11
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Re: Question about a simple RC low pass filter
A resistor inline limits current to the capacitor. The current is what 'fills up' the capacitor.
Thus a higher series resistance makes it take longer to charge to a given volt level.
The volt level on the capacitor opposes an incoming signal (concept of a low pass filter).
I have Youtube videos which portray capacitor action visually. Current through wires is portrayed. Dynamic charge level on a capacitor is portrayed.
www.youtube.com/watch?v=eIWEU4pObJw
Equivalent Series Resistance comes into play when capacitors are used to filter a power supply. A capacitor with low ESR has a short time constant, and it works well at filtering ripple V at 5060 Hz.
A capacitor with high ESR becomes 'sluggish' in comparison. It has a longer time constant. It becomes ineffective as a high pass filter.
Link to my Youtube video demonstrating this visually, over a range of ESR values:
www.youtube.com/watch?v=vm8wf7y94rM

18th July 2012, 01:45 #12
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