# Determining the type of AC filter.

Discussion in 'Homework Help' started by Petrucciowns, Jun 24, 2009.

1. ### Petrucciowns Thread Starter Active Member

Jun 14, 2009
62
0
By looking at the components in an AC filter how can you determine what type it is based on the following information?
The impedance of a capacitor at very high frequencies is low​
The impedance of a capacitor at very low frequencies is high.​
Inductors have a direct relationship with frequency​
The impedance of an inductor at very high frequencies is high​
The impedance of an inductor at very low frequencies is low​
The impedance of a resistor stays constant​
A series resonant circuit at the resonant frequency is low impedance.​
A parallel resonant circuit at the resonant frequency is a High impedance​

This has been causing be some problems and I would appreciate some help.​
Thank you.

2. ### scythe Active Member

Mar 23, 2009
49
6
Believe it or not, these are actually very good questions. When people understand these things well, the rest (bode plots, 3db cutoff frequencies, high vs low frequency filters, etc) usually follow.

Most of the time, it is the component's placement in the filter that will determine its effect on the rest of the circuit, in much the same way that a very, very large resistor (say "∞"Ω) will be negligible when placed in parallel with another component, but will remain a very, very large resistance when put in series with the same component. It is not the fact that the resistor is large, but how you use it that determines its function.

If this sounds confusing (and it probably is...), check out the schematic I attached.

Now if you grasp that concept, remember that the impedance for a capacitor is: 1/jωC, where "ω" is angular frequency and = 2∏f.
So, at very high frequencies, a capacitor by itself will have an impedance of about 0 Ω. Its behavior will resemble a wire, for impedance purposes. A wire will short something out entirely if it is placed in parallel with it, but won't act this way if placed in series with the same component.

This is why a capacitor placed in series with other components acts as a "high pass" filter. It has virtually no impedance and everything "passes" through it.

An inductor is opposite ( Z = jωL).

To analyze how these components will modify your circuit's behavior at high or low frequencies, plug in "0" or "∞" for your frequency(ω) and see if the components will act as an open circuit, or as a short.

Hope this helps, and that I didn't say anything retarded.

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3. ### Petrucciowns Thread Starter Active Member

Jun 14, 2009
62
0
Wow, that was an awesome explanation! I will have to go over it a few times to really understand it, but that definitely helps. Why is an infinite resistor in parallel with another resistor the value of the other resistor while in series the resistance is infinite?

Also when an inductor is closest to the source while a capacitor is at Vout I would plug in zero for the frequency , and since an inductors have a direct relationship with the frequency it will allow only low frequencies and it will ignore the other components afterwords. Is this correct?

What about a resistor in series with a capacitor or inductor with the resistor closest to the source, how does that work?

Thanks again for the awesome explanation. That is like post of the year!

Last edited: Jun 25, 2009