Calculating Q-Factor and Resistance for Series LC Filters

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Discussion Overview

The discussion focuses on calculating the Q-factor and resistance for series LC filters, addressing the appropriate resistance values to consider in these calculations. Participants explore the roles of inductor and capacitor resistances, as well as load resistance, in determining the Q-factor.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions whether the resistance used for calculating the Q-factor should be the resistance of the inductor and capacitor or the load resistance further down the circuit.
  • Another participant suggests that both the resistance of the inductor and capacitor and the load resistance should be considered, recommending the creation of an equivalent circuit to calculate effective DC resistance.
  • A different viewpoint introduces the concept of a loss resistance in parallel with the capacitor, indicating that this resistance must be converted into an equivalent series loss resistance for accurate calculations.
  • One participant emphasizes the importance of using resistance at the operating frequency, noting that DC and AC resistances can differ due to factors like skin effect and dielectric losses.

Areas of Agreement / Disagreement

Participants express differing views on which resistance values to include in the Q-factor calculations, indicating that multiple competing perspectives remain on this topic.

Contextual Notes

There are unresolved aspects regarding the definitions and calculations of resistances at different frequencies, as well as the implications of using equivalent circuits.

Mzzed
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I am trying to calculate the correct component values to use in a simple series LC filter, however I am not sure which value of resistance is to be included in these equations. When talking about a series LC filter, is the resistance required for calculating q factor the resistance of the inductor and capacitor? Or is it what ever the load resistance is further down the line in the circuit?

Thanks!
 
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Mzzed said:
the resistance of the inductor and capacitor? Or is it what ever the load resistance is further down the line in the circuit?
Both.
Create an equivalent circuit using the DC resistance of each component, load, and source resistance.
Calculate the effective DC resistance of all series and parallel branches combined (Thevenin Equivalent). Use that resistance to calculate the Q.
Note that in this sense, Q is defined only around the resonant frequency.

Cheers,
Tom
 
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Normally, the capacitor has a loss resistance Rp in parallel to the capacitance.
Therefore, in order to combine this resistance Rp with the series loss resistance of the inductor Ri, the value of Rp (relatively large) must be transferred into an equivalent (small) series loss resistance Rs.
 
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Thankyou both, exact answers I needed, really appreciate it!
 
Tom.G said:
Both.
Create an equivalent circuit using the DC resistance of each component, load, and source resistance.
Calculate the effective DC resistance of all series and parallel branches combined (Thevenin Equivalent). Use that resistance to calculate the Q.
Note that in this sense, Q is defined only around the resonant frequency.

Cheers,
Tom
Apart from simple textbook questions, I think you need to use the resistance at the operating frequency, because DC and AC resistance differ due to skin effect and dielectric losses.
 

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