Significance of RC time constant

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SUMMARY

The discussion centers on the significance of the RC time constant in a series RC circuit, particularly its role in determining the cutoff frequency, also referred to as the -3dB frequency. The consensus is that while the -3dB point may seem arbitrary, it is a practical choice due to its simplicity and effectiveness in circuit analysis. The time constant, defined as the product of resistance (R) and capacitance (C), is essential for calculating the energy stored in the capacitor and its impact on voltage and current over time. Understanding these concepts is crucial for effective circuit design and analysis.

PREREQUISITES
  • Understanding of series RC circuits
  • Familiarity with the concept of cutoff frequency
  • Knowledge of voltage and current relationships in capacitors
  • Basic principles of electrical engineering
NEXT STEPS
  • Study the derivation of the -3dB frequency in RC circuits
  • Learn about the implications of the time constant in filter design
  • Explore the differences between -3dB, -10dB, and -20dB points in signal processing
  • Investigate the role of RC time constants in other types of filters, such as active filters
USEFUL FOR

Electrical engineers, students in electronics, and anyone involved in circuit design or analysis will benefit from this discussion, particularly those looking to deepen their understanding of RC circuits and filter behavior.

jaydnul
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In a simple series RC circuit, when t=RC, the voltage across the resistor and capacitor is the same. Why is this physically significant and why do we choose this as the time constant that dictates the cutoff frequency?
 
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Review this first. We need some way to calculate the energy (as physical charge) in the capacitor over time into voltage or current values used in circuits.
 
Yes, but why is RC used when calculating cutoff voltage? If RC is arbitrary, then cutoff voltage is arbitrary which doesn't make sense to me.
 
Jd0g33 said:
Yes, but why is RC used when calculating cutoff voltage? If RC is arbitrary, then cutoff voltage is arbitrary which doesn't make sense to me.

I think you mean "cutoff frequency" like you posted in your Original Post, not some cutoff voltage... :smile:

And even "cutoff frequency" is a bit harsh to use for a simple single stage RC filter. A better term would be the -3dB frequency. Can you say why that would be a better term to use?
 
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Ya sorry, cutoff frequency.

Is the -3dB arbitrary?
 
Jd0g33 said:
Is the -3dB arbitrary?

Well, you could pick -10dB or -20dB instead, so sure, it's semi-arbitrary.

But "RC" is simpler than "some number * RC", and RC gives you -3dB.

Your question is reasonable though. There are some things in physics and EE that are fundamental, and others that we use for convenience. It's fine to ask about the distinctions as you learn more and more in your education. This is a good place to ask such questions. :smile:
 
The time constant in the RC filter is useful for looking at ...
Jd0g33 said:
Ya sorry, cutoff frequency.

Is the -3dB arbitrary?

Think about the electrical properties of an RC network and what -3dB means.
fil10.gif
 
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Ok, thanks!
 

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