Significance of RC time constant

[jaydnul]

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?

 

[nsaspook]

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.

 

[jaydnul]

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.

 

[berkeman]

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...

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?

 

[jaydnul]

Ya sorry, cutoff frequency.

Is the -3dB arbitrary?

 

[berkeman]

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.

 

[nsaspook]

The time constant in the RC filter is useful for looking at ...

Ya sorry, cutoff frequency.

Is the -3dB arbitrary?

Think about the electrical properties of an RC network and what -3dB means.

 

[jaydnul]

Ok, thanks!