Are Shunt Capacitors in the Signal Path?

[Edgar]

It occurs to me that the problem may be in the definition of terms. I am an engineer who specializes in signal processing. So I think of AC and DC in terms of the associated mathematics: a DC signal contains only components at 0 Hz, an AC signal contains at least one component that is not 0 Hz. That may not align with the popular definitions, where DC means that the signal is either always positive or always negative, and AC means that it crosses 0V at least sometimes.

[mboxler]

1 hour ago, Edgar said:

It occurs to me that the problem may be in the definition of terms.

 

I believe so.  When I was testing I2S output pins on an ESP32 project, I had no scope.  I knew, however, that the master clock pin should be varying from 0V to +3.3V at a 50% duty cycle.  I set my DMM to DC and measured between ground and the MCLK pin and measured 1.65V DC, so at least I knew I had a clock.  I'm pretty sure the measurement didn't work when I set the DMM to AC.

Edited March 13, 2022 by mboxler

[DrWho]

On 3/13/2022 at 10:07 AM, mboxler said:

However, if you change the signal to switch from +0V to +2V, is it still AC?  I think, with a series capacitor, current will flow the exact same way in both circuits???

Ah, totally understand the confusion. The mathematical definition of AC doesn't require a zero crossing. It just requires the slope of the line to be nonzero. The "alternating" aspect is the slope of the line changing between positive and negative (rising and falling edges). The "DC" aspect is the average value. Sliding your curve up and down changes the average value, but it doesn't change the slope that is constantly changing.

 

Edgar added a caveat where you have a signal that rises once and then flattens out, and calls that AC. Technically that's true in the math, but engineers don't typically call that AC. And if you want extreme technicality, then you have to require the circuit to be perfectly linear for Forier or Laplace transforms to be meaningful.... Which isn't true for circuits (like a power supply) that exhibit that behavior.

 

That's why I think it's better to think of AC as alternating slope, and DC as the average level over the period of interest. The AC level just tells you how far your signal deviates from the average level.

 

If you get into other waveforms like Edgar is describing, then I don't think it's helpful to call it AC or DC. I agree it's technically AC, but my circles will typically call it an impulse, or ramp, or integrator, or whatever description fits the context of the circuit. I think the important thing is that current flows through a capacitor when there is a nonzero slope on the voltage (back to the fundamental equations). Once the slope is zero, then the current is zero, and we call that DC.

[Edgar]

It's all about context, and I can lose sight of that as easily as anyone, I guess.

[KT88]

 

If I have learned from this thread posts correctly, in principle, the only thing that matters with a capacitor is that you create a change in the voltage so that it does not block current.
In this sense, a capacitor is lifeless as long as everything is static and it produces effects when it is no longer static. 

 

Maybe this is not technically described correctly, but the voltage change causes current to be able to flow. Almost like a gate for energy that you can open and close by changing the voltage.

[babadono]

Well.......

[mboxler]

On 3/15/2022 at 5:53 PM, KT88 said:

Maybe this is not technically described correctly, but the voltage change causes current to be able to flow. Almost like a gate for energy that you can open and close by changing the voltage.

 

Nice!  It's interesting how many ways one can describe what is happening.  My description, although similar, works for me...

 

A capacitor blocks everything, or to put it another way, electrons do not flow through a capacitor.  

 

When a capacitor is in a series circuit, current only flows when there is a change in voltage.  

The slower the voltage change, the less current will flow (think low frequency audio).

The faster the voltage change, the more current will flow (think high frequency audio).