Capacitors

Question

Electronics are becoming ever more prevalent in my uni course (I’m doing sound engineering). Knowing how your own equipment works and stuff is integral but what is a capacitor. I failed physics in school almost. What does it do. Help.

In: 31

Answer 1

Here’s a graph of a square wave at the input plugged into an RC (resistor and capacitor) circuit.

[https://cdn2.webdamdb.com/md_MbQx0zCMqo01.png?1535479309](https://cdn2.webdamdb.com/md_MbQx0zCMqo01.png?1535479309)

As you can see, the high voltage takes a bit to “fill up” the capacitor, and then when the input goes low the capacitor takes some time to “empty” which slows down the edge rate of the square wave.

This on its own is kinda meaningless. Why are capacitors useful? Well here, lets look at a signal that doesn’t have perfect signal integrity like our model from above:
[https://www.researchgate.net/figure/Overshoot-and-undershoot_fig2_37931458](https://www.researchgate.net/figure/Overshoot-and-undershoot_fig2_37931458)

This is an example of the input signal having too fast of an edge rate and as the electrical waves arrive at the endpoint of the signal, extra charge is collecting at the input pin of whatever device needs to examine the signal. A physical example is if you push water towards the drain while you’re in the bathtub, the waves collect unevenly at the end before they settle out to equilibrium. The electrical solution to this overshoot/undershoot “ringing’ problem is to add capacitance to slow down how quickly the waves can collect at the input pin.

Instead of slowing down the edge rate and distorting the signal (first example), we’re filtering out noise from our system so that we’re able to measure something closer to the intended signal that had been distorted by the non-ideal aspects of our system.

The physics-ey buzzwords like “low-pass filter” apply here. The ringing from the overshoot/undershoot would add extra high frequency noise that could sound like a whine to our ears if we heard it in speakers. Properly designed sound systems have the right amount of capacitance (and some other things too) to filter out unwanted whining or buzzing noise in our sound systems.

It would be fun to make a presentation or a lab demonstration of this phenomenon where you plug in a signal that has noise and then have the students tune the capacitance of an RC filter to get rid of the noise in the signal.

Source: Am electrical engineer who plays guitar and took a class on “acoustic engineering” as an elective while in college.