Electronic – What are coupling and decoupling capacitors

I was the one that asked that question. Here is my rudimentary understanding:

You attach capacitors across \$V_{CC}\$/GND to try to keep the voltage more constant. Under a DC circuit, a capacitor acts as an open circuit so there is no problem with shorting there. As your device is powered up (\$V_{CC}\$=5V), the capacitor is charged to capacity and waits until there is a change in the voltage between \$V_{CC}\$ and GND (\$V_{CC}\$=4.5V). At this point, the capacitor will discharge to try to bring the voltage back to the level of charge inside the capacitor (5V). This is called "smoothing" (or at least that is what I call it) because the change in voltage will be less pronounced.

Ultimately, the voltage will not ever return to 5V through a capacitor, rather the capacitor will discharge until the charge inside it is equal to the supply voltage (to an equilibrium). A similar mechanism is responsible for smoothing if \$V_{CC}\$ increases too far beyond its average (\$V_{CC}\$=5.5V perhaps).

As for why you need them, they are very important in high-speed digital and analog circuits. I can't imagine you would need one for an SN74195, but it can't hurt!

These are decoupling capacitors. They are there primarily for two reasons:

1. Power supplies take time to respond to a demand for more current from the device. The capacitors act as a local reserve until the power supply responds.
2. Digital logic devices demand current very abruptly (due to the steep logic edges). The inductance of the power supply traces makes it impossible to transfer a step in current from a power supply to the logic chip. To solve the problem, one places "decoupling" capacitors very close to the chip. As the remaining traces are very short, the edge problem is reduced.

The reason for the two different types of capacitors is as follows:

1. The device apparently requires a 10µF decoupling capacitor. Capacitors of this size are typically electrolytic capacitors. The problem is: they respond quite slowly compared to the edge time.
2. To solve the problem, one places a (typically) ceramic capacitor in parallel. To simplify the issue: they only exist in fairly small values.

Functioning: The ceramic capacitor (100nF) smoothes the edge time of any current requests from the device and the electrolytic capacitor (10µF) supplies the bulk of the current once it kicks in.