Capacitors are little storage devices for electrical charge. They help to provide energy when needed, filter signals, decouple circuits, time in circuits, manipulate control voltage, prevent noise in signals and more. Besides that, they block DC and allow the flow of AC in a circuit.
So let’s take two metal plates. Put them close together separated by some dielectric and apply a voltage across the plates. We got ourselves a basic capacitor.
No current will flow through the (ideal) capacitor. A dielectric (paper, air, polyester, mica, ceramic …) will not conduct electricity. However, a dielectric does transfer electrical force from one plate to the other.
The image below shows three common types of capacitors used in our circuits.
- ElectrolyticAluminium electrolytic caps can store the largest amount of energy. Disadvantages are the huge tolerance range (20% is quite common) and the tendency to leak DC. Not usable for precision applications. They are polarized. The negative lead of the cap has a white stripe printed on the capsule. Gets worse over time. Tend to leak, crack, even explode.
- CeramicMost widely used capacitor. Comes in a wide variety of capacitance values, but not as large as electrolytic caps. Better tolerances like 5% or 10%. Suitable for precision applications like filters, decoupling, blocking. Common dielectric types are C0G, NP0, X7R, Y5V, and Z5U.
- FilmOne of the advantages of polyester film caps is the capacitance vs. size ratio. Smaller footprint for a large capacitance. Better tolerances like 5% or 10%. Suitable for precision applications like filters, decoupling, blocking. More linear in frequency and voltage responses, compared to ceramic capacitors. Good choice for audio applications.
Charging and discharging capacitors
Due to the electrical force, electrons will accumulate on one plate. The electrons on the other plate are repelled by them. This way an electrical charge builds up inside the capacitor. No current will flow through the capacitor though.
Once the capacitor is fully charged up to the applied voltage level, it completely blocks DC. Take a look at the demonstration at the circuit below. This is what we call an RC charging circuit. Here, the R stands for the resistor and the C for the capacitor.
The cap charges up to the 5V from the power supply before blocking DC completely. The current flowing through the circuit while charging the cap is called charging current.
How long does it take to charge capacitors?
We calculate the time it takes to charge the 100 nF cap up to 63.2% with the formula below. This is called the RC time constant.
In our example, it takes half a second to charge the cap up to 63.2% of its maximum charge capacity. It takes about 5 times the RC time constant to fully charge the cap.
The same goes for discharging the capacitor.
Note that one RC time constant is needed to discharge the cap to 36.8% of its maximum capacity.
Capacitors in series
The circuit below shows three capacitors of different values in series.
We calculate the total capacitance of the capacitors in parallel combined, using the following formula. There’s a resemblance with the calculation of resistors in parallel.
Capacitors in parallel
The circuit below shows three capacitors of different values in parallel.
We use this formula to calculate the total capacitance of the capacitors in series combined. Here’s also a similarity with the calculation of resistors in series.