Welcome! This is the first of a three part series on capacitors. In this installment we’ll cover what capacitors are, what they do, and the different types of available capacitors. Part 2 discusses capacitors in DC circuits, including both steady state and transient response. In Part 3 we take a limited look at capacitors in AC circuits. Each installment will include an associated lab segment so that you can see the principles we cover in action.
I’ll post a combined video that covers this installment when I post Part 2: Capacitors in DC Circuits.
The Capacitor
A capacitor is an electronic component that stores energy in the form of an electric charge. Think of it as similar to a water storage tank. If we fill the tank, we can store the water as long as we like. When ready, we can open a valve and use the stored water to run a turbine. Capacitors work in a similar fashion. We can use a voltage source, such as a battery, to fill the capacitor with electric charge. Then, when we need some of the energy back, we can can draw the stored energy from the capacitor.
Unlike resistors, some types of capacitors are polarized (electrolytic and tantalum capacitors for example). Polarized capacitors are not tolerant of reverse voltage and will either breakdown or explode when reverse biased. Polarized capacitors have their leads clearly marked as to which is the positive and which is the negative.
Capacitors are used in all types of circuits. In power supplies, capacitors are used to smooth out AC and filter ripple. In audio circuits, capacitors are used both to block DC current (blocking capacitors) as well as to filter out noise or to allow the actual audio portion of the signal to bypass certain components (bypass capacitors). In other circuits, the capacitor’s ability to charge and discharge at settable rates is used to generate timing signals like a clock, or one-time pulse. Don’t worry if this seems complicated now, we’ll explain it all in short order.
In fact, the capacitor is probably the second most used component in electronic circuits, second only to the resistor.
Like resistors, capacitors come in many shapes, sizes, and types to satisfy the specific needs of the myriad of applications in which they are used.
How Capacitors Work
A capacitor is created using two conductive (usually metal) plates with a narrow gap separating them. The gap can be filled with air or with any of a number of other materials. The insulating material filling the gap between the plates is called the dielectric. Different dielectrics, along with different manufacturing methods, impart particular qualities to the capacitor such as temperature stability, energy storage capacity, leakage, power handling capability, and others.
The value of a capacitor, the capacitance, is measured in Farads (F) after Michael Faraday a British scientist who contributed to the study of electromagnetism and electrochemistry. He discovered the physical principles that define electromagnetic induction, diamagnetism and electrolysis.
Like resistors, capacitors come in a large number of standard values; from very low values (picofarads or x .000000000001 farads) to very large values (100s of farads) also called super capacitors. Very large value capacitors can store so much electrical energy that they are often used in place of backup batteries. Larger value capacitors are created by making the plates larger. In general, the larger the capacitance value and/or the voltage, the larger the physical capacitor.
In addition to a capacitance value, every capacitor has a voltage rating. It’s critically important that you don’t exceed the rated voltage or the capacitor could fail. Just like with resistors, a capacitor that is stressed way beyond its rated value could fail catastrophically and explode or burst into flame. In fact, it’s considered a best practice to “de-rate” (assume it’s much less than stated) the capacitor voltage and include a 50% safety margin, especially with electrolytic capacitors.
In addition to a rated voltage, capacitors also have a tolerance value commonly between 5% and 20% variance from stated value.
How to Read Capacitor Codes
Like resistors, capacitors are marked in a variety of ways including color bands or dots, numeric and alpha codes, or values written on the capacitor itself. The code system used varies by type of capacitor. For example, electrolytic, through-hole capacitors normally have the values written on the side of the capacitor. Ceramic disk capacitors use a numeric code to indicate capacitance value (it works the same as resistor codes where the last digit is the multiplier) and a letter code to indicate tolerance.
There are also codes for voltage rating and temperature coefficient. The temperature coefficient not only gives the rated temperature range but also defines how the capacitance value changes with temperature. As hobbyists, there are few applications where we need to factor temperature into a design. However, the voltage rating is critical and we always need to consider it.
Don’t worry if you can’t easily read a capacitor’s value right away. Like anything else, learning the codes takes time. I usually look up the code on the internet whenever I need to. In fact, there are a number of capacitor value calculators on the internet. I’ve listed one below. A quick search will turn up many more.
Dielectric Materials & Qualities
Capacitors are made from a number of different materials and manufacturing methods. Each type of capacitor has specific characteristics, stability, leakage, tolerance, voltage rating, etc. Each type of capacitor offers benefits and drawbacks that must be considered in concert with the application. For example, electrolytic type capacitors offer much higher capacitance values for a given size but also exhibit higher leakage and lower reliability than other types of capacitors.
Capacitive Energy Storage
When we hook a capacitor up to a battery, current flows through the circuit and the capacitor charges. Current continues to flow until the capacitor is fully charged and then it stops flowing. A capacitor in the fully charged state blocks DC current. A capcitor’s ability to block DC current has many uses in electronics. You’ll learn about them throughout these lessons.
If we quickly disconnect the battery from a charged capacitor, leaving an open circuit, the capacitor will hold the charge indefinitely.
Only a perfect or theoretical capacitor holds charge indefinitely. In reality, no capacitor is perfect and every capacitor exhibits something called leakage. Modern capacitors have fairly low leakage and we can ignore it in most cases. Leakage is often represented by a large resistor (multiple mega-Ohms) drawn in parallel with the capacitor’s plates. In addition, real capacitors exhibit impedance (milliohms in DC circuits) represented by a series resistor. For purposes of these early lessons we can ignore both.
If we then take the charged capacitor and insert it into a circuit, the energy stored in the capacitor is released and current flows through the circuit as the capacitor discharges. Once the capacitor is discharged, current stops flowing.
The rate at which the capacitor charges and discharges is determined by the capacitance (in farads) and the resistance of the current path (in Ohms). We’ll cover this in detail in the next lesson: Capacitors in DC Circuits.
Well, that’s enough for this installment. If you found it valuable, please like, share, and subscribe so you can keep up with future lessons.
Cheers
Dominick
