Capacitors

Table of Contents

Figure 1: A wide variety of capacitors [1]

What are Capacitors?
A capacitor is an electrical component that stores energy in the form of electrical charge like an electric battery [2][3]. The difference between a capacitor and a battery is that a capacitor cannot produce new electrons while a battery can [3]. Capacitors also discharge extremely quickly (less than a second) while batteries take minutes to discharge [3].
All capacitors contain at least two metal plates called electrical conductors. In between these plates, there is an insulator known as a dielectric [2][3].
A capacitor's ability to store charge is known as its capacitance [6] and it is measured in Farads [2][3]. One Farad holds a lot of electrical charge, so capacitors tend to have capacitance values that are measured in quantities as small as Picofarads and Microfarads [2][6]. The amount of charge stored by a capacitor can be calculated according to the equation Q = CV, where Q is the total charge, C is the capacitor's capacitance, and V is the voltage being applied to the capacitor [2].
Capacitors are commonly known as caps [2] and were formerly referred to as condensers [4].

Applications

Coupling
Capacitors can let AC signal pass from one part of a circuit to another while blocking DC signal [5][6][7]. This process is known as capacitor coupling [5]. Some real-life applications of coupling include communications systems, where they are used to block DC signals in the transmission lines, [6] and loudspeakers, where alternating current is converted into sound while preventing direct current from reaching the speaker [5].

Decoupling
Decoupling is essentially the opposite of coupling. It can be described as filtering out AC signals so that only the DC component of a signal can pass through [6][7]. Decoupling opposes and filters out voltage spikes in a circuit and stabilizes the circuit by providing or absorbing energy when necessary [6].

Energy Storage
Capacitors store electrical energy in the form of electrical charge. A capacitor can be connected to a power source to accumulate energy and then it can release this energy when it is disconnected from the power source [6]. A common application of using a capacitor to store energy is a camera flash. A capacitor is used to store energy in the form of charge and when the shutter button is clicked to take a picture, the capacitor discharges extremely quickly causing the flash [5][6].

Smoothing
Capacitors can smooth current to convert AC into DC [5]. AC can be visualized as a waveform that is not smooth and is varying between zero and peak voltage [7]. A capacitor can charge up as the AC heads towards peak voltage and then discharge once peak voltage is reached before starting to charge up again [5]. This allows a capacitor to supply voltage when necessary [7] in order to prevent output current from dipping by a large amount [5]. This allows the output current to act similarly to direct current [5].

Timing
Capacitors can be used in time-dependent circuits since they charge and discharge at regular intervals [5][7]. Thus, they can determine the time it takes for a circuit to operate [7]. Some real-life applications of capacitors being used for timing include LEDs and loudspeaker systems as well as any flashing light or beeping that occurs at regular intervals [5].

Materials and Types
The dielectric of a capacitor can be made from various different materials including [1][9][10]:
Aluminum
Ceramic
Glass
Niobium oxide
Paper
Plastic
Silver mica
Tantalum
Capacitors can be broken down into two categories: fixed and variable. Fixed capacitors have unchangeable capacitance values, while variable capacitors have capacitance values that can be changed [1].
Capacitors can be further classified by the material that makes up their dielectric [1].
Capacitors can be described as either polar or non-polar. Polar capacitors have a positive and negative terminal marked [9] and the positive terminal must be connected to the power [2] or else the capacitor could be destroyed [9]. Non-polar capacitors are able to be connected either way in a circuit [9].

Figure 2: Circuit symbols for standard and polarized capacitors [2]

Fixed Capacitors

Ceramic Capacitors
Ceramic capacitors are some of the most popular capacitors in the world [1]. The most common types of ceramic capacitors are the multi-layered chip capacitor (MLCC) and the ceramic disc capacitor. Billions of MLCC's are used everyday [9] and trillions are produced every year [11].
The capacitance of a ceramic capacitor will not be mentioned directly on the capacitor. They have a 3 digit number followed by a variable which identifies the capacitance. A guide on reading these codes can be found here [13].
They tend to have low capacitance values [9][7][11] and are non-polar [11]. They are often favored for their small size [11].
They come in two main classes: class 1 and class 2. Class 1 ceramic capacitors are more stable with regards to capacitance value and they are also the more accurate class. Class 2 ceramic capacitors have a high capacitance per volume and are therefore less stable and accurate [11].
Ceramic capacitors are also often described using three-character codes. Some common codes include C0G, X5R, X7R, and Y5V. C0G is a class 1 dielectric meaning it is very stable, however it takes up a lot of room. X5R and X7R are class 2 dielectrics, so they are slightly less stable than C0G capacitors. Y5V capacitors are actually in class 3, meaning they are even more unstable than the other 2 classes [12].
Due to their properties, ceramic capacitors have a wide range of applications. They are often used for radio frequency coupling and decoupling [11]. Class 2 ceramic capacitors are also used in laser power supplies, power circuit breakers, and furnaces [11].

Figure 3: Ceramic capacitors [11]

Electrolytic Capacitors
Electrolytic capacitors are also very popular [10] and have larger capacitance values than other capacitors [9][10][14].
Their dielectric is made of an electrolyte, which is a liquid or gel material with a large concentration of ions. Electrolytic capacitors tend to be polarized, meaning the voltage at the positive end is larger than the voltage at the negative end. For this reason, they can only be used in DC circuits [14]. (See Safety Considerations section)
The characteristics of electrolytic capacitors mean they have several downsides including leakage current (current that leaks through the dielectric [4]), inaccurate value tolerances, and limited lifetimes [1][14].
Electrolytic capacitors are commonly made from aluminum and tantalum [1][14], although other materials such as niobium can also be used [10].
Aluminum electrolytic capacitors have a lot of capacitance for their size and are cheaper while tantalum capacitors have lower capacitance and maximum voltage but are more stable, leak less, and last longer than aluminum ones [1].
In general, electrolytic capacitors have many applications. Aluminum electrolytic capacitors are commonly used for input and output smoothing [7][14] while the characteristics of tantalum capacitors make them useful for power supply filtering [15] and decoupling [1]. Tantalum capacitors can be found in motherboards and cell phones [15].

Figure 4: An aluminum electrolytic capacitor [14] Figure 5: A tantalum capacitor [15]

Film Capacitors
Film capacitors have dielectrics that are made out of plastic film [10][16]. Some common plastics used in film capacitors include Mylar (polyester) [1][9], polypropylene [1][16], polystyrene [1][9][16], and PTFE [16].
The film can be metallized if necessary [1][9][16]
They are similar to ceramic capacitors in terms of properties which means they have low capacitances [10]. Their capacitance can also be very precise [16]. They also look like ceramic capacitors as well, although they are a bit bigger [10].
They tend to be quite cheap, although this depends on the plastic that is used to make the dielectric [16].
They are also non-polar, allowing them to be used with AC signal [16].
They are very reliable and can retain their capacitance value for a long time which means they tend to last longer than other types of capacitors [16].
Some common applications of film capacitors include decoupling, voltage smoothing and energy storage. They can be found in power electronics devices, fluorescent light ballasts, and snubber capacitors (polypropylene) [16].

Figure 6: Film capacitors [16]

Mica Capacitors
Mica is a word used to describe group of natural minerals. Mica capacitors are constructed using mica in their dielectrics [17].
The mica is compressed by two metal sheets usually made of silver or aluminum [1].
Mica capacitors are used when a very reliable and stable capacitor with a low capacitance value is needed [9][17]. Their capacitance value does not change very much over time [17].
They tend to be expensive since silver is frequently used in their construction [1], reaching up to several USD per piece [17]. They are also bigger and bulkier than their counterparts [17].
Silver mica capacitors are non-polar [9]
They see much of their use in radio frequency power circuits as well as high-voltage applications and high-frequency filters [17].

Figure 7: Mica capacitors [14]

Supercapacitors
Supercapacitors, also known as double-layer capacitors or ultracapacitors, store much more electrical charge than normal capacitors [18]. They can have capacitances up to hundreds of Farads [10].
They charge and discharge at very fast rates [18].
They are becoming more popular [9], however they have not begun replacing batteries everywhere yet [18]. This is because they are more expensive than batteries [18] and they come in very low voltages [1]. They also release all of their energy all at once, so their lifespan is much lower than that of a battery [2].
For now, supercapacitors are used in places where memory must be retained after power is removed from a circuit [9]. They are also used in devices that need to be recharged quickly, such as camera flashes and MP3 players [18].

Figure 8: Supercapacitors [7]

Variable Capacitors

Air Capacitors
Air capacitors use air as a dielectric by having an air gap between two sets of conductive plates [19]. One set of plates is fixed while the other is connected to a shaft that can be rotated, which allows the capacitance to be changed [1][19].
They are non-polar, so they are safe to use in AC applications [19].
They are stable and do not leak much, but they are also bulky. They are becoming less popular because smaller capacitors can do their job in a smaller package [1].
However, they can still be found in old radios to help with tuning [1] and in the modern world, they can be found in MRI medical scanner among other applications [19].

Figure 9: Air capacitors [19]

Trimmer Capacitors
Trimmer capacitors use a small screw to adjust the distance between the conductive plates [1].
Their dielectric can be either air or ceramic [20].
They tend to be cheaper than normal variable capacitors [20].
Some places where trimmer capacitors can be found include mobile radios and aerospace transmitters and receivers [20].

Lifetime Expectancy
The end of a capacitor's life is usually defined as the point when the characteristics of the capacitor (ex. capacitance) change by a specific percentage [21]. A general rule of thumb is that the lifetime of a capacitor doubles for every 10°C decrease in temperature [22][23][24].
The shelf life of a capacitor is heavily determined by the conditions in which it is stored. Environmental factors such as humidity, temperature, and atmospheric pressure can greatly affect the shelf life of a capacitor. If capacitors are stored in harsh conditions, they will degrade much faster. Thus, it is important to follow storage instructions specified by the manufacturer of the capacitor. In addition, make sure to check the receipt time before using a capacitor. It is not advisable to use capacitors that have been in the store for a long time [23].
For aluminum electrolytic capacitors, make sure they are not exposed to moisture, UV rays, ozone, oil, and radiations while they are stored. They should be stored at around room temperature. If stored properly, modern day aluminum electrolytic capacitors have a shelf life of around 2 years [23].
The lifespan of an electrolytic capacitor can span anywhere from 1000 hours to 10000 hours or more [22]. Aluminum electrolytic capacitors have a rated lifespan, but this lifespan is typically on the lower end because it is calculated for cases of high stress. If the capacitor will be used in a lower stress environment, its lifespan can be de-rated. By taking into account the effect of certain stresses on the capacitor (heat, current, and voltage), the actual lifespan of an aluminum electrolytic capacitor can be much longer than its rated lifespan [21].
Tantalum capacitors are more stable than aluminum electrolytic capacitors and their capacitance value does not decrease with time, so they tend to have a longer shelf life. However, they should still be stored at low temperatures [23].
The shelf life of ceramic capacitors depends a great amount on how they are packaged and stored. They don't deteriorate much when stored for short periods of time but they can degrade a lot when stored for a long time, especially if they are stored in open air or are exposed to chlorine gas or sulfur dioxide [23].
A calculator that gives a rough estimate of the lifespan of a capacitor's lifespan can be found at illinoiscapacitor.com [24].

Safety Considerations
Capacitors dump their charge incredibly quickly which can make large and charged capacitors very dangerous and possibly fatal [3]. Capacitors can be exposed to high current levels that can cause heating and explosion. In addition, the liquid dielectric in some capacitors can be toxic [25].
Coming into physical contact or even just close proximity with a capacitor may result in electric shock from the discharged current of the capacitor [26]. Capacitors can build up and hold dangerous charge for some time even when de-energized (power is turned off) [25][26]. As a result, capacitors should be discharged before working with them. One way to do this is through the use of bleeder resistors, which are permanently connected resistors that force capacitor discharge if the equipment is powered off [25][26].
Electrolytic capacitors in particular have many safety concerns. Since they are polarized, they can be damaged and even explode if connected improperly [1][9][14]. The positive pin of the electrolytic capacitor, known as the anode, should be connected to a higher voltage than that of the negative pin, or cathode [2]. In addition, polarity should not be reversed [1] and the rated voltage should not be exceeded (ideally it should be a good deal below this amount [9]). Finally, electrolytic capacitors should never be connected to an AC source [1].

References
[1] B. Jenkins, “Types of Capacitors: Pros, Cons, & Applications,” Circuit Crush. [Online]. Available: https://www.circuitcrush.com/types-of-capacitors-applications/. [Accessed: 15-Mar-2021].
[2] S. Sattel, “Everything You Need to Know About Capacitors,” Autodesk, 02-Feb-2021. [Online]. Available: https://www.autodesk.com/products/eagle/blog/everything-need-know-capacitors/. [Accessed: 12-Feb-2021].
[3] M. Brain and C. W. Bryant, “How Capacitors Work,” HowStuffWorks, 10-Feb-2021. [Online]. Available: https://electronics.howstuffworks.com/capacitor.htm. [Accessed: 12-Feb-2021].
[4] “Capacitor Guide, your guide to the world of capacitors,” EEPower. [Online]. Available: https://eepower.com/capacitor-guide/. [Accessed: 24-Mar-2021].
[5] R. Garner, “List of Uses for Capacitors,” Sciencing, 13-Mar-2018. [Online]. Available: https://sciencing.com/list-uses-capacitors-8059446.html. [Accessed: 25-Mar-2021].
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[8] “Capacitance | Fundamentals | Capacitor Guide,” EEPower. [Online]. Available: https://eepower.com/capacitor-guide/fundamentals/capacitance/. [Accessed: 23-Mar-2021].
[9] I. Poole, “Capacitor Types: Types of Capacitor,” Electronics Notes, 05-Mar-2020. [Online]. Available: https://www.electronics-notes.com/articles/electronic_components/capacitors/capacitor-types.php. [Accessed: 15-Mar-2021].
[10] “Introduction to Capacitors: Basic Concepts, Working, Types and Applications in Circuits,” Components101, 21-Nov-2018. [Online]. Available: https://components101.com/articles/transistor-basics-types-and-working-in-circuits. [Accessed: 15-Mar-2021].
[11] “Ceramic Capacitor | Types | Capacitor Guide,” EEPower. [Online]. Available: https://eepower.com/capacitor-guide/types/ceramic-capacitor/. [Accessed: 05-Mar-2021].
[12] R. Keim, “X7R, X5R, C0G...: A Concise Guide to Ceramic Capacitor Types - Technical Articles,” All About Circuits, 30-Mar-2018. [Online]. Available: https://www.allaboutcircuits.com/technical-articles/x7r-x5r-c0g...-a-concise-guide-to-ceramic-capacitor-types/. [Accessed: 23-Mar-2021].
[13] “Ceramic Capacitor,” Components101, 30-Sep-2017. [Online]. Available: https://components101.com/capacitors/ceramic-capacitor-pinout-parameters-datasheet. [Accessed: 23-Mar-2021].
[14] “Electrolytic Capacitor | Types | Capacitor Guide,” EEPower. [Online]. Available: https://eepower.com/capacitor-guide/types/electrolytic-capacitor/. [Accessed: 05-Mar-2021].
[15] “Tantalum Capacitor | Types | Capacitor Guide,” EEPower. [Online]. Available: https://eepower.com/capacitor-guide/types/tantalum-capacitor/. [Accessed: 05-Mar-2021].
[16] “Film Capacitor | Types | Capacitor Guide,” EEPower. [Online]. Available: https://eepower.com/capacitor-guide/types/film-capacitor/. [Accessed: 19-Mar-2021].
[17] “Mica Capacitor | Types | Capacitor Guide,” EEPower. [Online]. Available: https://eepower.com/capacitor-guide/types/mica-capacitor/. [Accessed: 19-Mar-2021].
[18] "Supercapacitor | Types | Capacitor Guide,” EEPower. [Online]. Available: https://eepower.com/capacitor-guide/types/supercapacitor/. [Accessed: 19-Mar-2021].
[19] "Air Capacitor | Types | Capacitor Guide,” EEPower. [Online]. Available: https://eepower.com/capacitor-guide/types/air-capacitor/. [Accessed: 19-Mar-2021].
[20] "Trimmer Capacitor | Types | Capacitor Guide,” EEPower. [Online]. Available: https://eepower.com/capacitor-guide/types/trimmer-capacitor/. [Accessed: 19-Mar-2021].
[21] D. Williams, “Calculating the Lifespan of Electrolytic Capacitors with De-Rating - News,” All About Circuits, 26-Dec-2016. [Online]. Available: https://www.allaboutcircuits.com/news/calculating-the-lifespan-of-electrolytic-capacitors-with-de-rating/. [Accessed: 19-Feb-2021].
[22] G. Bocock, “Electrolytic capacitors determine the lifetime of a power supply,” XP Power. [Online]. Available: https://www.xppower.com/resources/blog/electrolytic-capacitor-lifetime-in-power-supplies#:~:text=Manufacturers%20of%20electrolytic%20capacitors%20specify,to%2010%2C000%20hours%20or%20more. [Accessed: 19-Feb-2021].
[23] T. Zedníček, “Get the lowdown on shelf life and storage of capacitors,” Passive Components Blog, 01-Mar-2018. [Online]. Available: https://passive-components.eu/get-the-lowdown-on-shelf-life-and-storage-of-capacitors/. [Accessed: 19-Mar-2021].
[24] “Capacitor Life Calculators,” Illinois Capacitor. [Online]. Available: https://www.illinoiscapacitor.com/tech-center/life-calculators.aspx. [Accessed: 19-Mar-2021].
[25] “Electrical Safety in Research Operations,” Virginia Tech. [Online]. Available: https://www.ehss.vt.edu/programs/ELR_capacitors.php. [Accessed: 01-Mar-2021].
[26] Office of Environment, Health, and Safety, “Injury Caused by High Voltage Capacitor Discharge,” UC Berkeley, 31-May-2005. [Online]. Available: https://ehs.berkeley.edu/news/injury-caused-high-voltage-capacitor-discharge. [Accessed: 01-Mar-2021].

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