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 [1][2].
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].
Fixed Capacitors
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 [18]. A general rule of thumb is that the lifetime of a capacitor doubles for every 10°C decrease in temperature [19][20][21].
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 [20].
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 [20].
The lifespan of an electrolytic capacitor can span anywhere from 1000 hours to 10000 hours or more [19]. 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 [18].
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 [20].
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 [20].
A calculator that gives a rough estimate of the lifespan of a capacitor's lifespan can be found at illinoiscapacitor.com [21].
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 [22].
Coming into physical contact or even just close proximity with a capacitor may result in electric shock from the discharged current of the capacitor [23]. Capacitors can build up and hold dangerous charge for some time even when de-energized (power is turned off) [22][23]. 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 [22][23].
Electrolytic capacitors in particular have many safety concerns. Since they are polarized, they can be damaged and even explode if connected improperly [6][7][8]. 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 they it should be a good deal below this amount [6]). Finally, electrolytic capacitors should never be connected to an AC source [1].
Applications
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Faculty Advisors: Allyson Giannikouris, Michael Lenover, Kim Pope, John Thistle