Aluminum Electrolytic Capacitors: An Introduction. Capacitor. Capacitor Science Module 1 Second Edition

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1 Second Edition 1 Aluminum Electrolytic Capacitors: An Introduction By RL Callins, B.SC. Capacitor As a term, capacitor refers to a broad range of devices which share a common general definition. The name of any given capacitor type often reflects the kind of materials used to produce the device. Capacitors may be simple in structure and materials used or they may be complex. The capacitor has its origins in the mid 1700 s when a scientist discovered that static electricity could be stored temporarily using a glass jar partially filled with water and containing a metal conductor inside the jar. The device became known as a Leyden jar. A common definition of a capacitor is as follows: two or more electrically conductive parallel surfaces which are separated by an electrical insulator and which is capable of storing energy. The initial discovery came as a real shock to the scientist as one of his hands provided the second conductive surface. It was modified to contain two metal surfaces separated by the jar of water. Among the early scientist to study the Leyden jar was Benjamin Franklin who pondered the question as to where the energy was being stored in the device. This article will focus on one type of capacitor, the aluminum electrolytic. Often referred to as an Ecap, the aluminum electrolytic is a relatively complex type within the set of devices termed capacitor. The aluminum electrolytic capacitor or E-cap is a very interesting phenomenon from both its electrical behavior and its range of usable features. It s considered an electrochemical device. Figure 1 on page 1 shows two aluminum electrolytic capacitors and a battery as a size reference. Figure 1

2 Polar and Nonpolar Capacitors 2 E-caps are made in one of two different electrical configurations. They may be either a polar or a nonpolar capacitor. There are polar capacitors for use in direct current applications [DC] and there are nonpolar capacitors used in alternating current applications. Polar capacitors are often recognized by polarity markings near their wire terminals. An example of this is displayed in figure 1[page 1]. The smaller tubular shaped device with a wire at each end is a polar capacitor, and markings have been employed to indicate that type. Notice the band on the left end and the ++++ signs on the right end of the capacitor. Often signs are used instead of a band. This is very similar to the markings on a battery, and indicates how it must be installed in the system. Again, polar capacitors are for use only in direct current applications. The larger tubular shaped device is a nonpolar capacitor. Notice the nonpolar capacitor has no markings near its wire terminals. This is a standard practice to indicate it is nonpolar. Again, nonpolar capacitors are designed and made for alternating current applications. Physical Structure Figure 2 represents a model for a basic capacitor. Figure 2 is presented as a visualization aid to understanding a basic capacitor. Given the general definition of a basic capacitor [two or more electrically conductive surfaces which are separated by an insulator ], the two vertical parallel lines represent the electrically conductive surfaces. The space between those lines represents the insulator. The two horizontal lines represent electrical connections such as the wire terminals in Figure 1. The electrically conductive surfaces for a capacitor which is purposely designed generally uses highly conductive metallic material such as aluminum or tantalum. And the insulator could be simply dry air which is employed in mechanically variable types. However, a solid insulator material is most often used. The actions which occur to make a capacitor work occur at a sub-microscopic level, and traditionally atomic theory has been used to explain the phenomena of a capacitor since the materials we can see and work with are composed of atoms. That is, matter is made up of atoms which are linked together. Figure 3 on page 3 is a conceptual model of an atom.

3 3 Figure 3 As shown in Figure 3 atoms are composed of electrons, neutrons, and protons. The center of the atom named the nucleus contains particles which have been named neutrons and particles which have been named protons. In the orbits outside the nucleus are particles termed electrons. The electrons are particles with a negative force or charge. The neutrons are neutral as they are neither negative nor positive. The protons contain a positive force or charge. Electrons naturally contain an energy form which was simply given the name negative. Likewise, protons naturally contain an energy form which is different from the energy of an electron. To reflect the difference in the energy form it was given the name positive. For an understanding of a capacitor, the electron is the most important particle. Also, it is always negative. Traditional atomic theory suggest that the negative electrons are held in orbits around the nucleus by the equally but opposite force of the positive protons. Positive and negative forces attract each other. The normal state of an atom in a metallic material is equilibrium. That is it has the same amount of positive force and negative force. The electrons in the orbits have the same amount of negative force as the protons have positive force. These energies are called charge. Therefore an atom in its normal state contains equal amounts of negative charge and positive charge. There are situations in which a stable atom becomes unstable; that is, its constituent particles no longer provide an overall balance between the opposite forces of positive and negative energy. This can occur when a force outside of a stable atom exerts an influence on one or more of the electrons in the outer orbits causing them to be pulled away. One type of outside force which can influence a stable atom is an electric voltage being applied to the conductive material of the capacitor. This will cause one or more of the electrons in the outer orbits to be pulled away from the atom. That is the electrons are moved and are traveling along a conductor. Furthermore, the theory suggest that the electrons in a conductive material travel by movement from one atom to the next atom, and to the next; and, that movement of electrons is referred to as an electric current or electricity. Consider an analogy of water moving through a pipe. If the electrons are viewed as the water then the pipe is the conductive material. Water will flow through the pipe so long as some force compels it to do so and there is more

4 4 Capacitor Science water to move. Likewise electrons will flow on a conductive material so long as a force compels them to do so and there are electrons which can be moved. Figure 4 Figure 4 is a conceptual model of a capacitor which does not contain energy. The atoms on both conductive surfaces are in equilibrium. That is, all the atoms which make up the conductive material contain the same amount of positive proton charge as the amount of negative electron charge. The two vertical parallel lines represent the two conductive surfaces within a capacitor. For an aluminum capacitor, these represent two pieces of aluminum separated. The two horizontal lines represent conductive wires attached to the two aluminum plates within a capacitor. The space between the parallel lines represents an insulator which in capacitor science is termed dielectric. The event is an uncharged capacitor. An uncharged capacitor is empty. There is no energy stored. By definition a capacitor is capable of storing energy. If a common household battery is connected to the conductive wires of the capacitor[see figure 5 page 5], electrons will start flowing [electric current].the negative charged battery terminal will push electrons onto one of the capacitor plates while the positive terminal of the battery will pull electrons off the other plate. This will cause an excess of electrons to accumulate on one plate and a deficit of electrons to occur on other plate.

5 5 Figure 5. A capacitor being charged by the influence of a battery. Atomic theory suggest that the atoms on the deficit electron plate will build a positive charge [+] since some of the electrons have been moved to atoms on the other plate. The atoms on the excess electron plate will build a negative charge [-] since there is now more negative electrons than positive protons. The battery is using its electrical energy to change the stable atoms on both of the capacitor plates into unstable atoms. An electric current is flowing. That is electrons are being relocated from the outer orbits of the atoms on one of the plates and being placed in an orbit of atoms on the other plate. This process continues until no more electrons in the atoms of the capacitor plates can be moved. Before this state is reached the capacitor is said to be charging. It is being filled with electrical energy, and an electric field is being established on and between the plates of the capacitor. Now let us remove the battery. There is no longer an external force influencing the capacitor. However, it now contains electrical energy. This fact can be verified by placing a voltage meter on the capacitors terminal wires.

6 6 Figure 6. Model for a charged capacitor. The right vertical line in figure 6 represents one conductive plate which has atoms with a deficit of electrons so the atoms have an overall positive charge denoted by the + sign. The curved vertical line represents the other conductive plate which has atoms with a surplus of electrons. That plate has an overall negative charge. The curve in the plate represent a negative charge, and it is not intended to suggest a curving of the plate. Remember that positive forces are attracted to negative forces. Also remember that an atom in a metallic material will seek a state of equilibrium. That is, it tends toward the same amount of positive force (protons) as negative force (electrons) to regain its stable state. However the insulator [dielectric] separating the two conductive plates will not allow the electrons to move from one plate to the other to regain a state of equilibrium. There exist an equal but opposite force attraction between the two plates. Figure 7. Model of the electric field between and around the two plates of a capacitor. The two spheres in figure 7 represent the two charged plates of a capacitor. The lines are the attraction energy of the negative plate and the positive plate. The capacitor has one conductive plate with a net positive charge [+] and one conductive plate with a net negative [-] charge. The net positive charge [+] is attracted to the net negative charge [-].The word net is used since the atoms still contain the positive charge of their protons which has not changed in magnitude. What has occurred is a relocation of negative charged electrons. The two opposite charges [ - +] separated by the insulator create a force of attraction referred to as an electric field. And without a conductive path on which the electrons can travel to allow the atoms to regain equilibrium, the unstable atoms remain with an excess of electrons on one plate [-] and a deficit of electrons on the other plate [+]. Theoretically, they will remain in that state of attraction indefinitely. This is a charged capacitor. The capacitor contains an energy field maintained by the unstable atoms seeking to regain their state of just enough electrons on each plate

7 7 Capacitor Science to be in balance. These phenomena are the storage of energy within the capacitor. The energy stored is in the form of static electricity. Before proceeding to other topics, let s review some key capacitor concepts. A basic capacitor is a part used in either electrical or electronic equipment. A basic capacitor is simply two or more electrically conductive surfaces separated by an insulator and it is capable of storing energy. The insulator which electrically separates the plates of a capacitor is called a dielectric. An electric charge is one or more atoms with either an excess of electrons [negative charge] or a deficit of electrons [positive charge]. A positive charge is attracted to a negative charge and vice versa. Electrons in a conductive material move from one atom to the next atom, and on to the next atom because some external force compels them to do so. That movement of electrons is referred to as an electric current or electricity. The energy stored in a charged basic capacitor is in the form of static electricity. An aluminum electrolytic capacitor is an advanced type of capacitor as it is an electrochemical device. Two fundamental properties of any capacitor are its capacitance and its working voltage. Moreover the values of capacity and voltage are commonly imprinted on the exterior of the device. Figure 8 on page 7 shows an example of these markings. Figure 8 is a picture of the capacitance and voltage markings. Notice the 1000UF and the 15V as these indicate the capacity rating of 1000 and voltage rating of 15.

8 Capacitance 8 Capacity or capacitance is a general reference to the amount of energy which can be stored inside the device when it is fully energized. Working voltage or voltage rating indicates the voltage at which the capacitor was designed to function properly. There are other important properties of complex capacitors which are discussed in another module. Capacity is defined by the mathematical equation C=Q/V. Here C denotes capacity, Q denotes magnitude of charge, and V denotes voltage applied. Capacity is equal to the magnitude of charge stored on each of the two capacitor plates divided by the voltage applied to the capacitor. As the equational definition indicates there is a relationship between capacity and voltage. They are not independent ratings. To be sure if voltage applied is changed, capacitance will be changed. As an introduction, it is sufficient to say that capacity is a general measure of the energy stored given a specific voltage. Likewise, it is sufficient to say that the voltage rating [working voltage] is the amount needed for the device to function as designed. This brings up an important point for capacitor replacement. The E-cap must be used at the correct working voltage rating to provide the marked capacity rating. If an E-cap rated at 15 volts is placed in a 12 volt application, it will not provide the rated capacity. Both capacity and voltage are measurements of different yet related specific properties. Capacity as a measurement is expressed in terms of the farad. As a unit of measurement, the term farad was first used in 1861, and it was named after scientist Michael Faraday. A one farad capacitor would be very large in its physical dimension, and most capacitors in use are rated as a fraction of a farad. For example, E-caps are rated in the range of microfarads. One microfarad is one millionth of a farad. The microfarad is denoted by the lower case Greek letter mu [µ] which looks similar to the English letter u. So uf or MFD denote the microfarad measure. Working voltage specifies the number of volts with which the capacitor should have applied to its terminals in order to energize it to the level of its rated capacity. For example a 1000uf 15v rated capacitor should have 15 volts applied to its wires to enable it to energize to its rated capacity of 1000 microfarads. Also, the working voltage rating specifies, generally, the maximum number of volts which the capacitor can handle without failure. The working voltage of a capacitor is customarily denoted simply by the letter v. The aluminum electrolytic capacitor is a very complex type of capacitor which does not solely rely on metal structures and an insulator [dielectric] for its ability to store energy. The E-cap utilizes a chemical solution to achieve more types of useful characteristics and a greater energy storage ability. In capacitor science this solution is termed electrolyte. [elec tro lyte] A basic metal capacitor is limited both in the amount and the form of energy storage. Two metal plates separated by a dielectric [insulator] can only store energy in static electrical form. Also the amount of energy is limited by the surface area of the metal plates and the distance separating the two plates. The more the distance between the two plates the smaller the amount of energy which can be stored.

9 9 Figure 9 Example of a dry air dielectric capacitor Some basic capacitors use only dry air as the dielectric separating the plates. If the plates are too close even a small charge on the two plates will cause an arc through the insulating air and all stored energy is lost. Some basic capacitors use a ceramic disk as the dielectric. While this is an improvement over dry air, the distance between the two plates is still too great to support a large energy storage. For instance, a ceramic capacitor which has a capacity rating of 1 uf is reaching the practical limit of that type capacitor. However the ceramic capacitor can have a high working voltage rating of several thousand volts. That high rating is a result of ceramics insulating ability. Electrolyte Aluminum electrolytic capacitors contain both the metal structure of a basic capacitor and electrolyte. Hence they are electrochemical devices which have the ability to store relatively large amounts of energy. For example the capacity ratings on E-caps range from one microfarad to several thousand microfarads. Two structural differences allow this to occur. First, the E-cap uses a microscopic thin dielectric in the form of aluminum oxide which allows the two plates to be placed very close together. Secondly, the E-cap stores energy in the form of chemical energy. The electrolyte [elec tro lyte] is the most important aspect of the aluminum electrolytic capacitor. It gives the electrolytic capacitor its defining attributes compared to other capacitor types. Furthermore premium grade capacitors contain electrolyte which is a product of intense research and is a compounded chemical solution. As such their formulas are considered guarded trade secrets. Electrolyte may appear as a liquid or a paste. Manufacturers of E-caps sometimes use a variety of electrolytes each providing the capacitor a different set of behaviors. The particular formula used will provide the set of characteristics useful for a specific application. As stated previously, electrolyte allows the capacitor to store energy as chemical energy. This allows the capacitor to store more energy. The E-cap is charged with electrical energy, stores it as chemical energy, and then converts it back into electrical energy. Although this seems similar to a battery, it is a very different phenomenon. Electrical energy Chemical energy storage Electrical energy

10 10 Current flow through a metallic conductor versus current flow through an electrolyte Metallic conductors such as aluminum, copper, or silver have current flow by the movement of electrons. If you recall, an electron is a negatively charged particle [part] of an atom. In metallic conductors electrons move from one atom to another atom and so on resulting in current moving through the conductor. However, electrolyte is a nonmetallic conductor. In contrast, current flows in an electrolyte by movement of ions. An ion is an unstable atom; whereas, an electron is a part of an atom. Ions are atoms which have gained or loss bonded electrons, and thus have a positive or a negative charge. If you recall an electron is always negative. Electrolyte provides a supply of negative ions by which current can flow. Furthermore an ion being an atom can carry more energy than the electron which is just a part of an atom. As a result, a capacitor with electrolyte can store more energy. Aluminum electrolytic capacitors can be made as either a DC current component or as an AC current component. In tubular form they may have the two contact wires coming from just one end or one wire from one end and the second from the other end of the tube. Capacitors made for work in AC current are called nonpolar capacitors. However, alternating current [AC] cannot be stored. Once the external source of the AC is removed from a nonpolar, the capacitor will discharge rapidly. Before closing, let s review some key electrolytic capacitor concepts. The dielectric used in an aluminum electrolytic capacitor is aluminum oxide. Electrolyte gives the capacitor its defining attributes. Aluminum electrolytic capacitors store energy as chemical energy. Electrolyte is a nonmetallic conductor which carries current by the movement of ions. The use of electrolyte in a capacitor enables it to store more energy. Different electrolyte formulas are used to provide the capacitor with different useful behaviors. provided a basic introduction to the aluminum electrolytic capacitor. More in-depth topics on electrolytics are presented in other modules. Revisited 31 May 2016 Published By Callins Capacitor Group Copyright Callins Capacitor Group

11 11 The Callins Capacitor Group is a non-profit organization which provides free authoritative articles concerning capacitor science. Fostering Applied Science Education