Physics
Capacitor Discharge
Capacitor discharge refers to the process of releasing stored electrical energy from a capacitor. When a capacitor is discharged, the stored charge is released through a circuit, resulting in a flow of current. This discharge can occur rapidly, releasing the stored energy in a short amount of time, and is commonly used in various electronic devices and circuits.
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eBook - ePub- Adrian Waygood(Author)
- 2018(Publication Date)
- Routledge(Publisher)
electric field .So we can say thata capacitor is a circuit component that stores energy.Perhaps we should replace the terms, ‘charging’ and ‘discharging’ with ‘energising’ and ‘de-energising’? Well, unfortunately, the terms ‘charging’ and ‘discharging’ are too well established for us to change them at this stage!Despite this, many textbooks continue to insist that a capacitor ‘stores charge’, but this is misleading because, as we have learnt, there is no increase in the overall charge on the plates whenever a capacitor is charged. All that has happened is that charge has been separated , and has been moved from one plate across to the other; no additional charge has been introduced or stored .Actually, these textbooks aren’t really ‘wrong’ because what they mean (as opposed to what they say !) is that a capacitor is a circuit component which ‘stores separated charges’ which, of course, is quite correct!Circuit symbols for capacitors and capacitance
Capacitors may be fixed value or variable value . A special type of variable capacitor is the trimmer
eBook - ePubReactive Power Compensation
A Practical Guide
- Wolfgang Hofmann, Jürgen Schlabbach, Wolfgang Just(Authors)
- 2012(Publication Date)
- Wiley(Publisher)
12 Discharging Devices for Power Capacitors 12.1 Chapter OverviewThis chapter underlines the importance of components for unloading charged capacitors after disconnection from the grid. For this purpose resistances with fixed connection to the capacitor are mainly in use. Sometimes inductances are used as well if quick discharging is required. During normal operating periods the inductance to be connected in parallel with the capacitor forms a high impedance (12.2 Basis at LV ApplicationsXL= 2 · π · f · L ) due to the power system frequency f . After disconnection, only the ohmic resistance of the coil is effective for a rapid discharge of the capacitor. This provides significant protection with regard to re-energizing the capacitor to be discharged. Furthermore, it avoids any electric shocks to persons touching a disconnected capacitor, which still may be in a charged condition due to missing discharging devices. The capacitor would keep the voltage for a long time because the discharging procedure runs via the insulating resistance between the foils.After disconnection of a power capacitor the instantaneous voltage at the time of disconnection is stored for a long time if there are no discharging devices connected in parallel. This voltage could be any value between 0 V and plus or minus peak voltage. In order to prevent any accidents or high inrush currents the capacitor has to be discharged within a defined time according to DIN VDE 0560. In automatically controlled compensation banks the stored voltage in the capacitor is to be discharged down to less than 10% of the rated voltage before re-energizing again. It must be ensured that the connection between the discharging device and the capacitor is fixed without any fuses or switch.
- Magno Urbano(Author)
- 2019(Publication Date)
- Wiley(Publisher)
8 Capacitors : And Electric Charges8.1 Introduction
In this chapter, we will examine capacitors, basic electronic components very popular in all kinds of circuits.8.2 History
The capacitor or originally known as condenser1 is a two‐terminal electric component that can store potential energy in the form of electric field.Technically, a capacitor is composed of two metal plates, separated by a medium. This medium, called dielectric, can be any nonconductive element like air, oil, plastic, etc.Historically the idea for the capacitor is based on the Leyden Jar.28.3 How It Works
Capacitors are basically two metal plates in parallel. The plates are put very close to each other, without touching. These plates are at rest and have roughly the same number of electrons (see Figure 8.1 ).Two metallic plates in parallel, at rest.Figure 8.1Something magical happens when a battery is connected to the plates. Suddenly, an electric field (F) is formed between the plates, as shown in Figure 8.2 .A battery is connected between the plates.Figure 8.2The electric field makes the plate connected to the battery’s negative pole (A) to accumulate an excess of negative charges (electrons) and the other one to accumulate positive charges, or lack of electrons (B).Because both plates are very close and charges with the same polarity repel each other, this excess of electrons in plate A will create a force that will repel electrons on plate B, expelling them from the plate and forcing them to migrate to the battery’s positive pole.At this time, one may have the illusion of current flow between the plates. The truth is that the electric field generated by plate A forced electrons from plate B to move off the plate to the battery’s positive pole. No electron crossed through the dielectric from plate A to B.
eBook - ePub- S. Bobby Rauf(Author)
- 2020(Publication Date)
- CRC Press(Publisher)
anode . This separation of charge and the quantity of charge separated determine the electrical potential – or voltage – developed across the electrodes of the capacitor, as well as the amount of electrical energy stored in a given capacitor. Therefore, as energy storage devices – with a potential difference – capacitors are analogous to mechanical pressure energy storage devices like air receivers and pneumatic cylinders; pressure differential wherein can be used to perform mechanical work.Construction of a simple capacitor is depicted in Figure 1.6 . As shown in the figure, a simple capacitor can be constructed with two parallel square plates, of equal size, separated by a dielectric substance like air, glass, mica, paper, etc. The separation between the two plates (electrodes), “r ,” in conjunction with the area of the plates determines the “capacitance ” of the capacitor. Capacitance, “C ,” of a capacitor is defined as the charge storage capacity of the capacitor.Capacitance can be defined, mathematically, through Eq. 1.16.FIGURE 1.6 A simple parallel plate capacitor.whereC = ∈(1.16)A r- C = Capacitance is quantified or specified in farads.
- A = The area of cross-section – or simply area – of the capacitor electrode plates.
- Є = Permittivity of the dielectric medium between the plates.
- Є = Єr ∙ Є0
where Є r = Relative permittivity of a specific dielectric medium and Є 0 = Permittivity in vacuum or in air = 8.854 × 10−12 farads per meter (F·m−1 ).One farad is rather large amount of capacitance for most common capacitor applications. Therefore, many capacitors – especially, at the circuit board level – are specified or labeled in terms of smaller units, such as mF (milli-Farad), µF (micro-Farad), or nF (nano-Farad). The capacitor shown in Figure 1.7
eBook - ePub- W. Bolton(Author)
- 2015(Publication Date)
- Routledge(Publisher)
V is across the capacitor. When the two- way switch is moved to the discharge position, i.e. a path between the plate with the excess of negative charge and the plate with the deficiency, i.e. the positively charged plate, the current rises immediately to the same maximum as during the charging but in the reverse direction; it then decreases with time. The CRO shows that the p.d. across the capacitor drops with time from its initially fully charged value to become zero when the capacitor is fully discharged.If the experiment is repeated with more resistance in the charge and discharge circuits, it takes longer to charge or discharge a capacitor. If the capacitance is made larger then the time to charge or discharge is increased. The term time constant is used for the product of the capacitance and resistance, i.e. CR, and the bigger the time constant the longer it will take to charge or discharge.14.6.1 Capacitor in d.c. and a.c. circuits
When a capacitor is in a d.c. circuit, once it has become fully charged then the current in the circuit is zero and so it ‘blocks’ the passage of a d.c. current. However, with an a.c. circuit, we can think of the continual changes in the direction of the applied current as charging the capacitor, then discharging it, then charging it, then discharging it, and so on. During charging and discharging, there is a current in a capacitor circuit. Thus, with a.c. we have an alternating current in the capacitor circuit. A capacitor does ‘not block’ an a.c. current. Thus, if we have a signal which might be a mixture of a direct current and an alternating current and we put a capacitor in the circuit, the d.c. component will be blocked off and only the a.c. component transmitted.13.7 Energy stored in a charged capacitor
Consider a constant current I being applied to charge an initially uncharged capacitor C for a time t . Since current is the rate of movement of charge then the charge moved onto one of the plates and off the other plates will be It . If this movement of charge results in the p.d. across the capacitor rising from 0 to V then the charge = It = CV and thus I = VC/t. The average p.d. across the capacitor during the charging is ½V and so the average power to the capacitor during charging = 1 x ½V
eBook - ePubEnergy Storage
Systems and Components
- Alfred Rufer(Author)
- 2017(Publication Date)
- CRC Press(Publisher)
The relaxation phenomenon is a dynamic variation of the properties of the supercapacitor due mainly to charge migration inside of the porous electrodes, misopores, mesopores, and macropores. During a fast charge (discharge), ions will first enter (leave) macropores and then mesopores. The diffusion of ions in misopores is characterized by longer time constants.During the aging process, the relaxation phenomenon is reinforced by impurities affecting the dimension of the pores. After a fast charge (discharge), nonhomogeneous repartition of charges appears on the electrodes. The diffusion of the charges for reaching a homogeneous distribution depends on the size of the pores and the size of the ions. The observable phenomena are• Voltage decrease (after charge), even if the current is set to zero• Voltage increase (after discharge), even if the current is set to zero5.3 DESIGN OF A SUPERCAPACITIVE BANK5.3.1 SERIES AND PARALLEL CONNECTIONS OF ELEMENTS FOR LARGER POWER AND HIGHER CAPACITY
For storage applications of significant power, the relatively low voltage of the supercapacitors gives only a limited power level and also a limited amount of stored energy. The example of a large element of 3000 F and 2.7 V illustrates this limitation when the power by a discharge at 200 A leads to only 540 W, and the energy content is around 10 kJ. These limitations lead to the realization of storage banks with series and parallel connections. The design of a storage device must determine the total number of needed supercapacitors and their series/parallel arrangement. The operating voltage level will determine the number of series-connected elements in each branch.5.3.2 DEFINING THE NEEDED ENERGY CAPACITY
For a given application, the needed energy capacity of a storage device is evaluated for a basic cycle of the use of energy. Generally, the known specification of an application is the needed power profile, defined over the complete cycle (time).
- S. Bobby Rauf(Author)
- 2021(Publication Date)
- River Publishers(Publisher)
anode . This separation of charge, and the quantity of charge separated, determine the electrical potential — or voltage — developed across the electrodes of the capacitor. The electrical potential difference between the capacitor plates, or electrodes, signifies the storage of electrical energy in the capacitor. Therefore, as an energy storage device, with a potential difference, capacitors are analogous to air receivers and pneumatic cylinders that store pressure energy, in the form of higher pressure relative to the atmospheric pressure, which can be used to perform mechanical work.Construction of a simple capacitor is depicted in Figure 1.6 . As shown in Figure 1.6, a simple capacitor can be constructed with two parallel square plates, of equal size, separated by a dielectric substance like glass, mica, etc. The separation between the two plates (electrodes), “r”, in conjunction with the area of the plates determines the “capacitance” of the capacitor. Capacitance, “C”, of a capacitor is defined as the charge storage capacity of the capacitor.Fig. 1.6 : A simple parallel plate capacitor.Capacitance can be defined, mathematically, through Eq. 1.16 , below.whereC = ∈A r,Eq. 1.16C = Capacitance quantified or specified in farads (F).A = The area of cross-section — or simply area — of the capacitor electrode plates.∊ = Permittivity of the dielectric medium between the plates, and,∊ = ∊r .∊0 ,where∊r = Relative permittivity of a specific dielectric medium and∊0 = permittivity in vacuum or in air = 8.854 × 10−12 F per meter (F m−1 ).One F is a rather large amount of capacitance for most common capacitor applications. Therefore, many capacitors — especially, at the circuit board level — are specified or labeled in terms of smaller units, such as mF (milli-Farad or 10−3 ), μF (micro-Farad or 10−6 ), nF (nano-Farad or 10−9 ), or pF (pico-Farad or 10−12 ). The capacitor shown in Figure 1.7 is rated 470 μF and designed to operate at a maximum of 35 VFig. 1.7 : A cylindrical 470 micro-Farad capacitor.The mathematical relationship stated as Eq. 1.16 stipulates that capacitance is directly proportional to the area A of the capacitor plates and inversely proportional to the separation r between the plates. In other words, if larger capacitance or charge storage capacity is desired, one must increase the area of the plates and/or decrease the separation between the capacitor plates. In addition to serving as a “constant ofproportionality” for the equation, permittivity € injects the property or characteristic of the dielectric medium into the computation of capacitance through the dielectric medium’s characteristic €r
