The post Types of Capacitor appeared first on BINARYUPDATES.

]]>Table of Contents

Capacitors comes in variety of shapes, sizes and electrical ratings. You may see same type of capacitor in axial, radial as well as surface mount (SMD) type. Depending on value, capacitors fall into two main categories: fixed value capacitors and variable capacitor. These types can be further categorize on the basis of polarity and dielectric material used.

Many types of fixed capacitors are used in electronics as well as electrical circuits. They are designed to have fixed value of capacitance. These capacitors can be classified according to polarity. The polarized and non-polarized fixed capacitor can be further classified by dielectric material used. Usually, fixed capacitor are named as per dielectric material used in them.

They are non-polarized. Their value ranges from pF to µF. They are available in wide variety of working voltage (from few volt to kilo volt). Ceramic capacitors are divided into two categories, namely disc capacitors and multi-layer capacitors. The disc type capacitors have pretty simple construction. They have a small ceramic disc coated with silver on both side, hence also called as **disc capacitors**. This disc and silver coating acts as ceramic electric and electrodes respectively. The disc and silver electrode assembly is coated with insulator for protection. The capacitance value of disc capacitors ranges in between 0.5 to 1600 pF. The dielectric can also be in plate shape for plate, tabular ceramic capacitor. These capacitors capacitance ranges in between 1 pF to 1 µF. the breakdown voltage ranges in between 500 V to 20 kV. The multi-layer ceramic capacitors are called as **MLCC** – Multi-layer Ceramic Capacitor, used to achieve high capacitance. The high dielectric constant increases capacitance of ceramic capacitors while keeping physical size small. These capacitors perform well at high frequencies. These are general purpose capacitors and mainly used to remove noise (E.g. in Microcontroller key de-bouncing circuit, with MAX232 IC, with crystal oscillator). As ceramic capacitors are non-polarized; they can be used in both DC as well as AC circuits.

They are also known as film caps or power film capacitors. Film capacitors are made of plastic (or paper, metal film) film covered by metallic electrodes, placed into a winding, with leads attached, and then enclosed in casing. The various film capacitors get their name on the basis of dielectric used. Capacitors with polyester (mylar), polystyrene, polycarbonate or teflon as dielectric material are generally called as plastic capacitor. The capacitance of foil or metalized capacitor lies in between 100 pF to 100 uF, while paper capacitor value is in between 1 nF to 1 µF. They have higher working voltage than ceramic capacitors. The voltage range is around 200 V to 1600 V for paper type and 50 V to 600 V for foil type film capacitors. They are widely used in power electronics because of their low cost and superior characteristics such as temperature stability, low self-inductance and ESR. Film capacitors are not polarized, hence can be used in both AC and DC circuits.

These capacitors has mica as dielectric material covered with thin silver layer. Hence these capacitors are also called as silver mica capacitors. Mica capacitors are available in the range between a few pF to thousand pF with voltage rating in between few hundred volt to thousand volt. The dielectric in mica capacitor is used as stacked sheets. The capacitance of mica capacitors ranges from 10 pF to 5000 pF and breakdown voltage is similar to ceramic capacitors. Mica capacitors offer high precision, reliability and stability. They are available in small values and are generally used at high frequencies and in situations where low losses and low capacitor change over time are required.

Electrolytic or Polar capacitors are widely used in electronics circuit because of their low cost, high capacitance and ease of availability. They comes cylindrical metal shape with outer plastic sheath. These types of capacitors is used as ripple filter in power supply, as a filter to bypass low frequency signals. Electrolytic capacitors are usually measured in microfarads and rarely in farads. These capacitor are polarized, hence they are mostly used in circuits where both AC and DC signals are used.

Aluminium is used in construction of aluminium electrolytic capacitors. These capacitors are available with capacitance in between 1 µF to 47000 µF. They have maximum breakdown voltage around 400 V. They have high ripple current capability, high leakage, poor tolerance and lifetime. Aluminium electrolytic capacitors have Poor performance at high frequencies due to ESR. The size of electrolytic capacitors increases with increase in capacitance. These capacitors are widely used in audio amplifiers to reduce hum noise. The aluminium electrolytic capacitor has one special construction arrangement on top of it. You may be wondering that, why such marking is there? Well, this marking related to protection of you. Imaging, what will happen if electrolytic capacitor is connected with wrong polarity? The reverse polarity connection creates gas and increases temperature in capacitor. This permanently damages and may explode capacitor. Thanks to electronics component designers, electrolytic capacitors have thin casing (marking) on top side which break upwards and allows this gas pressure to get release and prevent capacitor from getting explode.

A tantalum metal is used in construction of tantalum electrolytic capacitors. These capacitors are available with capacitance in between 47 nF to 330 µF. Generally they have low working voltage in between 1.5 V to 40 V. Tantalum electrolytic capacitors have low ripple current capability, low leakage and highly tolerant to reverse and over voltage. They have poor performance at high frequencies. The high capacitance in small size makes tantalum capacitor, first choice for electronics circuit designer to use in complex circuits such as mother board. They are also useful in military application and extremely stable audio amplifiers.

They are designed to have variable value of capacitance. In this type area between two plates is adjusted to change capacitance of capacitor. The construction of tuning capacitors consists of two important mechanical movements i.e. angle of spindle movement and plate movements. In variable capacitor, conductive plates in air capacitor are meshed (cross together). The stator (stationary) plates pairs with movable plates through spindle movement. Capacitance is varied by spindle movement (shaft rotation) to make the movable plates mesh with the stator plates. The change in capacitance by such mechanical structure can be of following types – linear (spindle movement ∝ capacitance), logarithmic (spindle movement ∝ percentage change of frequency), even (spindle movement ∝ capacitance and frequency) and square law (square of spindle movement ∝ capacitance). Variable capacitors are generally used in LC circuits for tuning frequency in radios, hence such capacitors also called as tuning capacitors.

These are simplest variable non-polarized capacitors. The capacitance of air capacitor is small, about 100 pF to 1 nF. Air capacitors uses air as dielectric in two conductive plates. The operating voltage of air capacitor is in between tens to a thousands of volts. The breakdown voltage of air as dielectric is lower hence there is a change of electrical breakdown in capacitor. This leads to faulty working of capacitor. Hence sometimes vacuum is created between capacitor plates which has dielectric constant nearly the same as that of air. The breakdown voltage is higher for vacuum hence less chance of electrical breakdown. Sometimes the air capacitor is also called as “Gang capacitor”. A gang capacitor is a combination of two or more variable capacitors mounted on a common shaft to. This adjustment allows simultaneous change in capacitance of grouped capacitors. You can see in picture the gang capacitor has many output leads, this leads get gang (grouped) by adjustment screw for changing the capacitance. It is used in AM and FM radio circuits.

Similar to trimmer resistors capacitors also have trimmer or preset capacitors. They are non-polarized. Trimmer capacitors are used when there is no need to vary capacitance again after initial adjustment. This capacitor has dielectric placed between two parallel facing conductive plates. Generally, trimmers capacitance is change by changing overlap area between plates with provided adjustment screw. Trimmers uses a sheet of dielectric material such as mica, mylar etc. The maximum value of trimmers is in between few pF to about 200 pF. These capacitors are design to handle low to moderate voltages, and are highly efficient. To vary capacitance of trimmer capacitors it is advised to use non-metallic tools, since use of metal may affect capacitance value.

Each capacitor type has its own set of specifications and characteristics. Hence, one has to be careful while choosing a capacitor. The specifications of capacitor can be observed from information printed on its outer body and its characteristics can be understood by finding details about its composition and physical structure. Let us see which factors are need to be consider while selecting a capacitor.

*1. Equivalent series resistance *– Every metal have some amount of resistance. The capacitor has metal leads and they have tiny resistance (about 0.01 Ω). This resistance together with current through capacitor, creates heat i.e. power loss.

*2. Precision* – Capacitors do not have exact or precise capacitance. The variation in value of capacitance is called as tolerance of capacitor. This value depends on type and ranges from ±1% to ±20% of the actual capacitor value.

*3. Voltage rating* – Depending on type, capacitors have maximum rate voltage that can be applied across it. This voltage rating may vary from 1V to 100V.

*4. Size* – The capacitor size is related to capacitance value and it physical size. Higher the value of capacitance and voltage rating bigger is its size.

*5. Stability* – The stability of capacitor defines the change in value of capacitance with temperature and time.

*6. Leakage current* – Practically, there is tiny value of current (in mA or nA) leaking through capacitor. This leakage leads to decrease in capacitor stored energy and it gradually discharge capacitor.

*7. Aging* – The capacitance of capacitor decreases over time, this is known as aging.

*8. Application* – Depending on type capacitors have variety of applications. E.g. filter circuit, tuning circuit, bypass capacitor etc.

This is it for this post. I believe now you’re familiar with different types of capacitor and their significance. In next post, we’ll learn about color coding of capacitors. Thanks for reading. Keep visiting.

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]]>The AC signal continuously varies with respect to time. There are different types of AC signals e.g. sine, triangular, square etc. Let us first clear two most important concepts related to capacitor in AC circuits.

*Capacitor passes AC signal and blocks DC.*

This statement is not 100% true. The capacitor blocks DC except for a time while it is charging or discharging. Capacitor also blocks AC signal to some extent. This nature of capacitor towards AC signal is refer as **reactance of capacitor**. Reactance works in the same way as resistance in DC circuits.

*Capacitor is short circuit to AC and open circuit to DC.*

You may thinking that if capacitor is passing AC much better than DC, then it must be acting as either short or open circuit with AC signal. The answer is No. The capacitor neither act as open nor short circuit. Then question arises, how capacitor passes AC without being short or open circuited. Let’s discuss…!

The** water analogy **can be compare with working of capacitor with AC. Let’s consider capacitor plates as two water tanks T1 and T2, filled same at half of their full capacity. The two pipes used to fill/emptying tanks, acting as capacitors leads. A pump is use in between these two pipes is similar to voltage source. Here tank T1 gets filled with negative voltage and emptied with positive voltage. The tank T2 works exactly opposite to T1. The emptied tank is similar to capacitor plate with negative charge. The water flowing through pipes is similar to current flowing through capacitor.

Now first consider our voltage source is DC. The DC signal is constant and it can be positive or negative. For negative voltage pump will drain water from T1 and supply it to T2. After some time water flow stops, T1 get emptied and T2 get filled completely. Hence there is no continuous flow of water from T1 to T2 with DC.

Now, the voltage is replaced with AC. The AC signal continuously varies in between positive and negative. The tanks get emptied or filled for respective voltages. But this time, signal polarity is continuously changing from positive to negative and vice-versa. Hence neither T1 nor T2 get emptied completely and water flows continuously in both directions through pipes. This exactly happens when capacitor is working with AC. The charge on capacitor plates is changing continuously with alternating current. Hence it result in flow of electrons through capacitor.

There is one important similarity in resistor and capacitor. Resistance of resistor opposes current flow by heat dissipation. The ability of capacitor to oppose current flow (both AC and DC) is known as reactance of capacitor. Reactance opposes current flow without heat dissipation. The resistance of capacitor to current is apparent in nature i.e. it is observe only at some point. Both resistance and reactance are measured in ohms. The term reactance comes from fact that, reaction of capacitor plates to current flow i.e. plates carries either positive or negative charge when voltage applied to capacitor.

The frequency is important parameter of AC signal. You may have read that, *capacitor acts as an open circuit at low frequencies and short circuit at high frequencies*. This statement is based on a face that **frequency is inversely proportion to capacitive reactance**. This voltage fluctuation is directly proportional to current through capacitor. The slow the input voltage fluctuates, less is the electron flow through capacitor and vice-versa.

Recall the mathematical representation of capacitor time constant.

Time (τ) = R x C

Hence increase in capacitance increase required time to charge, which implies low frequency (slow input voltage fluctuation) and less electron flow though capacitor.

The mathematical expression of capacitive reactance is,

As discussed earlier resistance and reactance both have same unit – ohm. There is also an ohms law for capacitive reactance. The important note is while applying ohms law for capacitive reactance frequency must be constant. Let’s find out how capacitive reactance varies in ohms law.

For C = 0.1µf, f = 100 Hz, V = 5V | For C = 0.1µf, f = 10 kHz, V = 5V |

_{I = }^{V} ⁄ _{Xc = }^{5} ⁄_{(15.91k)}
I = 0.314 mA |
_{I = }^{V} ⁄ _{Xc = }^{5} ⁄_{(159.15)}
I = 31.4 mA |

These examples show that change in input signal frequency changes capacitive reactance. Hence frequency of input signal should be constant while applying ohms law for capacitor with AC. This is it for now. I hope now you know How Capacitor works with AC. In future post, we will discuss about types of capacitor. Thanks for reading and don’t forget to leave a comment. There is lot to come about capacitors in this electronics series. Keep visiting.

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]]>**EAGLE** (**E**asily **A**pplicable **G**raphical **L**ayout **E**ditor) **PCB Design Software** which use to draw circuit schematic and a board lay out to design PCB. EAGLE comes with **schematic editor**, a **PCB editor** and an **Auto Router Module**. The software is very popular among students and professionals. The reason behind this is EAGLE comes with extensive part libraries which includes ready to use electronic parts. The library editor is also available to design new parts for your circuit or modify existing. EAGLE is capable to design from simple Single layer to complex Multi-layer PCB. EAGLE is very intuitive and built for professional PCB Design. Autodesk (*Former owner: CadSoft*) provides great support and documentation.

Let’s begin with setup and installation of Eagle for PCB Design. EAGLE is free to download. In this lesson, we will be designing simple Voltage Regulator Circuit. We can download and install free version of EAGLE. This version is more than enough for us. It allows us to design PCB up to 2-layer. The video given below will take you through step-by-step to setup EAGLE. Also we will introduce you how to **add**, **move**, **arrange** and **name a component** while working with schematic drawing.

**Specifications of EAGLE PCB design software (FREE VERSION)**

- Board Area: 80 cm
^{2} - Limited to 2-Schematic Sheets, 2-Signal Layers
- Operating Systems: Windows, Mac and Linux
- This version is for Students, Makers, and Professionals.

There are few components libraries we’ve used in this tutorial. You can download them from following links.

At this point, we’re ready with setup and installation of EAGLE. In the next section, we will start drawing actual schematic for Voltage Regulator Circuit.

**There are four major steps to get PCB Design:**

- Draw Schematic for Electronic Circuit
- Generate board file and route PCB tracks
- Run DRC (Design Rule Check)
- Generation of Gerber Files/Output Files

Let’s draw the schematic for voltage regulator circuit. We will learn from creating fresh new EAGLE project. The video below will show you how to work with Eagle Schematic Editor and explore different features that eagle provides for PCB Designer. Also while drawing circuit we’ll explore different section of EAGLE workspace such as **Tool Box**, **Command Line**, **Sheet Display**, **Grid Setting**, **Tool Bar** *(which include tools like: move, copy, mirror, rotate, group, delete and many more)*. The designing schematic sheet divided into several subsections, each and every step will be covered in the video below. The entire schematic circuit we want to draw is given below for your reference.

In this section, we’ll learn how to turn our previously drawn schematic sheet into board layout to complete PCB Design process. Basically we will learn how to work with board file using Eagle PCB Design Software. This process include **Creation of board file**, **Routing the traces, Run DRC ***(Design Rule Check)* and **Generation of Gerber files**. Later on you can send these files to board house and get PCB Manufactured. All the necessary files are free to download, which will be posted in the end of this post.

This is how we can turn our circuit schematic from rough sketch to actual fully functional PCB Printed Circuit Board. We hope you have enjoyed reading and watching videos of EAGLE PCB Design Tutorial. If you have any question or suggestions, then feel free to leave a comment. We’ll keep adding more and more to this post. Keep visiting Thanks.

In this section we’ve provided all the project files that you may need while following this post and videos.

Please Click Here to Download PCB Design Project: Voltage Regulator Circuit.

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]]>- How capacitor works with DC input?
- What is the final voltage of capacitor after getting charged?
- How much time capacitor takes to charge/discharge?

Let’s discuss about solution to above questions.

Capacitor performs three tasks in dc circuits i.e. taking charge, holding charge and delivering charge at certain time. When capacitor is connected to dc voltage source, capacitor starts the process of acquiring a charge. This will built up voltage across capacitor. Once capacitor has acquire enough charge, current starts flowing and soon capacitor voltage reaches at value approximately equal to dc source voltage. When capacitor has almost full voltage across it, no more current flows though capacitor. This take some time. But there is an interesting fact. The capacitor will not acquire 100% charge at same instant when dc voltage is given to it. The capacitor gets first part of total charge quickly, second part slowly, third part more slowly and so on. Hence we can say that capacitor charges non-linearly.

You can imagine this situation with **bus analogy**. Compare bus with capacitor, vacant seat with space and people with electrons. In bus, every person try to acquire a seat. If fewer seats remaining, people need more time to find vacant seat. Similarly, electrons try to acquire space on plate of capacitor. Here, electrons require some time to get on plates. Rewind the construction of capacitor. For input dc voltage, first plate charges to input voltage. As there is no conducting path in between two plates, second plate take some time to get charge.

This time defines charging time of capacitor. So, we need to find out parameters on which charging time of capacitor depends. According to ohms law, if circuit resistance is increased, less current is available to charge a capacitor. This increases time require for capacitor to charge. As capacitance and voltage are inversely proportional to each other, increase in value of capacitance takes a longer time for capacitor to charge itself. So, with these relations we can say that the charging time of capacitor depends on both resistance of circuit and capacitance of capacitor. This is **time constant** of capacitor. But, the process of measuring charging time of capacitor is complex, since capacitor will never charge at same rate.

The charging time or time constant is denoted as τ (tau). It defines time taken by capacitor of “C” farads in series with resistance of “R” ohms, to acquire first part of total charge. Time constant can be mathematically define as,

Charging time = Resistance x Capacitance

τ = R x C

The time constant is the time capacitor needs for either voltage or current to increase to **63.21 %** of maximum or decrease to **36.79 %** of initial value.

Here is the equation for voltage across capacitor at any instant of time during charging.

Where *V _{c}* = capacitor voltage,

E.g. for R = 10 MΩ and C = 0.1 µF, time constant is 1 second. This doesn’t mean that capacitor will be fully charge in 1 second. It means that capacitor will be charge to 63% of input voltage in 2 seconds. If we continue to apply the voltage, capacitor takes 63% of the voltage difference between current voltage and input voltage. This process will repeat itself till capacitor acquires full charge. We get value 63% or 0.63 when we put one time constant in above equation. We can calculate current at any instance (time) in capacitor using ohms law. Consider same circuit as discussed earlier. Here is the current equation during charging of capacitor.

The table below shows values of capacitor charging voltage and current for respective time constant.

Switch position |
Time constant (τ) (in seconds) |
Capacitor charging voltage (V) (in volts)_{c} |
Capacitor charging current (I)_{c} |

OFF | 0 | 0 |
10 µA |

ON | 1RC | 63.2120 |
3.6787 µA |

ON | 2RC | 86.4664 |
1.3533 µA |

ON | 3RC | 95.0212 |
0.4978 µA |

ON | 4RC | 98.1684 |
0.1831 µA |

ON | 5RC | 99.3262 |
0.0673 µA |

ON | 8RC | 99.9664 | 3.3546 nA |

ON | 11RC | 99.9983 | 0.1670 nA |

ON | 14RC | 99.9999 | 8.3152 pA |

ON | 17RC | 99.9999 |
0.4139 pA |

The term 1RC, 2RC etc. defines number of times a constant voltage that must be applied to capacitor. The table above reminds important fact related to capacitor i.e. **the capacitor will never store complete charge given to it**. For every time constant capacitor voltage increases slowly (except first) but it will never equal to the input voltage. The current flowing through resistor capacitor circuit is decreases as time (τ) increases. Here is the graph showing behavior of charging voltage and current of capacitor.

The graph of capacitor charging voltage and current is exponentially rising and falling in nature respectively. The curve shows how much time capacitor need to get almost full charge. The exponential rise of voltage and exponential decay of current in capacitive circuit is not same or it is not in **phase**. Note that the x axis of graph is changed with respect to value on y axis to have a clear view change in voltage or current. The graph is not as per specific scale. In 5RC seconds, charging current *I _{c}* ≈ 0 and charging voltage

There are multiple ways to discharge a charged capacitor. The easiest way is to use LED or resistor in series with capacitor. We need to take extreme care while selecting resistor or led for capacitor to discharge. It is good practice to refer specifications like wattage, value in case of resistor and forward current, voltage in case of LED before use. The discharging circuit for capacitor is shown below.

Here are the equations for voltage across capacitor and current in capacitor at any instant of time during discharging.

The table below shows values of capacitor discharging voltage and current for respective time constant. During discharging, the voltage from which capacitor starts discharging is last charge

Switch position |
Time constant (τ) (in seconds) |
Capacitor charging voltage (V)_{d} |
Capacitor charging current (I)_{d} |

OFF | 0 | ≈ 100 V | 10 µA |

ON | 1RC | 36.7879 V | 3.6787 µA |

ON | 2RC | 13.5335 V | 1.3533 µA |

ON | 3RC | 4.9877 V | 0.4978 µA |

ON | 4RC | 1.8315 V | 0.1831 µA |

ON | 5RC | 0.6737 V | 0.0673 µA |

ON | 8RC | 0.0335 V | 3.3546 nA |

ON | 11RC | 1.6701 mV | 0.1670 nA |

ON | 14RC | 30.5902 µV | 8.3152 pA |

ON | 17RC | 4.1399 µV | 0.4139 pA |

During discharging, the capacitor voltage and current decreases quickly at 1RC second and after that there is slow decrease in both quantities. Here is the graph of capacitor discharging voltage and current. Both graphs are exponentially falling in nature. In 5RC seconds, discharging current *I _{d}* ≈ 0 and discharging voltage

This is it for now. I hope now you know How Capacitor works with DC. In future post, we will learn about capacitor in AC circuits. Thanks for reading and don’t forget to leave a comment.

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]]>This post is about **What is Capacitor**, **Why we need Capacitor in Circuit** and **Working of Capacitor**. Capacitor is the second important member in the triad of passive electronic components. It is very hard to find any electronic or electrical circuit without capacitor. Capacitor stores charge and can act like a battery. It is necessary in filter circuits to minimize voltage spikes, smoothing changes in voltage. Like resistors, capacitors can also create voltage divider network.

Capacitance is the property which store input energy in the form of **electrical charge** and return almost all store energy to other circuit elements. Capacitor is passive electrical component having property of capacitance. Using water analogy, capacitor is similar to a bucket holding water. Capacitance defines capacitors capacity (ability) to store electrical energy, just as capacity of bucket to hold water. More the capacitance more is capacity to store charge. The amount of electrons store in capacitor is also known as Capacitance. The unit of capacitance is **Farad (F)**. A 1 F capacitor is very rarely found in circuits. Usually, capacitor is use in micro to pico Farad range.

Capacitor is a two-terminal passive component having property of capacitance. This property electrifies (charging with electricity) capacitor with input voltage. As capacitors condense (store) electricity, hence it is also known as **condenser**.

With this definition we can define capacitance as,

The capacitor symbol looks like two parallel lines with one line is either curve or flat. There are two types circuit symbols of capacitor; **Non-polarized and polarized**.

(Polarized component is asymmetric. Polarity makes component unidirectional. It gives component a unique position of placement in circuit.)

The basic structure of capacitor consists of two parallel metal foils (very thin sheets of metal). The metal foils act as electrode. Dielectric material is use as as insulator to separate metal foils. The term “di” in dielectric refers to placement in between two (di) foils and electric means it holds electric field. The circuit symbol of capacitor nearly looks like basic structure of capacitor.

The insulating material such as glass, rubber, ceramic, plastic, paper etc are use as dielectric material. The material use as metal foils are tantalum, aluminum, mica etc. The operation and structure of capacitor defines its capacitance as per following two relations,

Where, ϵ (epsilon) is permittivity (a kind of resistance present when electric field is establish in a medium), *ϵ _{0}* is permittivity of air (vacuum) with constant value of 8.85 × 10−12 Farad/meter,

The electric charge is the backbone of component like capacitor. The construction of capacitor shows that there is a gap in between two metal foils. Hence, when electric current passes through capacitor, because of the gap, charge get “freeze” on metal foils. The foil with more electrons get net negative charge and foil with less electrons get positive charge. The dielectric material present in between metal foils do not allow charges to get attracted to each other. Hence these steady charges forms electric field. This structure resembles with the battery. Hence capacitor act as battery. Capacitor and battery stores electrical energy in the form of electrical charge and chemical energy respectively.

Let’s take a simple example to understand how capacitance allows capacitor to act like a battery.

There are two switches controlling two parts of the circuit. A capacitor is shown in cylindrical shape and looks like small DC battery. Watch animation carefully..!!!

When upper switch is close and lower switch is open, capacitor is connected with battery and get isolated from other circuit elements. This starts **charging** of capacitor. Capacitor is kept in charging position for some time, this produces certain amount of voltage inside capacitor.

An LED is present in circuit to demonstrate **discharging** of capacitor. When upper switch is open and lower switch is close, capacitor get isolated from battery and is connected with other circuit elements. Now capacitor is acting as voltage source. Take a close look at light intensity of LED and voltage drop in capacitor. At start, capacitor easily provides minimum required forward voltage to LED. But when capacitor voltage start to decrease; light intensity of LED also decreases. Finally LED get turn OFF but a small voltage is still remaining in capacitor which is less than minimum forward voltage of LED.

You might have read or listen about strange behavior of capacitors toward DC signal. It is said that capacitor blocks DC, actually it should be written as capacitor blocks **DC current**. The question arises why so? There are two answers for this question. If we look at the construction of capacitor, there is a tiny gap in between two conducting materials. This gap block the path of direct current and don’t allow it to flow through capacitor. This may look like very general and simple explanation. But, there is also a mathematical proof.

The current flowing through capacitor depends on capacitance as well as change in voltage. The mathematical expression for this is,

There is a derivative (varying something with respect to time) of voltage in current equation. In case of AC signal, voltage is not steady (continuously varying) and oppositely in DC signal, voltage is steady. According to rules, a derivative of steady (constant) value is zero. Hence for DC signal above equation gives value of I equal to zero. This is the reason behind capacitor blocking DC but passing AC current.

I hope you enjoy reading this post and now you know what capacitor is and why we need capacitor in circuit. Capacitors are key component of filter part present in every power supply circuit. In next post we’ll learn about time constant and combinations of capacitor. Thanks for reading.

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