What is a capacitor?
A capacitor is a two-terminal passive component that stores electrical charge. This component consists of two conductors separated by a dielectric medium. The potential difference when applied across the conductors polarizes the dipole ions to store charge in the dielectric medium. The circuit symbol of a capacitor is shown below:
Fig. 1: Symbol of a capacitor
The capacitance or potential storage by the capacitor is measured in Farads, which is symbolized as 'F'. A Farad is the capacitance when one coulomb of electrical charge is stored in the conductor upon application of a potential difference of one volt.
The charge stored in a capacitor is given by
Q = CV
Where Q – charge stored by the capacitor
C – Capacitance value of the capacitor
V – Voltage applied to the capacitor
Note the other current formula, Eu = dQ/dt
Taking the derivative with respect to time,
dQ/dt = d(CV)/dt
From the above statement, we can express the equation as
I = C (dV/dt)
As you turn on the power supply, current begins to flow through the capacitor, inducing positive and negative potentials across its plates. The capacitor continues to charge until the capacitor voltage equalizes with the supply voltage, which is called the capacitor charging phase. Once the capacitor is fully charged at the end of this phase, it is open circuited to DC. It starts discharging when the capacitor power is turned off. The charging and discharging of the capacitor is given by a time constant.
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The voltage across the capacitor is given by
Capacitors are widely used in a variety of electronic circuit forms like
store charges, as in a camera flash circuit
· smoothing the output of power supply circuits
Two-stage coupling of a circuit (coupling an audio stage with a speaker)
· filter networks (tone control of an audio system )
· delaying applications (as in 555 timer IC controlling the charging and discharging )
· tune radios to specific frequencies
· phase change.
The conductors offer a resistance in series and if the capacitor is constructed in a tubular structure then some inductance is also induced. The dielectric medium between the plates has an electric field strength limit and also passes a small amount of leakage current which results in a breakdown voltage.
There are different types of capacitors , they can be fixed or variable. They are categorized into two groups, polarized or non-polarized. Electrolytic capacitors are polarized. Most low value capacitors are not polarized. The capacitor symbol for each group is shown below:
Figure 2: Image showing types of capacitors
Construction and Types
Construction and Types:
The capacitor consists of two conducting plates separated by an insulating medium known as a dielectric. The capacitance depends on the surface area of the plates, the distance between the dielectric medium and the dielectric constant of the object. The greater the area of the plates, the closer they are to each other and the greater the value of the dielectric constant, the greater the value of the capacitance. High capacitance capacitors are now available in small sizes. This was achieved by employing a number of techniques, such as multiple sets of plates, placing the plates very close together, placing a thin layer of dielectric between them, and developing special insulating dielectric materials.
The capacitance of a capacitor is also affected by the shape or structure of the capacitors. Capacitors are available in different shapes, such as radial conductor type, which are rectangular or cubic, or axial conductor type, which are tubular or cylindrical.
The variable type of capacitors can vary the capacitance by changing the distance between the plates or the effective area of the capacitor.
Polarized type capacitors must be connected according to their polarity, otherwise the capacitor may be damaged due to incorrect connection.
Low value capacitors are not polarized and can be connected in any way. They are not damaged by heat during soldering, except the polystyrene type capacitor. They have high voltage ratings of at least 50V, usually 250V or more
Many small value capacitors have their value printed but no multiplier, so you need to use experience to figure out what the multiplier should be!
For example:
· 0.1 means 0.1µF = 100nF.
Sometimes the multiplier is used in place of the decimal point:
For example – 4n7 means 4.7nF .
For example – 4n7 means 4.7nF .
Figure 3: Image of various types of capacitors
The various types of capacitors are given below:
1. Fixed capacitors
· Film capacitors such as glass capacitor, mica capacitors, silver mica capacitor, ceramic capacitor, paper capacitor, metallized paper capacitor, polyester capacitor, polystyrene capacitor, metallized polyester capacitor, polycarbonate capacitor, capacitors polypropylene, Teflon capacitors, porcelain capacitor.
· Electrolytic capacitors such as aluminum electrolyte, tantalum electrolyte, aluminum-tantalum electrolyte
2. Variable capacitors
2. Variable capacitors
Fixed Capacitors
1. Fixed Capacitors
The. F film capacitors
Film capacitors consist of a relatively large family of capacitors, with the difference in their dielectric properties. These include polyester (Mylar), polystyrene, polypropylene, polycarbonate, metallized paper, Teflon, etc. Film-type capacitors are available in capacitance ranges from 5pF to 100uF, depending on the actual type of capacitor and its voltage rating. Film capacitors come in various shapes and case styles, such as:
· Wrap and fill (oval and round) – The capacitor is wrapped in tight plastic tape and the ends are filled with epoxy to seal them.
· Epoxy housing (rectangular and round) – The capacitor is enclosed in a molded plastic housing filled with epoxy.
· Hermetically sealed metal (rectangular and round) – The capacitor is enclosed in a metal tube or tin can and sealed with epoxy.
Note: All of the above case styles are available in Axial and Radial leads.
B. Paper Capacitor:
Paper capacitors are made from paper or oil-impregnated paper and layers of aluminum foil rolled into a cylinder and sealed with wax. These capacitors were commonly used, but are now replaced by plastic or polymer type capacitors. Paper capacitors are bulky, highly hygroscopic and absorb moisture, which causes loss to the dielectric, degrading its overall performance, being the main disadvantage of this type of capacitor. The other variants include oil-impregnated paper capacitor, polyester paper and Kraft paper.
Fig. 4: Image of paper capacitors
Figure 5: Image showing the construction of paper capacitors
Fixed Capacitors – 2
w. Metallized paper capacitors:
Metallized paper capacitors are smaller in size than conventional paper capacitors. However, these capacitors are only suitable for low current applications and are now replaced by metallized film capacitors.
Fig. 6: Image of Metallized Paper Capacitors
d. Mica Capacitor:
The mica capacitor uses mica as a dielectric medium. Mica is inert in nature and therefore the physical and chemical properties do not change as it ages. Provides good temperature stability and resistance to corona discharge, i.e. electrical discharges due to ionization around the conductor. However, the cost is very high and due to inadequate sealing the capacitor is highly subject to humidity which increases the power factor.
Fig. 7: Image showing the construction of the mica capacitor
Figure 8: Image of mica capacitors
Fixed Capacitors – 3
It is. Silver mica or metallized mica capacitor:
This is a type of mica capacitor that has the additional advantage of reducing moisture infiltration. These capacitors are expensive and are often used in low HF and VHF radio frequency circuits as low value precision capacitors, especially in oscillators and filters. The reasons why these capacitors are still in use regardless of the high cost, large size and availability of other low-cost capacitors are due to their notable features such as:
Low tolerance of +/- 1%
· positive temperature coefficient of 35 to 75 ppm/C
· larger range from a few pF to two or three pF
· little voltage dependence,
· high stability
· Good Q factor.
However, these capacitors are not widely used today.
Fig. 9: Image of the silver mica capacitor
f. Glass Capacitor:
These capacitors are manufactured using glass dielectrics and are very expensive and are used for highly accurate, stable and reliable operation in harsh environmental conditions. These are resistant to nuclear radiation and are available in the range of 10pF to 1000pF.
Figure 10: Image of the glass capacitor
Fixed Capacitors – 4
g. Ceramic capacitor:
Non-polarized type ceramic capacitors, also known as 'disk capacitors', are widely used today. They are available in millions of cost and performance varieties. The characteristics of the ceramic capacitor depend on:
· Type of ceramic dielectric used in the capacitor that varies in temperature coefficient.
· Dielectric losses.
The exact formulas of the different ceramics used in ceramic capacitors vary from one manufacturer to another. Common compounds such as titanium dioxide, strontium titanate, and barium titanate are the three main types available, although other types such as through-hole mounting leaded disc ceramic capacitors, resin-coated ceramic capacitors, multilayer surface mount chip and microwave lead-free disc ceramic capacitors that are designed to be placed in a slot on the PCB and soldered in place.
They are made by placing silver-coated ceramic plates on both sides and assembled to form the capacitor. The surface mount version consists of a ceramic dielectric in which several interspersed precious metal electrodes are contained. This structure gives rise to a high capacitance per unit volume. The internal electrodes are connected to the two terminations, either by palladium silver alloy (AgPd) in the ratio of 65:35, or silver dipped in a barrier layer of plated nickel and finally covered by a layer of plated tin (NiSn).
The Electronic Industries Alliance (EIA) has broadly classified the ceramics used in these capacitors into 3 classes – class 1, class 2 and class 3. The lower the class, the better their overall characteristics, but it depends on the size cost. Each class defines the working temperature range, temperature deviation, tolerance, etc. Typical values range from 10pF to 1uF. Capacitance values are labeled by three-digit codes, where the first two digits represent a number and the third digit is the multiplier digit.
For example: 103 means 10*10 3 pF which is 0.01uF
or
104 which is 10*10 4 pF which is 0.1uF
The tolerance is indicated by a letter such as j=5%, K=10% and M=20%.
These capacitors are commonly used as timing element in filter circuits and balancing oscillator circuits in radio frequency applications, coupling and decoupling networks.
The three classes of ceramics decided by the EIA are:
The . Class 1 – Class 1 ceramic capacitors are the most temperature stable forms of ceramic capacitor. Common compounds used as dielectrics are magnesium titanate for positive temperature coefficient (PTC) or calcium titanate for negative temperature coefficient (NTC) capacitors. Using combinations of these and other compounds it is possible to obtain a dielectric constant between 5 and 150. They have an almost linear characteristic and their properties are almost independent of frequency within normal limits. Temperature coefficients between +40 and -5000 ppm/C can be obtained.
Class 1 capacitors offer the best performance in terms of dissipation factor. A typical value might be 0.15%. It is also possible to obtain very high accuracy class 1 capacitors (~1%) instead of the more usual 5% or 10% tolerance versions. The highest precision class 1 capacitors are designated C0G or NP0.
The EIA has defined a set of codes to have a managed way of ceramic capacitor performance. The codes for class 1 and class 2 capacitors are different.
Class 1 codes are as follows:
Fig. 11: Table listing class 1 codes for ceramic capacitors
· The first character is a letter that gives the significant number of the change in capacitance with respect to temperature in ppm/C.
· The second character is numeric and provides the multiplier.
· The third character is a letter and gives the maximum error in ppm/C.
A common example of a class 1 capacitor is a C0G. This has 0 deviation, with an error of 30PPM/C.
Fig. 12: Image of class 1 ceramic capacitors
B. Class 2 – Class 2 capacitors are better in size but have less accuracy and stability. As a result, they are typically used for decoupling, coupling and bypass applications where accuracy is not of primary importance. A typical class 2 capacitor can change capacitance by 15% or more over a temperature range of -50°C to +85°C and can have a dissipation factor of 2.5%. It will have medium to low accuracy (from 10% to +20/-80%). However, for many applications, these numbers would not pose a problem.
Class 2 codes are as follows:
Figure 13: Table listing class 2 codes for ceramic capacitors
. The first character is a letter indicating the minimum operating temperature.
· The second is numeric, which provides the peak operating temperature.
· The third character is a letter that provides capacitance change over this temperature range.
Common examples of class 2 ceramic capacitors are:
· The X7R capacitor that operates from -55C to +125C with a capacitance change of up to 15%.
· The Z5U capacitor that operates from +10C to +85C with a capacitance change of up to +22% to -56%.
Fig. 14: Image of class 2 ceramic capacitors
w. Class 3 – Class 3 ceramic capacitors are small, with less precision, stability and low dissipation factor. This type of capacitor cannot withstand high voltages.
Barium titanate which has a dielectric constant of about 1250 is used as a dielectric. A typical cla is Capacitor 3 will change its capacitance by -22% to +50% over a temperature range of +10C to +55C. It can also have a dissipation factor of around 3 to 5%. It will have quite low accuracy (typically 20% or -20/+80%). Therefore, class 3 ceramic capacitors are typically used as decoupling or in other power supply applications where accuracy is not of primary importance. However, they should not be used in applications where spikes exist as they cannot withstand high voltages.
SMT ceramic capacitors are also available in standard packages that have the following designations given in the table below.
Figure 15: Table listing standard SMT ceramic capacitor packages
Fixed Capacitors – 5
H. Plastic Capacitors
I. Polyester or PET capacitor:
Polyester or PET capacitors are plastic capacitors available in lead packaging that replace paper capacitors. These capacitors are made from polyester films of small size and available at low cost. These have operating voltages of up to 60,000 V DC, operating temperatures of up to 125 °C and low moisture absorption. They are mainly used as capacitors and integrators of low frequency signals. They are preferred where cost plays an important role because they have high tolerances of 5 to 10%.
Figure 16: Image of plastic capacitors
ii. Polystyrene Capacitors:
These are large size capacitors available in leaded tubular packaging. They have high stability, negative temperature coefficient (NTC), high precision and low moisture absorption. The operating temperature is limited to +85 C. These are mainly preferred for low frequency applications as the tubular structure induces inductances that degrade performance at high frequencies.
Figure 17: Image of polystyrene capacitors
iii. Kapton Polyimide Capacitor:
These capacitors are similar to polyester or PET capacitors made from Kapton polyimide film. They are expensive but offer high operating temperatures of up to 250 C. These capacitors are not suitable for RF applications.
Figure 18: Image of the Kapton polyimide capacitor
4. Polycarbonate Capacitors:
These are high-performance capacitors that are less affected as they age. These are characterized by good insulation resistance and dissipation factor. The operating temperature ranges from -55 to +125 C. The dielectric constant is 3.2% and the dielectric strength is 38 KV/mm. The dissipative factor is 0.0007 at 50 Hz and 0.001 at 1 MHz. Water absorption is 0.16%. They are mainly used for filters, couplings and timing applications. These can be directly replaced by Polyethylene Naphthalate (PEN), Polyphenylene Sulfide (PPS), Polyimide (PI) and Polytetrafluoroethylene (PTFE).
Figure 19: Image of polycarbonate capacitors
v. Polypropylene Capacitors:
They are used where higher tolerances are required than PET film capacitors. They are available in leaded packages and are used for low frequency operation. They have high operating voltages and are resistant to breakage. However, they are damaged by transient overvoltages or voltage inversions.
Figure 20: Image of polypropylene capacitors
saw. Polysulfone Capacitor:
These capacitors are similar to polycarbonate capacitors but can withstand full voltage at comparatively higher temperatures. These capacitors are very expensive and not readily available. Stability is limited as moisture absorption is typically 0.2%
viii. TEFLON or PTFE fluorocarbon capacitor:
These plastic capacitors are large and expensive. Due to low losses and higher stability, these are used for some critical applications. The operating temperature ranges up to 250 C. The dielectric used is Polytetrafluoroethylene.
Figure 21: Image of TEFLON or PTFE fluorocarbon capacitor
viii. Polyamide Capacitor:
These plastic film capacitors are large and expensive. The operating temperature ranges up to 200 C.
Ix. Metallized polyester or metallized plastic capacitor:
These capacitors have metallized plastic films that provide self-heating advantage and also reduce the size of the capacitor compared to the conventional plastic or polyester capacitor. However, they are limited by maximum current capacity. They are available in leaded packaging.
Figure 22: Image of metallized plastic capacitor
Fixed Capacitors – 6
1. Electrolytic capacitors
I. Aluminum Electrolytic Capacitor:
These polarized capacitors are made of oxide film on aluminum foil. These are cheaper and easily available. The value range typically varies from 1uF to 47,000uF and large tolerance of 20%. Voltage ratings range up to 500V. They have high capacitance-to-volume ratio and are used for smoothing in power supply circuits or coupling capacitors in audio amplifiers. They are available in surface mount and leaded packages. The capacitance value and voltage ratings are printed in uF or encoded by a letter followed by three digits. The three digits represent the capacitance value in pF, where the first two digits represent the number and the third is the multiplier digit. The letter codes are as follows:
Figure 23: Table listing letter code for aluminum electrolytic capacitors
Fig. 24: Image of aluminum electrolytic capacitors
ii. Tantalum electrolytic capacitor:
These capacitors use tantalum oxide that allows the manufacture of small-sized electrolytes. They are more expensive than aluminum electrolytes and have lower maximum voltage, up to 50 V, and are preferred where size is important. Its typical values range from 47uF to 470uF. These may use layered sheets of tantalum oxide or porous anode with sulfuric acid as electrolyte between sheets of tantalum in a wet tantalum electrolyte or solid tantalum electrolytes. Its SMT formats are available in standard packages where the package designations have been defined by the EIA.
Figure 25: Image showing the construction of the tantalum electrolytic capacitor
Fig. 26: Image of tantalum electrolytic capacitors
iii. Supercapacitor:
Supercapacitors, also called double-layer electrolyte capacitors, are made of a thin electrolyte separator flanked by activated carbon ions. These have capacitance values as high as the order of a thousand farads. They are used as a temporary power source as a replacement for batteries.
Fig. 27: Image of Supercapacitors
Variable Capacitors
2. Variable capacitors
The variable type of capacitors can vary the capacitance by changing the distance between the plates or the effective area of the capacitor.
The. Air Gap Capacitors:
These capacitors use air as a dielectric medium. The distance between the plates can be varied to change the capacitance. The offered capacitance values are high and can be used with high voltages. They are used for high frequency operations in communication systems.
B. Vacuum Capacitors:
These capacitors have glass or ceramic encapsulation and vacuum as dielectric. Its complex construction makes it very expensive. Theoretically, they have less losses and are used in RF applications.
Fig. 28: Image showing the construction of the Trimmer
Fig. 29: Image showing working principle of variable capacitor
Fig. 30: Image of variable capacitors
Capacitor color code
Capacitor color code:
Figure 31: Image showing color coding for capacitors
A color code has been used on polyester capacitors for many years. It's now obsolete, but of course there are still many out there. The colors should be read as the resistor code.
· The top three color bands provide the value in pF.
· The fourth band is for tolerance.
· The 5th band is for voltage rating.
For example:
I. brown, black, orange means 10000pF = 10nF = 0.01µF.
Note: There are no spaces between the colored bands; therefore, two identical stripes appear as one wide stripe.
ii. wide red, yellow means 220nF = 0.22µF.