Capacitor - is a device which has the ability to store electrical energy in the form of an electrical

charge. It is constructed of two or more parallel conductive plates that are electrically
separated by an insulating material such as waxed paper, mica, ceramic, plastic or some form of
a liquid gel, called, the dielectric. It blocks steady state or DC current while providing lesser
opposition to AC current.

Capacitor construction
 Capacitor symbol

Types of Capacitors

Capacitance, C – the ability or measure of the capacitor to store electrical charge. It is also a
measure of how fast or slow the capacitor will store charge. The unit of capacitance is Farad (F).
There are two ways to measure capacitance.

C = Q/V, Farad, F
Where:
C = Capacitance, in Farad, F
Q = Stored Charge, in Coulomb, C
V = Voltage Across the plates, in Volt, V

In terms of its construction,

C = εoεrA/d, Farad, F
Where:
C = Capacitance, in Farad, F
εo = absolute permittivity or permittivity of vacuum
= 8.854 x 10-12 F/m
εr= relative permittivity or dielectric constant of the insulator or dielectric,
unitless
A = area of conducting plate, m2
 d = distance between plates, m

Relative permittivity, εr or dielectric constant, k, of some insulators

Example: calculate the capacitance of a capacitor if it is able to store 10μCoulomb of charge

over a potential of 10V?

Given:
Q = 10 μCoulomb = 10x10-6 Coulomb = 0.00001 Coulomb
V = 10 Volt

C = Q/V = 0.00001 Coulomb/10 Volt

= 0.000001 Farad or 1x10-6 Farad or 1 μF

Example: calculate the capacitance of a capacitor with a plate dimension of 1cm by 1cm and a
plate separation of 0.5cm and uses Mylar as the dielectric material.

Given:
d = 0.5cm, in meter = (0.5cm)(1m/100cm) = 0.005m
A = 1cm x 1cm = 1cm2 = (1cm2)(1m2/10,000 cm2) = 0.0001m2
εo = 8.854 x 10-12 F/m
εr= 3.1
 C = εoεrA/d
= (8.854 x 10-12 F/m)(3.1)(0.0001m2)/(0.005m)
= 0.549 x 10-12 F or 0.549 picoFarad or 0.549 pF

Charging and Discharging a Capacitor

a S1 Rc

b
+ +
V Rd C Vc
-

Charging and Discharging Circuit of a Capacitor

To charge a capacitor, a voltage must be applied across its plates. Capacitor is usually
charge through a resistor, in this case, Rc, as shown in the figure below. When the switch is at
position a, the capacitor will charge up to the level of the source voltage, V. The time needed to

Charging Current

a S1 Rc
b
+ +
V C Vc
-

Charging a Capacitor
charge a capacitor is determined by a factor called time constant or tau (τ), whose value is the
product of the charging resistance and the capacitance of the capacitor .

τ = RC, second (s)

It would take approximately a time equals to 5τ to fully charge a capacitor if the
capacitor is initially uncharged.
t ≈ 5τ time for full charging

When the switch is moved to position b without a discharging resistor, as shown in the
figure below, the capacitor will not be able to discharge, and will store and continue to hold on
to the stored potential.

Capacitor Isolated from the Source

The stored potential across the capacitor can be discharged through a resistor, in this
case, Rd, and Rc, as shown in the figure below. Capacitor discharges when the source voltage
that causes it to charge is removed or diminished. When isolated from a voltage source, a
capacitor will act

a S1 Rc
b
+ + Discharging
Rd Current
V C Vc
-

Discharging a Capacitor

as a voltage source and discharges its stored potential through a load. The approximate time for
the capacitor to fully discharged is equal to five times the product of the resistance where it
discharges and its capacitance.
 τ = (Rd+Rc)C, second (s)
Important Capacitor Parameters

Capacitance, C = known as the rated nominal capacitance, its common values ranges
from few thousand microfarad (μF) to picoFarad (pF).
Working Voltage, WV = the maximum continuous DC or RMS AC voltage that can be
applied to the capacitor before destroying the dielectric. It is usually specified at a
temperature. Typically, the working voltage decreases with increase in temperature.
Common working voltages typically ranges from few volts up to few thousand volts.
Tolerance = is the extent to which the actual capacitance can deviate from its rated
nominal value. It is expressed either as capacitance deviation or as percent deviation
above and below the nominal capacitance.
Leakage Current = the current that will “leak” or flow through the dielectric when
capacitors are exposed to very high potential. Typically ranges from few microampere
(μA) to nanoampere (nA) range.
Working Temperature = provides the temperature range a capacitor must be operated
for it to work satisfactorily and depends on the type of dielectric used.
Temperature Coefficient = the change in capacitance due to change in temperature. It is
expressed as parts per million per degree Celcius (ppm/°C). There are two types of
Temperature Coefficients, namely;
Positive Temperature Coefficient (PTC) = capacitance increases with increase in
temperature.
Negative Temperature Coefficient (NTC) = capacitance decreases with increase
in temperature.

When temperature coefficient is expressed, the format is “Pxxx” or

“Nxxx”, with P and N denoting the nature, whether positive or negative, and xxx
denoting the value in ppm/°C. For example, a Temperature Coefficient of “P150”
means the capacitor is PTC and has a coefficient of 150 ppm/°C.

Types of Capacitors

Capacitors can generally be categorized as Non-Polarized (Non-Polar) or

Polarized (Polar) or Electrolytic capacitor. Non-Polarized capacitors do not have implicit
polarity, hence they can be connected in either orientation. Polarized capacitors on the
other hand have implicit polarity for each terminal, hence they should be connected to a
potential at an appropriate orientation.
Types of Non-Polarized Capacitors

Names of capacitors indicates the type of dielectric used. There are many types
of non-polar capacitors.

Ceramic Capacitor - are made by coating two sides of a small porcelain or

ceramic disc with silver as electrodes. Ceramic capacitors have values ranging from less
than ten microfarads to few picofarads.

Ceramic Capacitors

Mica Capacitor

Clamped Mica Capacitors - are constructed from thin sheets of mica and metal
(usually copper) foil. These sheets and foils are stacked together and clamped. They
were then encapsulated in insulating material. The tolerance and stability of clamped
mica capacitor are worse than other capacitors because the surface of mica is not flat
and smooth.

Nowadays, There are obsolete and are replaced by silver mica capacitor.

Silver Mica Capacitor - is made of sheets of mica with electrode being coated on
its both sides. Multiple layers are added together to increase its capacitance. It is then
dipped in an insulator Epoxy for protection against humidity and air.

They are very stable and have low losses. They have a low tolerance of about +/-
1%. Its capacitance depends very little on the applied voltage. The encapsulation
protects the electrodes from corrosion. Thus, they maintain a longer lifespan.

They are costly and have a larger volume as compared to ceramic capacitors. It
can operate on heavy voltage ranging from 100 v to 10 KV with capacitance ranging
from 47 pF to 3000 pF.
 They are still used in modern electronic circuits because of its high voltage and
power handling capabilities such as RF broadcast transmitter, amplifiers, high voltage
inverters, and resonance circuits.

Silver Mica Capacitor

Film Capacitors - also known as Polymer Film or Plastic Film capacitor is a type of
capacitor which uses a film of usually plastic and sometimes paper as a dielectric. Its
construction has two types or formats of configuration, namely;

Metalized Capacitor - are those types of capacitors that use a metalized

dielectric film, which is made by depositing a metal layer over the dielectric film.
The metal used can be Aluminum or Zinc. Such configuration provides self-
healing property and the film can be wound together to achieve capacitance up
to 100uF.

Film/Foil Capacitor - are constructed by sandwiching a dielectric film with

metal foils. The metal is usually Aluminum which acts as the electrodes. Such
type of configuration enables the capacitor to handle high surge currents.

The film capacitors are divided into different types of capacitors based on
the type of dielectric film.

Paper Capacitors - It is the first film capacitor where an oil-

impregnated paper was used as a dielectric between Aluminum foils. The
main drawback of paper film/foil capacitor was that it soaks moisture,
which degrades its performance over time. They were fairly bulky.
Nowadays, Metallized paper films are used as a dielectric with a self-
healing property. The paper is combined with polypropylene film to
increase its voltage rating and improve its performance.

Power capacitor that uses paper as a dielectric, is filled with oil to

fill the air gaps increasing its breakdown voltage.

Polyester (PET) Film or Mylar Capacitors - uses a dielectric made

of polyethylene terephthalate (PET), which is a thermoplastic polar
polymer. They are constructed in both metalized film and Film/Foil
structure as well.

Its ability to resist moisture makes the capacitor to be used

without coating. Its high permeability and dielectric strength offer high
volumetric efficiency. However, its capacitance temperature coefficient is
a little higher than other film capacitors. It can operate in temperature up
to 125° C. This also allows it to be used as an SMD capacitor. They
operate on a maximum voltage of about 60 KV. They have a tolerance of
5% to 10%.

Mylar Capacitor

Poly Propylene (PP) Film Capacitors - uses Polypropylene, an

organic polymer material as a dielectric. They are manufactured in both
configurations i.e. metalized film and film/foil.

They are even more resistant to moisture than polyester film

capacitor, thus they don’t need any protective coating. Their capacitance
is less dependent on temperature and frequency as compared to
polyester but its operating frequency is lower with a max limit of 100
KHz. Its max operating temperature is 105° C. They have high operating
voltage with a maximum voltage rating of 400 KV.

They are used in high-power induction heating, AC motor run

capacitor and power factor correction capacitor.

Poly Propylene (PP) Film Capacitors

 Polyethylene Naphthalate (PEN) Film Capacitors - uses
Polyethylene Naphthalate (PEN) which belongs to the polyester family as
dielectric. These capacitors are only available in metalized dielectric
structure.

The main advantage of PEN capacitors is its high-temperature

stability of about 175°C. Due to high-temperature stability; they are
manufactured in SMD packaging.

It has low volumetric efficiency because the PEN dielectric has

lower permeability & strength compared to PET. However, its
capacitance dependence on its temperature and frequency is similar to
PET capacitors, which is why they are used in applications where
temperature dependencies are not required.

There are used for coupling, decoupling & filtering purposes.

PEN Film Capacitors

Polyphenylene Sulfide (PPS) Film Capacitors - These film

capacitors are available in only metalized film form. Their capacitance
depends very little on the temperature & frequency compared to other
film capacitors.

It offers a very stable response for the temperature below 100° C.

Its dielectric can withstand temperature up to 270° C. Therefore, they are
also made in SMD packaging. However, they are costly compared to
other film capacitors.

They are used in applications where high operating temperatures

exist.

Polytetrafluoroethylene (PTFE) Film Capacitors - also known by

its trademark name Teflon, uses a synthetic polymer
 Polytetrafluoroethylene (PTFE) as the dielectric. They are manufactured
in both metalized & Film/Foil types.

They are fairly bulky & expensive. The temperature dependence

for its capacitance is a little higher than the Polypropylene (PP) film
capacitor. But they are very resistant to a temperature of about 200°C
with very low losses.

They are used in high-quality applications for aerospace & military

equipment.

Polycarbonate (PC) Film Capacitors - these film capacitors use

polycarbonate dielectric which is manufactured in both metalized and
film/foil structure as well.

They offer very high stability & very low loss. It is almost
independent of the temperature in the range of -55° to +125° C. The
Polycarbonate film provides high tolerance which increases its reliability.

Power Film Capacitors - have the similar construction as film

capacitor. The layers are wound together to attain a larger size and
capable of handling high power. They are used in high power AC & DC
applications.

Capacitance Reading

Alpha Numeric Coding

Uses numbers and letters to represent the actual or coded value of the
capacitance, tolerance, working voltage and temperature coefficient.

The most common is the use of numeric codes usually found in ceramic disc
capacitors and polyester film capacitors. The table below shown below is used to
determine the capacitance.
Procedure:

For a capacitor with one or two digit coding, the value is equals to
number formed by the corresponding digit/s and the unit is in picoFarad (pF).
 2-digit ceramic capacitor

Example: for a ceramic capacitor coded as 18, what is the value of the
capacitance?

Solution: Capacitance = 18 pF = 18x10-12 F

For capacitors with 3 digit coding, the first two digits represents the
number while the third digit is the multiplier as shown in the table above.

3-digit ceramic capacitor

Example: for a ceramic capacitor coded as 104, what is the value of

the capacitance?

Solution: Capacitance = 10x104 pF = 100,000 pF = 100,000 10-12 F =

100nF

If a letter is used after the digits, it could represent either the tolerance or
the scientific notation of unit in Farad.
 Example: for a ceramic capacitor coded as 104K, what is the value of
the capacitance?

Solution: Capacitance = 10x104 pF +/- 10% = 100,000 pF +/- 10,000pF

Working voltage can either be directly printed in the body as shown in
the figure above or the polyester capacitor shown below.

Polyester Capacitor

Code as shown in the table can is also used, as shown in the example
below.

Color Coding
Table for Color Code for Capacitor
 Color Code for Voltage Rating

Metalized Polyester Capacitor

Disc Ceramic Capacitor

Example: Determine the capacitance of a capacitor color coded as

a. Brown, Black, Orange, Brown

C = 10x103 pF +/-20%, 100V

b. Yellow, Violet, Orange, White, Red

C = 47x103 pF +/- 10%, 250V