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Feature article
All About
Capacitors
Capacitors are probably the most misunderstood of the passive components, due
to the many different types available, their many parameters and greatly varying
performance. This article should give you an understanding of the most common
types, how they differ, and how to choose the right ones for your design.
By Nicholas Vinen
C
apacitors come in all shapes and
sizes. Some are much smaller than
a grain of rice, while others are huge
and used in banks to launch aircraft
weighing many tonnes into the air!
Because there are so many different
types, it can be very confusing trying
to choose one. Even if you know what
capacitance and voltage rating you
need, there could be hundreds or even
thousands of matching parts. Some of
those might not work at all in your
circuit, while others might work but
not very well, and some will be very
expensive. You need to narrow the
choice down to just a handful and
then pick one.
We have tried to break the following
descriptions into digestible sections,
despite their complexity. If you find
yourself overwhelmed, give yourself
time to digest what you have read so
far, then read the rest later.
Capacitor dielectrics
Fundamentally, a capacitor is just
two conductors (originally flat plates)
separated by an insulator (the “dielectric”). But because the area of the plates
required for any significant capacitance
36
is quite large, modern capacitors are
typically arranged as many layers of
smaller conductors and insulators connected in parallel, allowing for a more
compact package.
In some cases, the ‘plates’ are not
even flat but instead are spiral coils,
or 3D structures such as the etched
surface of a metal foil or granular
materials.
Etched or granular materials have a
much higher capacitance per volume,
as capacitance is proportional to surface area and inversely proportional to
the distance between the plates.
This creates a tradeoff; thinner dielectrics give more capacitance, but
have a lower breakdown voltage, so
the maximum voltage applied to the
capacitor must be kept lower. This is
the main reason that a capacitor with
a higher voltage rating, but the same
capacitance, tends to be physically
larger; its dielectric layer(s) need to
be thicker.
The type of insulating (dielectric)
material used has a strong effect on capacitor behaviour, and for this reason,
capacitors are mostly categorised by
the dielectric type. Different dielec-
tric types have their own trade-offs in
terms of capacitance, voltage ratings,
linearity, current handling and more.
Some widely used dielectric materials for capacitors are:
• Ceramics (typically metal oxides)
• Metal oxide layers (in electrolytic
capacitors)
• Plastic films
• Mica
• Paper
• A Helmholtz plane of solvent molecules (as in ‘double layer’ super/
ultracapacitors)
The most common types of capacitors in use today are ceramic and
electrolytic, followed by plastic film
types. These three types of capacitors
have important sub-categories which
strongly affect their behaviour.
One property of all dielectric materials is the dielectric constant (“K”).
The larger this number, the higher the
capacitance for a similarly constructed device. K can vary with temperature, voltage, age and other properties.
While high K values make for greater
capacitances in a small volume, there
are significant penalties in other areas,
as we describe below.
Practical Electronics | December | 2024
All About Capacitors
Ceramic capacitors
If you look at the PCB of just about
any modern electronic device, you
will find it covered in ceramic capacitors. They are cheap, reliable,
perform very well and are available
in a wide range of capacitances and
voltage ratings.
Because modern ceramic capacitors are fabricated in bulk, they can
have anywhere from one to many
thousands of layers. This gives them
a wide capacitance range, from fractions of a picofarad up to hundreds
of microfarads, in a small package –
see Figs.1-3.
Ceramic capacitors are typically
robust and long-lasting, and are not
polarised (they can handle negative
or positive voltages).
Ceramic capacitors are available
with voltage ratings from just a few
volts up to several kilovolts. Ceramic
capacitors with voltage ratings above
500V tend to use different types of ceramic to those below 500V, and have
slightly different properties.
The most common ceramics used
are based on titanium dioxide (TiO2)
or barium titanate (BaTiO3) with additives to tweak their properties.
As there are so many different possible combinations, they are arranged
in various categories based on their
performance. The categories are based
on the initial tolerance of the capacitor
(ie, the variation of real samples from
the rated value), how the capacitance
changes with temperature (the temperature coefficient) and how it changes
with applied voltage (the voltage coefficient).
The most common type codes are
NP0 or C0G (different names for the
same category), JB, SL0, X5R, X5S,
X6S, X7R, X7S, X8L, Y5V and Z5U.
To take three examples, NP0/C0G
types have very close tolerances and
no or minimal capacitance variation
with temperature or voltage. They also
have a low dielectric constant, so they
are relatively large for a given capacitance value and voltage rating. As a
result, they are also quite expensive.
Fig.1: the range of
capacitances and voltages
available in 3.2 x 2.5mm
SMD ceramic capacitors
today. Both larger and
smaller sizes are available,
extending the range of
values down to 0.1pF (1.6
x 0.8mm) and up to 470µF
(4.5 x 3.2mm). Note how
some types of ceramic
dielectric are available to
higher working voltages,
and others to a higher
maximum capacitance.
(original source:
Wikipedia)
Fig.2: the structure of typical
SMD and through-hole ceramic
capacitors. SMD ’chip’ ceramics
are made of many layers; throughhole disc capacitors may have a
single layer construction, as shown
here, or increasingly these days, a
similar internal structure to a multilayer SMD capacitor. Multi-layer
through-hole capacitors are usually
encapsulated in epoxy, while
the single-layer disc types can be
encapsulated in ceramic. (original
source: Johanson Dielectrics)
Fig.3: the manufacturing process for multi-layer SMD ceramic
capacitors. To keep the cost low, they are made in large sheets and
after lamination is complete, the sheets are sliced up into individual
capacitors. Those are then fired (similarly to ceramic pottery) and
the end terminals are added, which provide a way to solder to the
capacitor while also electrically joining every second layer.
(original source: Johanson Dielectrics)
Practical Electronics | December | 2024
37
Feature article
A set of ceramic capacitors ranging
from 47pF to 2.2µF.
Table 1: Class 2 capacitor codes
First letter
Middle number
Last letter
X = -55°C
4 = +65°C
P = ±10%
Y = -30°C
5 = +85°C
R = ±15%
Z = +10°C
6 = +105°C
L = ±15%, +15 / -40% above 125°C
7 = +125°C
S = ±22%
8 = +150°C
T = +22 / -33%
9 = +200°C
U = +22 / -56%
lower temperature
upper temperature
change in capacitance over given temperature range
V = +22 / -82%
Fig.4: a cross-section of one layer of
a standard aluminium electrolytic
capacitor. The anode and cathode are
both made from etched aluminium
foil, for a large surface area. A thin
layer of aluminium oxide is formed
on the anode, which acts as the
dielectric layer. The conductive
electrolyte allows electrons to flow
between the cathode right up against
that oxide layer, so only the oxide
layer separates the two halves of the
capacitor, maximising capacitance
per area. (original source: Wikipedia)
Fig.5: like ceramic capacitors,
electrolytics are made up of many
layers to give higher capacitance, but
they are typically wound in a coil
rather than made from flat layers
(with some exceptions). Once the
leads are attached, the coil is inserted
into a can with a rubber bung almost
sealing it. We say almost because a
small amount of airflow is needed to
prevent the electrolyte from drying
out. (original source: Wikipedia)
38
On the other hand, Y5V ceramics are
very compact for a given capacitance
value and voltage rating, but they have
a very poor initial tolerance, and their
capacitance is reduced dramatically at
elevated temperatures and higher applied voltages. The benefit is that they
are quite cheap to produce.
Dielectrics like X5R and X7R are
in between those two; they are larger
than Y5V types for a given capacitance
and voltage, but smaller than NP0/
C0G. Similarly, their tolerances are
intermediate, as are their production
costs. Therefore, these capacitors are
very popular as a reasonable ‘middle
ground’.
Note that NP0/C0G ceramics are
almost ideal capacitors. They have
very stable capacitance values over
temperature and voltage, low ESL
(equivalent series inductance) and
ESR (equivalent series resistance), a
low dissipation factor and excellent
linearity. Their only real disadvantages
are a low maximum capacitance value
(due to their relatively high volume)
and high cost.
We’ll describe the meanings of those
performance parameters in some detail
later in this article.
If the two/three-letter schemes given
above look like gobbledygook to you,
that might be because there are multiple different naming/categorisation
schemes.
The most common schemes are from
the EIA, which consist of a letter, a
number and a letter. But they don’t
always mean the same things.
For the most common type of
ceramic capacitors (Class 2), the first
letter and following number refer to the
minimum and maximum temperature
range, while the third letter gives the
tolerance of the capacitance over this
range – see Table 1.
Better-performing capacitors are
the ones with a smaller capacitance
change over a broader range, like those
with an X7R dielectric.
Electrolytic capacitors
In an electrolytic capacitor, one
plate is a metal foil while the other
‘plate’ is a conductive liquid or gel
solution, known as the electrolyte.
The metal foil is etched to increase
its surface area greatly, and the liquid
or gel is in intimate contact with this
foil, separated only by a very thin
oxide layer. Therefore, electrolytic
capacitors have very high capaciPractical Electronics | December | 2024
All About Capacitors
tance values for their volume (see
Figs.4 & 5).
For this reason, “electros” are typically used for ‘bulk’ bypassing or
filtering applications. Their asymmetry, and the fact that the oxide
layer is formed by electrons flowing
from one ‘plate’ to the other, means
that they are generally polarised. So
one of their leads must always be
at a higher voltage than the other
(although there are ways around this
– described later).
Electrolytic capacitors typically
range from a bit under 1µF up to
100,000µF or more, with voltage ratings from a few volts to several hundred volts.
Traditionally, the metal foil used
was aluminium. However, other metals
can be used, giving certain performance advantages. For example, tantalum, while more expensive, generally results in a capacitor which can
handle more current and with a lower
ESR (see Figs.6 & 7).
Further refinements to the electrolytic capacitor came with the discovery that an organic polymer gel could
be used as the electrolyte, giving a
roughly ten-fold decrease in overall
ESR (Figs.8 & 9).
As such, organic polymer (‘solid’)
aluminium electrolytic capacitors
outperform standard tantalum capacitors, and solid tantalum capacitors perform even better again, approaching that of some ceramics but
with a better capacitance-to-volume
ratio (see Fig.10).
Other, more exotic types of electros
are hybrid polymer electros (which
combine liquid and polymer electrolytics) and niobium polymer electros;
niobium is cheaper than tantalum but
performs similarly.
Tantalum and solid electrolytics
tend to occupy the space between
traditional electros, which are still
Fig.6: while tantalum electrolytics work on the same
principle as aluminium types, the construction
method is necessarily quite different due to the
properties of tantalum. Tantalum particles are
sintered to form a porous slug, which is then
immersed in a manganese dioxide electrolyte.
Graphite and silver in contact with the electrolyte
form the cathode connection, while tantalum wire
acts as the anode. (original source: Wikipedia)
Fig.7: this gives you
an idea of how the
tantalum particles are
sintered and attached
to a tantalum wire
lead to form the body
of the capacitor.
(original source:
Wikipedia)
Fig.8: polymer or ‘solid’ aluminium
electrolytic capacitors use an organic
polymer (plastic-like) electrolyte
which has roughly ten times the
conductivity of a wet electrolyte.
This allows for more compact
electrolytic capacitors with much
higher ripple current ratings and
lower ESR values. Other techniques
like comprehensive lead stitching
are often combined with the polymer
electrolyte to maximise performance.
(original source: Wikipedia)
A set of electrolytic capacitors ranging
from 10µ
10µF to 68mF (68,000µ
(68,000µF). Note
how the can-type electros have a
stripe to indicate the negative lead,
while the rectangular SMD types have
a stripe showing the positive lead.
Figs.9(a) & (b): an alternative construction for polymer electrolytic capacitors
which uses the same cathode construction as a sintered tantalum capacitor.
This halves the number of dielectric layers, significantly increasing capacitance
at the cost of a more complicated manufacturing process and more expensive
inputs. Polymer tantalum capacitors are made much the same as regular
tantalums, just with a different type of electrolyte. (original source: Wikipedia)
Practical Electronics | December | 2024
39
Feature article
Fig.10: an impedance
vs frequency graph
comparing four different
types of electrolytic
capacitor and a multilayer ceramic capacitor
(MLCC) with the same
self-resonant frequency.
The tantalum-polymer
capacitor comes closest
to the MLCC in terms
of performance at the
resonant frequency, while
being superior at lower
frequencies, likely due
to a higher capacitance
value. (original source:
Wikipedia)
Fig.11: traditional
electrolytic capacitors are
wound with four layers:
two metal foils and two
paper separators which
are soaked in electrolyte.
Note the multiple tabs
which ensure that current
doesn’t have to flow too
far to reach any point
on the foils. Anode and
cathode tabs are lined up
together so they can be
welded to the appropriate
leads. (original source:
TDK)
Fig.12: for highperformance (eg, lowESR) capacitors, the lead
tabs are stitched into the
aluminium foils, with the
metal of the lead tab and
foil being joined at multiple
points throughout the foil
to provide a low-resistance,
low-inductance path for
current to flow.
widely used for bulk filtering, and
ceramics, which are used more for
local or high-performance supply
bypassing.
For example, tantalum or polymer
electros might be used in switch-mode
power supply circuitry, where very
high pulse current handling and good
filtering (low ESR) is critical.
Non-polarised (NP) or bipolar (BP)
electrolytic capacitors are effectively two polarised electrolytic capacitors connected back-to-back. You can
create an NP electro from two polarised electros by joining either the
negative or positive leads together,
although the internal construction of
an NP/BP may be somewhat different
in practice. But the result is the same.
This works because when one of
the two capacitors is reverse-biased,
it acts a bit like a diode, and the other
capacitor does all the work. When
the voltage reverses, the capacitors
swap roles.
Strangely, you can often get better
performance by connecting two polarised electros back-to-back, at a lower
cost than a dedicated NP capacitor!
This is probably due to economies
of scale; polarised electros are made
by the squillions while NP capacitors
are used in fairly specialised roles.
Another thing to note about electros regarding polarity is that it is
safe to apply a reverse polarity voltage long-term as long as it is low (ie,
below the threshold where it starts to
conduct). This means that polarised
electros are quite suitable for use as
AC-coupling capacitors even if the
polarity of the voltage across them
is not known, as long as that voltage
never exceeds about ±1.5V DC.
Electrolytic construction
Fig.13: SMD electrolytic capacitors come in different forms, but the can style
uses very similar construction to a through-hole radial capacitor. The main
differences are that the can sits on a plastic platform with the leads bent
horizontally under it, so that the capacitor sits on the PCB and the leads rest on
their respective pads. (original source: Panasonic)
Traditionally, electrolytic capacitors are ‘wound’. Two long strips of
aluminium foil are chemically etched
to increase their surface area, then a
strip of paper (or some other porous
insulator) soaked in electrolyte is sandwiched between them.
Each conductive strip has one or
more tags, for ultimately attaching
leads, projecting from one side (see
Figs.11-13).
This sandwich is then wound into
a roll, with a second paper layer to
separate the anode and cathode. With
the leads in place, the roll assembly is
inserted into a can. More electrolyte
is added, and a rubber bung to seal it.
40
Practical Electronics | December | 2024
All About Capacitors
Fig.14: SMD polymer electrolytic capacitors are available in various packages
including cans like regular electros. The main difference is the use of a
separator sheet impregnated with a conductive polymer instead of paper soaked
in an electrolyte solution. (original source: Panasonic)
Current is passed through the capacitor to form the required insulating
layer, up to a voltage somewhat above
the desired rating (which determines
the oxide layer thickness).
The process is slightly different for
tantalum and polymer capacitors; SMD
tantalum and polymer capacitors in rectangular prism packages may be made
in layers rather than wound. But the
result is much the same: a metal conductor with a large surface area separated from a conductive electrolyte
only by a very thin oxide layer (see
Figs.14-16).
If the leads were only connected to
the conductive foils at one point each,
the ESR and ESL of the capacitor would
be poor, as current must flow along a
spiral path to reach the inner and outer
layers of the capacitor.
For this reason, all but the most
basic electros usually have extra conductive paths giving current a ‘short
cut’ to move between the layers of the
capacitor.
Higher performance electros also
have the tabs ‘stitched’ into the foil at
multiple points to reduce resistance
and improve conductivity between
them, in addition to having numerous
tabs spread throughout the roll, that all
join to one of the two leads.
Plastic film capacitors
Fig.15: SMD tantalum electros are made similarly to through-hole types, but the
sintered tantalum grains are formed in a rectangular prism shape to create a
more compact and convenient package. (original source: Wikipedia)
Fig.16: the same
type of semirectangular SMD
package can also
be used to house
polymer aluminium
electrolytic
capacitors. (original
source: Wikipedia)
Some through-hole tantalum capacitors ranging from 3.3µ
3.3µF to 47µ
47µF. For these
capacitors, polarity is indicated by a plus sign (+)
on one side of the body.
Practical Electronics | December | 2024
Plastic film capacitors are not used
as much commercially these days since
ceramic capacitors are much cheaper
and are available with very low ESR
and ESL figures. However, in cases
where linearity or safety are essential, plastic films are still widely used.
Most plastic film capacitors have
better linearity than all but the best
(NP0/C0G) ceramics or mica capacitors, and they can be designed to fail
gracefully (going open-circuit).
Plastic film capacitors are available
from a few dozen picofarads up to a
few tens of microfarads, and have voltage ratings ranging from around 16V
up to several thousand volts.
The failure mode is vital in mains
applications, where capacitors are connected between Active and Neutral or
Active and Earth. If they were to fail
short circuit, a fire could result, or it
could be a shock hazard. While ceramic X/Y-class capacitors exist, generally, higher-value mains-rated (X/Y)
capacitors use either polyester (PET),
polypropylene (PP) or polycarbonate
(PC) films.
41
Feature article
Fig.17: plastic capacitor construction is similar
to ceramic, with alternating layers of plastic (the
dielectric) and conductive metal film or foil in between,
staggered to create many capacitors in parallel
(original source: Wikipedia).
Like ceramic capacitors, the plastic dielectric used has a significant
effect on capacitor properties. And
similarly, the plastics with the lowest
dielectric constants that result in the
bulkiest capacitors (like polypropylene and polystyrene [PS]), tend to have
the best performance figures, such as
Fig.18: plastic capacitors can be made from stacks of sheets,
similarly to ceramics, or from rolled-up strips, similarly to
electrolytics. The stacking process tends to be more timeconsuming and expensive, but it can give better density, so it is
used for some SMD plastic capacitors.
good linearity factors and low dissipation factors.
Other plastics used for capacitors
include polyphenylene sulfide (PPS),
polyethylene naphthalate (PEN) and
polytetrafluoroethylene (PTFE).
One interesting property of metallised plastic film capacitors is ‘self-
healing’. This is where a physical defect
or the application of excessive voltage
might damage the capacitor, but it will
not fail entirely; instead, a small area
of the metallisation burns away, reducing the capacitance by (hopefully)
a tiny amount – usually not enough to
affect its function.
Fig.19: the roll manufacturing process for metallised plastic capacitors. While metal foil can be used instead of
metallisation, it tends to result in a bulkier capacitor with inferior properties. (Source: Wikimedia user Elcap)
42
Practical Electronics | December | 2024
All About Capacitors
A set of film capacitors from
50pF to 1µF.
This effect is taken advantage of to
provide the higher safety margin required for mains-rated capacitors.
Plastic capacitor construction
Like ceramic capacitors, plastic film
capacitors usually need many layers to
give a usable capacitance value. However, they are not normally formed by
deposition methods. Therefore, they
must be either stacked or wound (see
Figs.17-19).
Stacked capacitors are made in the
way you might guess: with alternating
layers of conducting foils and dielectric
films. The conducting foils are staggered
so that when they are joined along each
edge, they form interlocking ‘combs’
and thus effectively, many singlelayer capacitors in parallel. Making capacitors that way is time-consuming
and expensive, though.
Rather than using metal foils, the
dielectric can also be coated with a
film of conductive material which
provides a thinner and more uniform
layer, improving performance and allowing more layers to be packed into
the same space for higher density.
Wound plastic film capacitors are
made similarly to electrolytics as described above. A sandwich is created
with the dielectric film between two
strips of metal foil, with the strips
slightly offset. This assembly is wound
up into a roll, and in most cases, the
roll is squashed flat to better fit into a
rectangular prism shape.
A metallisation layer is sprayed onto
the ends of the roll to connect the layers,
and leads are attached. This is why the
conductors are offset; each layer is only
exposed at one end of the roll, or else
the sprayed metal layer will short out
the capacitor. After this, the capacitor
is typically impregnated with silicone
oil or another insulating fluid to prevent moisture ingress.
The terminals are then attached and
the capacitor is encapsulated, sometimes by being dipped, other times by
being sealed into a pre-formed plastic case.
To make plastic film capacitors with
a voltage rating above 630V DC, partial
metallisation can be used to effectively
form multiple capacitors in series using
the same basic techniques. This can
extend voltage ratings up past 3000V
DC (see Fig.20).
As well as small-signal capacitors
and those for mains filtering, plastic
dielectric capacitors are also used in
motor run applications. Many motor
start capacitors are electrolytic types,
but electros are not suitable for handling the continuous current and high
voltages that motor run capacitors are
subjected to.
So they are typically made with polypropylene or similar plastic dielectrics
and thick metal films to handle high
currents continuously.
Electrical double-layer
capacitors (EDLs)
Fig.20: the basic method of plastic film capacitor manufacturing doesn’t work
very well for applied voltages above about 630V. Higher voltage ratings are
possible, but the capacitor needs to be internally separated into several elements
connected in series, so that the dielectric material only has a fraction of the
applied voltage across it. (original source: Wikipedia)
Practical Electronics | December | 2024
These are often known as supercapacitors or ultracapacitors. They are
a variation on electrolytic capacitors,
with extremely high capacitances but
usually low voltage ratings, and often
very high internal resistances (and
thus low operating currents). Silicon
Chip published an article on ultracapacitors in April 2008 (siliconchip.au/
Article/1793).
EDLs use similar conductive polymers with a very high surface area for
both the positive and negative electrodes, with a common electrolyte in
contact with both. Anions and cations
in the electrolyte form insulating Helmholtz layers in direct contact with the
surfaces of both electrodes.
These layers are only one atom thick,
and as mentioned at the start of the
article, capacitance is proportional to
43
Feature article
Fig.21: as the name suggests, a double-layer (EDL) capacitor
effectively has two dielectric layers, one at the surface of the
anode and one at the cathode, with a conductive electrolyte
between the two. The advantage is that these layers are
super-thin, just one molecule wide, giving extremely high
capacitances in a small package. However, this thin dielectric
layer results in a very low voltage rating, typically either 2.7V
or 5.5V (source: Wikimedia user Elcap).
surface area and inversely proportional
to dielectric layer thickness. You can’t
get a much thinner layer than one atom
(see Fig.21).
Given the large surface area of the
electrodes, EDL capacitors can have
values exceeding one Farad in a package not much bigger than a can 19mm
in diameter and about 16mm tall.
The fact that the current must pass
through two polymer layers plus an
electrolyte, neither of which is especially conductive, is why the current
delivery of EDLs is generally limited.
The extremely thin dielectric layer is
the reason why voltage ratings of only
2.7V or 5.5V are common.
Both of these problems can be mitigated by connecting many EDL capacitors in parallel (to improve current handling) or series (to increase
the voltage rating, at the expense of
capacitance). Higher voltage EDLs
usually have multiple internal EDLs
in series.
You might be using an ultracapacitor
without realising it; Mazda introduced
its i-ELOOP system in vehicles from
2011, and it is now in many vehicles.
This system recovers kinetic energy
during braking to rapidly charge an
ultracapacitor, then uses that energy
to charge the vehicle battery over a
longer period.
Other types of capacitor
Silvered mica capacitors unsurprisingly use mica, a type of mineral, as
the dielectric. Mica was chosen both
for its good dielectric properties and
because its crystalline structure makes
it very easy to cleave into super-thin
sheets; just what you need to achieve
a decent capacitance. A thin layer of
silver is applied to each side, and voila,
you have a capacitor with excellent
linearity and low leakage.
An example of an
806pF 300V mica
capacitor. The 1%
rating means its
actual value will
be in the range of
~798-814pF.
Mica capacitors have mostly been
replaced by ceramic or plastic film
types, as both are significantly cheaper to manufacture and achieve similar
performance.
Some still value mica caps for audio
circuits. Besides good linearity, another property of mica capacitors is that
they usually have tight tolerances due
to their predictable thickness, measurable surface area and low temperature
coefficient.
Another type of non-polarised capacitor that was widely used but is
now far less common (although still
available) is the paper capacitor (sometimes known as an MP [metallised
paper] capacitor).
These have also mostly been supplanted by ceramic or plastic film capacitors. The main disadvantage of
paper capacitors is that they can absorb
moisture from the air and fail; older
types have also been known to catch
fire! Modern capacitors usually combine paper and plastic (usually PET
or polypropylene) to overcome these
disadvantages. Their main advantage
is low cost.
One benefit that paper capacitors
retain is that they usually have zinc
metallisation compared to the aluminium metallisation of plastic capacitors.
This provides better ‘self-healing’ capabilities due to its lower-energy evaporation process.
Variable capacitors
Variable capacitors work either by
varying the amount of overlap between
two sets of metal plates, or by chang-
Fig.22: the simplest type of variable capacitor, used in many vintage radios, is just two sets of interleaved metal plates
where the amount of overlap can be adjusted. Air is the dielectric. Miniature trimmer caps tend to use a plastic or mica
dielectric and bring the two plates closer together or further away to vary the capacitance.
44
Practical Electronics | December | 2024
All About Capacitors
Some varcaps (variable capacitors)
from various older radios.
ing the spacing between two plates
(possibly separated by a plastic or
mica dielectric). In many cases, the
dielectric is air.
As such, the range of capacitance for
a typical tuning capacitor is generally
from a few picofarads up to a few hundred picofarads (see Fig.22).
They are typically used as part
of an RF oscillator or filter circuit,
so low picofarad values give the required time constants with reasonable values for other components
(usually inductors).
Oddball types
The capacitors described above
probably cover 99% of the capacitors you might come across, but other
types exist. That includes those with
a glass or silicon dielectric, or even
a vacuum!
Capacitor parameters
In addition to its construction/dielectric (ceramic, aluminium electrolytic, tantalum electrolytic, plastic film,
plastic foil etc), a capacitor is described
by its capacitance, tolerance, voltage
rating(s), ripple current rating(s), leakage current rating(s), operating temperature and expected lifespan.
Furthermore, each capacitor type
has several associated performance
metrics, which may be fixed or vary
with parameters like temperature, applied voltage, signal frequency, age
etc. These include the ESR (equivalent series resistance), ESL (equivalent series inductance), dissipation
factor (DF or delta [Δ]), temperature
coefficient (tempco), voltage coefficient, linearity and more.
We’ll describe all of these, starting
with the parameters which typically
form part of a model or part code.
Practical Electronics | December | 2024
Capacitance: the nominal capacity of the device, measured with no
or little charge (typically around 0.51.0V across the capacitor) and at room
temperature.
For very low-value capacitors (fraction of a picofarad to a few picofarads), the measured capacitance can
be affected by the connected PCB
tracks/pad or wires, or even the device’s lead length.
Tolerance: how close you can expect
the capacitance to be compared to the
nominal value. If you have a 10µF
±10% capacitor, if its value is less
than 9µF or more than 11µF, then it
would be considered faulty.
However, during actual use, its
capacitance could vary outside this
range, as explained below.
Voltage rating(s): the applied DC
voltage across the capacitor terminals can safely vary from 0V up to
this figure. For non-polarised types
like ceramic or plastic film, it can
also be negative, meaning the full
range of operating voltages is effectively doubled (ie, -50V to +50V for
a 50V capacitor).
Some capacitors have a higher
‘surge’ voltage rating which will not
damage them if applied for a limited
period, but that is less common these
days. Note that you sometimes need
to keep the voltage below the rating
for good performance; more on that
shortly.
Ripple current rating(s): all capacitors have some intrinsic resistance
and therefore heat up as current passes
through them; current flows through
a capacitor during both charging and
discharging.
For example, if a capacitor is used
to filter the output from a bridge rectifier turning AC to DC, it supplies
the full load current most of the time
(when the bridge is not conducting).
But it also must absorb large pulses of
current to recharge when the bridge
comes into conduction, 50 or 100
times per second.
Such circuits must be designed to
avoid exceeding the RMS ripple current rating of the filter capacitor(s) or
else they can rapidly overheat and fail.
There are generally different figures given at low (50/100Hz) and
high (100kHz) frequencies, due to the
changing impedance of the capacitor,
from both its capacitance and its ESL
(see below). The ripple current rating
is usually higher at higher frequencies.
Leakage current rating(s): some capacitors (eg, electros) can have a fairly
significant leakage current through
the capacitor even when the voltage
is steady. This is usually proportional
to the applied voltage.
Ceramic, plastic film and mica capacitors also have leakage currents,
although they are generally very low
and are often (but not always) of no
concern. This is important in some
applications, like sample-and-hold
buffers, where the voltage across a
capacitor must remain stable for relatively long periods.
Operating temperature: critically
for electrolytic capacitors, this is the
maximum temperature at which the
capacitor is guaranteed to meet the
stated performance figures. It is also
the temperature at which the expected lifespan (if given) is calculated.
Capacitor lifespan roughly doubles
for each 10°C below the rated temperature, and halves for each 10°C above
it. Typical ratings are 85°C, 105°C and
125°C. We recommend using 105°Crated capacitors with an expected
lifespan of at least a few thousand
hours to avoid early failures.
Lifespan: usually stated in thousands of hours MTBF (mean time between failures), with 1000 hours at the
lower end and about 10,000 hours at
the upper end. If you can find a capacitor rated to last for 10,000 hours
at 125°C, it’ll probably outlast the rest
of the circuit!
Performance metrics
Equivalent series resistance (ESR):
this is a crucial metric for most capacitors as it has a strong effect on how
well the capacitor can smooth DC voltages, and how much heat is generated
at higher currents.
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Feature article
Fig.24: the variation with temperature of the dielectric constant, K, for several
ceramic materials. You can see that X5R and X7R have a much higher K than
C0G/NP0, making for higher capacitances in a smaller volume, with only a
slight variation over the temperature range. The vast variation for Y5V and
Z5U makes them unattractive. While they give a high capacitance at room
temperature, at very high or low temperatures, the K value drops below that of
both X5R and X7R. (original source: Digi-Key)
matically reduce the ESL of larger
capacitors.
The combination of capacitive reactance, ESR and ESL produces a
characteristic valley-like impedance
curve for most capacitors, as shown
previously in Fig.10.
Dissipation factor (DF): also known
as tan(δ), is the reciprocal of the ratio
of the ESR and capacitive reactance,
and as such, is typically a number
close to but slightly less than one.
It can be easier to tell how close to
ideal a given capacitor is by looking
at the DF rather than ESR, and it can
also make it easier to compare the
performance of capacitors with different values of capacitance.
Temperature coefficient (tempco):
how much the capacitance changes
with temperature. In an application
where the capacitance is used to define
a frequency (eg, in an oscillator or
filter), this must be minimised to prevent frequency drift with temperature.
Hence, NP0/C0G ceramics or plastic
film capacitors are generally preferred
in those roles (see Figs.23 & 24).
This is very important to keep in
mind with high-K dielectrics like
Y5V ceramics; they can lose 80% or
more of their capacitance at elevated temperatures! That’s made even
worse by…
Voltage coefficient: how much the
capacitance changes as the capacitor
is charged.
This causes two main problems.
One is a loss of effective capacitance;
combined with the poor tempco of
Y5V ceramics, a 10µF 6.3V Y5V capacitor at 80°C, charged to 5V, might
have less effective capacitance than
a 1µF 50V X5R capacitor under the
same conditions! (See Figs.25 & 26).
This is why we steer well clear of
cheaper Y5V ceramics.
The other problem with a high voltage coefficient is that it dramatically
impacts linearity.
So, perhaps unintuitively, standard
aluminium electrolytic capacitors are
better for audio coupling than most
high-quality multi-layer ceramic capacitors. NP0/C0G ceramics are the
exception, but they are huge and horrendously expensive at the sort of
values required for most audio coupling applications.
Linearity: this is not something
you will find on most data sheets,
and there is no standard way of representing it. But it is a real effect that
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Practical Electronics | December | 2024
You can think of a real capacitor
like an ideal capacitor with a resistor
in series; the lower the value of that
resistor, the less it is ‘isolated’ from
the circuit it is connected to.
Typical ESR values are a few ohms
for a low-value electrolytic capacitor,
down to a few milliohms for a large,
low-ESR electrolytic, tantalum, polymer or ceramic capacitor.
Equivalent series inductance (ESL):
just like ESR, you can imagine that all
capacitors have a low-value inductor
internally connected in series with
the capacitance.
This has little effect at low frequencies, but can make the effective impedance of the capacitor so high that
it is useless at higher frequencies.
ESL is critical for applications like
bypassing multi-GHz ICs such as CPUs
and RF devices. Smaller capacitors
generally have a lower ESL, and certain construction methods can dra-
Fig.23: by definition, the temperature coefficient of an NP0 ceramic capacitor
is zero (or very close to it). On the other hand, Y5V ceramic capacitors vary in
value wildly with temperature. Z5U is a little better at lower temperatures, but
still poor at high temperatures. X5R and X7R are the ‘go to’ ceramic dielectrics
because they are cheaper and more compact than NP0 capacitors, but have a
much more modest temperate coefficient than Y5V or Z5U. (source: Wikipedia
& Johanson Dielectrics)
All About Capacitors
varies significantly between different
capacitor types.
The easiest way to measure it is
to form a simple RC filter (low-pass
or high-pass) with a relatively lowvalue, linear (thin film) resistor and a
capacitor. You then feed a very pure
sinewave into the filter, at a frequency
near the -3dB point, and measure the
distortion figure of the voltage across
the capacitor.
The resulting % THD is inversely
proportional to the capacitor’s linearity. A very linear capacitor like a polypropylene or NP0/C0G ceramic capacitor will introduce an unmeasurable
level of distortion (below 0.0001%).
Other plastic film types like polyester are slightly worse, resulting in a
measurable but not worrying level of
distortion (say 0.0005%).
Other capacitors like high-K ceramics, electrolytics and so on could
give distortion measurements of 1%
or more, reflecting the fact that their
I/V curves are not straight.
In some cases (eg, electros), the
curves can even have hysteresis, meaning they are a different shape for charging and discharging.
This is most important in audio
circuits, although other circuits (eg,
RF) might be sensitive to linearity too.
Note that electros are fine for audio
coupling, even though they are not terribly linear, as the applied AC voltage
in that role is so small that it isn’t a
significant effect (unlike the voltage
coefficient of many ceramics, which
makes them unsuitable for that role,
despite probably being more linear
than electros).
Ageing: capacitors can change value
over time, usually decreasing due to
degradation of the dielectric. Typically, those with a tighter initial tolerance will tend to maintain their capacitance better over time (see Fig.27).
This is apart from an actual failure
of the component, which might manifest as a much lower capacitance,
higher ESR, higher leakage (especially
at voltages approaching the rating) or
some combination of the three.
Fig.25: the measured value of a range of 4.7µF X5R and X7R capacitors with the
application of a range of DC voltages. Note how the physically larger capacitors
tend to retain their capacitance better as they are charged to a similar voltage.
(original source: Maxim)
Fig.26: the change in capacitance over voltage for several different ceramic
dielectrics. While the 100V and 400V capacitors seem to perform poorly,
consider that the X-axis is a percentage of the voltage rating. Due to this
effect and the temperature coefficient, Y5V or Z5U capacitors can easily fall
below 10% of their rated values, and below even 5% at temperature extremes!
(Original source: Wikipedia)
Further reading
• Types:
https://w.wiki/q86
• Ceramic:
https://w.wiki/q87
• Electrolytic: https://w.wiki/q88
• Tantalum: https://w.wiki/q89
• Polymer:
https://w.wiki/q8A
• Film:
https://w.wiki/q8B
• Supercaps: https://w.wiki/q8C PE
Fig.27: if you need another reason to avoid Y5V ceramics, here is a comparison
of the loss in capacitance due to aging with the more robust X7R types.
According to the originators of this graph, Johanson Dielectrics, the value of
Y5V capacitors drops at roughly three times the rate of a comparable X7R
capacitor (original source: Johanson Dielectrics).
Practical Electronics | December | 2024
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