Capacitor Primer: delete if you don't want

[harrybissell at prodigy.net]

Here's a repost of the cap primer for those who asked for it.

Sorry if you have seen it already.

:^) Harry


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Capacitors: A Field Guide to Types and Habitats				Harry Bissell

	A capacitor stores a direct current electrical charge. It will
pass an AC waveform and block a DC level. It consists in simple
form of two parallel conductive plates separated by an
insulating material called a Dielectric. The IDEAL capacitor
would have infinite ability to block DC (infinite resistance),
would have zero series resistance, infinite voltage handling
ability, and zero series inductance, and zero temperature
coefficient.



Dielectric Strength: The ability of the dielectric to withstand
a certain voltage without an arc or short circuit occurring. The
higher the dielectric strength, the greater the voltage the cap
can be used for.



Dielectric Absorption: An effect where some of the charge stored
in a capacitor does not immediately return after discharge, but
slowly leaks back at a later time. Some dielectric materials
have a kind of sponge effect. Even after you wring the water out
of a sponge, it stays wet for some time after. Dielectric
Absorption is not a desirable characteristic.



Equivalent Series Resistance (ESR) All practical capacitors have
some resistance in series with them because of the lead wires,
materials, etc. Low ESR is a desirable characteristic.

Self (series) Inductance: Practical capacitors have some
inductance due to the construction techniques used. The minimum
inductance would be the same as a single conductor (wire) the
same length as the capacitor. Often this value is much higher.
With an inductor in series with the cap, there is a series
resonant circuit formed. The capacitor will resonate or "ring"
at this frequency. Series inductance is not a desirable
characteristic. The lowest series inductance caps are usually
surface mount ceramic and tantalum types.



Temperature Coefficient: Practical capacitors vary somewhat with
changing temperature. Some vary only a slight amount, some a
great deal. The temperature coefficient (change in capacitance
vs change in temperature) is not always linear.

 

Polarity: Capacitors that may be safely operated with only one
DC polarity are called "polar" Capacitors which can be operated
without regard to polarity are "non-polar". Sometimes "polar"
caps are combined in special ways to allow a limited ability to
cope with revering polarity are called "bipolar"



Ripple Current: The amount of AC current a capacitor can
withstand and how often it can withstand it. Not all capacitors
are rated for ripple current. This spec is mostly applied to
capacitors that deal with AC currents, or DC caps with large AC
components (often found in switch mode power supplies, and
industrial equipment such as motor drives etc.).



Dielectric Materials: Most capacitors are named after the
material used for their dielectric.



Air:  The original dielectric material. Old time radio receivers
used variable capacitors made of stacks of parallel plates, with
air between them. Air is a good dielectric, which varies only
slightly with humidity and airborne particles. Air dielectric
caps are impractical for values beyond a few hundred picofarads
because of their increasing size. Air caps are nonpolar.

Ceramic Monolithic: Ceramic materials make a good dielectric.
They can be used for values from low picofarads up to usually
around one microfarad. The small values are usually
"monolithic", they are made of a single dielectric (usually disc
shaped) with metal plated on both sides of the disk, and leads
attached. They are also rated by temperature coefficient. 



COG or NPO (negative-positive zero) capacitors are the most
stable with temperature, but usually are available only in the
picofarad range. They are larger for a given capacitance value.
There are special temperature coefficient caps for compensation
of circuits. These are rare.



X7R capacitors have a greater temperature coefficient, but are
available in larger values. They are smaller in size than the
NPO. The tempco is not linear and hard to adjust for, but the
values are usually plus/minus a few percent from 0-100 degrees
Celsius.



Z5U capacitors give the largest values in the smallest size
package. The tempco is terrible, often falling to -50% of the
value at -20 degrees and +100 degrees Celsius (relative to 25C).
Use only for non-critical applications like power supply
bypassing.



Ceramic Stacked: There are capacitors that are manufactured by
paralleling several monolithic caps in a single package. They
have larger values than a disk of similar ratings. The
performance is still related to the dielectric material, they
could be good or poor in tempco and voltage rating.



Ceramic (general): For small values and high voltages ceramic
caps are hard to beat. They are usually quite low in self
(series) inductance, and ESR is not usually an issue. Be aware
of their temperature coefficients. Some ceramic materials
exhibit some piezoelectric effects, they can be electrically
sensitive to vibration and shock. Most ceramics have low
dielectric absorption. Ceramic caps are as a rule "nonpolar"



Mica:

A very good, very stable dielectric made of the mineral Mica,
with plated or deposited plates on either side (often silver,
hence the name "silvered mica") These are a better performer
than the ceramic caps in the NPO series. Usually mica is not
practical in values larger than picofarads. Voltage ratings
range into the kilovolts. Mica caps are nonpolar.



Tantalum: A dielectric capable of giving very high capacitance
in a very small space. Tantalum caps are made of two
constructions, Wet and Dry. Wet tantalum caps have a liquid
electrolyte which causes the "dielectric" to be formed. They
have been almost completely replaced by "Dry" tantalums. Dry
tantalums use a spherical powered tantalum material with a
dielectric coating on the outside. They are "sintered" together
to form an almost solid material with very high capacitance
value per unit volume. They have reasonably low self inductance
and very low ESR.



Warning: Tantalum caps are RABIDLY polar. Reversing the DC
polarity even for a brief period of time will cause them to heat
and self-destruct. They are most common in low voltages and
values below a few microfarads. High voltages and large
(>5-10uF) values get very expensive. Their low ESR and
self-inductance makes them perfectly suited to power supply
bypass duties. Leakage current is usually very low, much better
than electrolytics. They should be avoided in audio coupling
(bipolar) circuits.



Electrolytic: 

 Electrolytic caps are named for the chemicals that cause the
dielectric to exist. Electrolytic caps have plates wound from a
long, thin strip of aluminum foil. The dielectric is a thin
(several atoms thick) coating of aluminum oxide (an excellent
insulator). The aluminum oxide is formed by a chemical reaction
between the electrolyte and the aluminum, in the presence of an
electric field. This formed dielectric gives the capacitors some
unique advantages and disadvantages. Electrolytic caps have very
large capacitances per unit space, since the dielectric is so
thin. The dielectric can tailored to allow voltages up to about
450 VDC, the upper limit for electrolytic caps.    The
disadvantages of the electrolytic come from the electrolyte, and
how the dielectric is formed. The electrolyte will dry up in
time, causing the capacitors to gradually decrease in
capacitance. Pushing the capacitor beyond its ratings (either
voltage, polarity, or ripple current) will increase the pressure
in the cap until it either vents (and loses electrolyte) or
explodes. The other problem is that if the electrolytic cap is
not used for a long time, the dielectric becomes thinner,
decreasing the voltage it can withstand. The dielectric needs an
electric field (charge) in order to maintain its strength.
Electrolytics that have been unused (either in storage or in
unused equipment) can have their dielectric layers restored by
slowly applying increasing levels of DC voltage. The procedure
can take days.     Electrolytics suffer from accelerated aging
at elevated temperatures. A rule of thumb is that their life is
cut in half for each 10 degree Celsius rise above ambient (25C).
For all these reasons, electrolytics have a limited life and the
user may expect to replace them at some time in the future.
Excess hum on a power supply, unstable rolling picture on a
monitor are often signs of an electrolytic nearing the end of
its useful life.    Electrolytic capacitors have a substantial
amount of leakage and Dielectric Absorption. This can be a
problem in timing circuits, and often limits their use. Some
designs (like the 555 timer) minimize these faults by operating
the capacitor at voltages where this is less of a problem. The
temperature stability of electrolytics is poor and seldom
specified 



Polar Electrolytics: The most common variety of electrolytic
capacitor. These are available in sizes from about 1uF up to
fractions of a Farad. Voltage ratings exist from about 5 volts
to 450 VDC. There are special varieties that have extended
performance ratings, usually specified for higher temperatures,
higher ripple currents, lower ESR, and enhanced reliability. All
these ratings usually result in a physically larger component.
There are special "sealed" electrolytics for timing
applications, but they are rare.

  

Bi-polar Electrolytics: These are a special variety of
electrolytic capacitor that are used in applications that have
mixed polarity, such as audio coupling applications. Physically,
they are made of two polar electrolytics connected in series at
their negative terminals. They are usually twice as large as a
polar electrolytic for the same voltage and capacitance value.
Common uses are in "economy" grade speaker crossovers.



Supercaps (battery): These are electrolytic capacitors that are
specialized for long discharge times with very low current
loads. They are used for memory backup applications in place of
Ni-Cad or Lithium batteries.

The major use for electrolytic capacitors is for power supply
filtering and bypassing. Minor uses are for audio coupling in
unipolar operation (where one terminal is always more positive
than another), non-critical timing applications.



Film Capacitors:  Film capacitors are the most often
misunderstood members of the capacitor family. They are named
after the material used as the dielectric. They come in two
physical varieties, Film and Foil construction, and Metallized
film construction. They are available in values from around
.0005uF to several microfarads, and voltages from around 10VDC
to several thousand VDC. For special applications sizes up to
several thousand microfarads are available, but they are very
large and very expensive. Most film caps have good to very good
temperature stability, most are low in dielectric absorption. As
a rule they are Non-Polar capacitors and have good AC response.
Self inductance ranges from low to high, depending on the
geometry of the construction.     In their simplest form, they
are constructed from two pieces of foil separated by a "film" of
the dielectric material. This is called Film and Foil
construction, and is larger and more rugged in most cases. Some
film caps have a metallization layer, usually aluminum,
deposited on both sides of the film. This is called Metallized
Film, and is usually smaller and more expensive. Both types have
similar performance.



Polyester (Mylar) Film: The most common dielectric material in
use. It is low in cost and can be found in Film and Foil are
Metallized varieties. Common Voltage ranges go from about 50VDC
to 200VDC (up to 1000VDC) Mylar suffers from Dielectric
Absorption  (.20%), which makes it unsuitable for applications
such as VCO timing capacitors, and Sample and Hold applications.
Polyester caps do not have a linear temperature coefficient,
they get progressively higher in capacitance in capacitance at
elevated temperatures, and progressively lower at lower
temperatures. The overall shape of the curve is similar to a
horizontal letter "S". Between 25 - 85 degrees Celsius they have
a rising temperature coefficient. They are useful for audio
coupling, semi-critical timing circuits, tone controls, general
uses. Temperature range is to 125 degrees Celsius.



Polystyrene Film: The Holy Grail of Film Capacitors, polystyrene
has the most desirable electrical characteristics. With
temperature coefficients as low as 30-40ppm (special) and
typically less than 120ppm (standard), they have excellent
linearity of temperature coefficient over the entire temperature
range most equipment ever sees on this planet. Dielectric
Absorption (.02%) is the lowest found in any capacitor variety.
This makes the polystyrene cap the first choice for all critical
timing circuits, such as VCO and VCF timings, and for all Sample
/ Hold circuits. Polystyrene caps cannot tolerate high
temperatures (85 degrees Celsius max.), so they are not
available in the metallized variety. Careless soldering can
destroy them easily, and they are often poorly suited to
automated production equipment.     There is a persistent rumor
that the only manufacturer in the world that made capacitor
grade polystyrene film has ceased production. There is still
stock in most values on the market, but people are well advised
to save a few for top of the line circuits. There are some new
polycarbonate caps which will approach the performance of
polystyrene.



Polypropylene Film: Polypropylene capacitors are often used in
place of polystyrene.  Slightly larger than polyester (Mylar),
they are superior electrically and slightly larger in size. They
have a linear, negative temperature coefficient of -150ppm (and
special -250ppm for compensation of coils in filter
applications). If the negative temperature coefficient is
desirable, they are used in place of polystyrene. Dielectric
Absorption is .02%, and the temperature range is up to 105
degrees Celsius max.



Polycarbonate Film: Polycarbonate capacitors are another good
choice for polystyrene replacement. Tempco and Temperature
Stability are not as good as polystyrene. They are linear over a
limited temperature range (25-85 degrees Celsius). Beyond this
range they are not linear (similar to polyester capacitors).
Polycarbonate caps withstand much higher temperatures (125
degrees Celsius max.) than polystyrene, so then can be made in
Metallized Film construction. Voltage ranges are available to
several thousand volts. These caps are best suited for critical
coupling and timing applications, and can be used wherever
Polystyrene is the preferred choice, with a slight decrease in
performance. Polycarbonate caps have a moderate amount of
moisture sensitivity, and Dielectric Absorption is around .08%.
Polycarbonate caps are about 12% larger than polyester (Mylar).



Polysulfone Film: The same specs as polycarbonate, but with even
higher temperature ratings. These are used where high
temperature performance is mandated (150 degrees Celsius max.)
and are rarely used for general purposes. They have a reasonably
linear temperature coefficient from 25 degrees Celsius and
above, but fall slightly at lower temperatures. Polysulfone caps
have a slight amount of moisture sensitivity, and Dielectric
Absorption is around .08%. 



Teflon Film: Teflon performs as well as polystyrene in every
regard, and is good for high temperatures as well. They are
twice the size of a polyester (Mylar) capacitor, and high price
makes their use uncommon for all but the most mission-critical
applications (read aerospace). The difficulty in making anything
adhere to the film makes metallization impractical. Teflon is
impervious to moisture and has dielectric absorption of around
.02%



Applications Notes:

Capacitor "Self Inductance"

    The inductance of a practical capacitor limits the frequency
range that it can be used for. At some frequency, the inductance
of the capacitor will form a series resonant circuit. At this
frequency the capacitor is useless as a filter. The series
inductance of the capacitor also raises the impedance, limiting
its ability to pass AC current.     For this reason, DC power
supplies often use a parallel combination of capacitors. For low
frequency filtering (60-120Hz) a large value of capacitance is
necessary. A polar electrolytic of several hundred to several
thousand microfarads is the common choice. The large series
inductance of this capacitor makes it effective only up to
several hundred kilohertz. Above this, the high impedance makes
it appear as "not there" to higher frequency components. To
extend the frequency range, a small tantalum cap (several uF) is
often added. This has much lower series inductance, so it is
effective up to the low megahertz. The value can be small,
because the lower frequencies are being filtered effectively by
the large electrolytic. At even higher frequencies, a small
ceramic (.01 to .1uF) is added in series. The same rules apply,
the cap can be even smaller because the frequency of interest is
even higher. Ceramic caps for decoupling use can be almost any
variety (X7R, Z5U etc.)     This will make a power supply filter
that performs equally well at eliminating AC line hum, radio
stations, microprocessor noise, etc. Remember, the filter not
only stops noise from riding into your circuit, it stops your
noise from reaching other circuits (yours, your neighbors' etc).

    The length of traces and wire that separate individual
components on a printed circuit board or a system also have
"series inductance". This means that if there is a sudden, local
demand for current, the voltage at that point in the circuit
will drop until the current flow makes it through the current
limiting inductance. For this reason, capacitors are placed very
near the power terminals of active components. This limits the
series inductance, so a small capacitor can act as a filter for
local noise, as well as a small current reservoir. These are
known as decoupling capacitors, because the make the local node
act independently of  (or decoupled from) the rest of the
circuit. The inductance of the wiring is usually small, so the
main electrolytic filter caps are still able to handle the low
frequencies. At the chip level, a small tantalum or electrolytic
(1-10uF) in series with a ceramic (.01-.1uF) is usually more
than enough. Lack of decoupling can cause circuits to perform
badly or sometimes, not to work at all. The opposite of
decoupling is "coupling", meaning that there are unintended
connections and interactions between circuit functions.

Coupling Capacitors: Capacitors are often used to block
different DC voltages in successive stages of audio equipment.
This function requires the capacitor to have low leakage, low to
moderate self inductance, and to be able to handle the AC and DC
levels involved. If the DC level of one stage is ALWAYS higher
than another, a polar capacitor (electrolytic) may be used. This
is often the case in guitar "stomp boxes" where the input and
output are ground referenced, and the circuit runs on a positive
supply.  If this condition can't be guaranteed, then a Bipolar
electrolytic may be used.    In all coupling cases, a film
capacitor is a superior choice if it is practical to use one.
Any film type can usually be used here. In high impedance
circuits, a fairly low value coupling capacitor will give good
low frequency response. If the impedance is low, however, the
electrolytic (polar or bipolar) may be the only choice.    
Tantalum capacitors are almost always a poor choice for coupling
applications. A reversal of polarity for any reason will destroy
the capacitor. Electrolytics can stand a little abuse in this
regard, but they should not be reverse biased for extended
periods of time.

Timing Capacitors: For all frequency sensitive applications,
film caps are the best choice. Polystyrene is the best, followed
by polycarbonate, and polyester (Mylar). Mylar capacitors should
always be avoided in Sample/Hold  and VCO applications, and used
with caution in VCF circuits. If the capacitance value is very
small then mica or COG/NPO ceramics may be used.

 

This paper is copyright 1999 by Harry R. Bissell Jr.		
harrybissell at prodigy.net 

It may be reproduced freely if my name stays on it.  

Don't want money... Got Money.... Want Recognition