© by Tony van Roon
What Exactly Is A Capacitor?
Like my other tutorials, lets start
with a bit of interesting history first to understand where the 'capacitor' or condensor came from and how it was
developed and then work our way up to our century.
In 1745 a new physics and mathematics professor at the University of Leyden
(spelled Leiden in modern Dutch), Pieter van Musschenbroek (1692 - 1791) and his assistants Allmand and
Cunaeus from the Netherlands invented the 'capacitor' (electro-statice charge or capacitance actually) but did not
know it at first. His condenser was called the 'Leyden Jar' (pronounced: LY'duhn) and named so by Abbe Nollet. This
Leyden jar consisted of a narrow-necked glass jar coated over part of its inner and outer surfaces with a conductive
metallic substance; a conducting rod or wire passes through as insulating stopper (cork) in the neck of the jar and
contacts the inner foil layer, which is separated from the outer layer by the glass wall. The Leyden jar was one of
the first devices used to store an electric charge. If the inner layers of foil and outer layers of foil are then
connected by a conductor, their opposite charges will cause a spark that discharges the jar. Actually,
van Musschenbroek's very first 'condenser' was nothing more than a beer glass!
By modern standards, the Leyden jar is cumbersome and inefficient. It is rarely used except in exiting laboratory
demonstrations of capacitance, and exiting they are! Benjamin Franklin was aquainted with the Leyden Jar experiments
also so he decided to test his ideas that 'charge' could also be caused by thunder and lightning. Franklin tested his
theories, in Philadelphia in June 1752, via his now famous 'Electrical Fluid Theory' to prove that lightning was an electrical phenomenon. What he
did was fly a kite which had a metal tip. The kite was tied with wet conducting thin hemp cord and at the end he
attached a metal key to which a non-conducting silk string was attached which he held in his hand; when he held his
knuckles near the key he could draw sparks from it. Although his experiment was completed successfully and the results
as he had calculated before, the next couple people after him who tried the hazardous experiment were killed by
lightning strikes. I guess Franklin was extremely lucky with his hazardous experiments. I myself believe in some sort
of "time-line" in which inventions are invented 'no matter what'.
A similar device was invented independently by Ewald Georg von Kleist, Dean of the Kamin Cathedral in Pomerania,
at about the same time (October 1745), but these facts were not published immediately at that particular time. As a
matter of fact, van Musschebroek announced his discovery in January, 1746. However, a letter dated February 4, 1745 appearing
in Philosophical Transactions suggests that the jar existed in van Musschenbroek's laboratory almost a year before
that date. There is still some residual controversy about this but the generally held opionion is: "Trembley, the editor,
or the composter of the letter in PT either misdated the letter, or failed to translate properly into the new style (NS).
Until 1752 the English began their legal year on March 25 so that, roughly speaking, their dates where a year behind continental
ones for the first quarter of every continental year. This makes sense because there would be no reason for van Musschenbroek
and his staff to delay announcing for 11 months, especial given the potential claim to prior discovery by Von Kleist.
Look at the picture at the right; the worlds first illustration of the working of a Leyden Jar, by Abbe
Jean-Antoine Nollet!
Trembley's letter is fascinating as it is one of the earliest first-hand accounts of this new discovery. He happened
to be in Holland about the time of the discovery and his letter was the first word to England of the marvelous new jar.
Georg von Kleist tried using an electrostatic generator to place a charge on an iron nail inside a small glass bottle.
Again later in 1745, a lawyer by the name of Anreas Cunaeus who frequently visited one the laboratories at the
University of Leiden, was trying to electrify water. He used a chain hanging into a flask of water, and brought the
end of the chain into contact with an electrostatic generator. In both cases, after disconnecting the generator, the
experimenter touched the metal nail or chain inside the flask with one hand while the other hand still surrounded the
outside of the container, and got zapped with an electric shock as a result.
But van Musschenbroek and von Kleist were certainly not the only ones playing with static discharge or electromagnetism.
The Greeks, by means of Greek philosopher Thales of Miletus, had already determined that fact in around 600 BC by charging
up Lodestone with a piece of amber and a sheeps skin. Lodestone (sometimes called incorrectly Loadstone) was
used in ancient times for navigation at sea.
Another Greek philosopher, Theophrastus, stated that this power is
possessed by other substances about three centuries later.
The first scientific study of electrical and magnetic phenomena, however, did not appear until AD 1600, when the
researches of the English physician William Gilbert were published. Gilbert was the first to apply the term electric
(Greek elektron, "amber") to the force that such substances exert after rubbing. He also distinguished between
magnetic and electric action.
Capacitors (also called condensers)
are funny things, creating enormous problems when troubleshooting for a fault and yet are absolutely necessary for
almost every electronic circuit. They come in a variety of sizes, shapes, models, or if you so desire they can be
manufactured by your specifications.
They also come in a variety of materials, to name a few: Aluminum foil, Polypropylene, Polyester (Mylar), Polystyrene,
Polycarbonate, Kraft Paper, Mica, Teflon, Epoxy, Oil-filled, Electrolyte, Tantalum, and the list goes on. Latest product
(in research) is Niobium. The value of a capacitor can vary from a fraction of a pico-Farad to more than a million µFarad (µ means 'micro').
Voltage levels can range from a couple to a substantial couple hundred thousand volts. The largest capacitor in my own collection is 150.000
µF at 10Volts. A big sucker measuring about 10 x 5 inches! Does it still work? You bet! It will still zap the
soles of your shoes... I use it on occasion to recondition shorted NiCad batteries which I use for my Radio Control
gear.
The basic unit of capacitance is the Farad. Clumsy and not very practical to work with, capacitance is usually
measured in microFarads, abbreviated µF or mfd, or picoFarads (pF). The unit Farad is used in converting
formulas and other calculations.
A µF (microfarad) is on millionth of a farad (10-6 F) and a pF picofarad is one-millionth of a microfarad (10-12 F).
What exactly is a 'Capacitor'?
A capacitor is a device that stores an electrical charge or energy on it's plates. These plates (see Fig. 1), a positive and a negative
plate, are placed very close together with an insulator in between to prevent the plates from touching each other. A
capacitor can carry a voltage eaqual to the battery or input voltage. Usually a capacitor has more than two plates
depending on the capacitance.
The 'Charge' is called the amount of stored electriciy on the plates, or actually the electric field between theses
plates, and is proportional to the applied voltage and capacitor's 'capacitance'.
The Formula to calculate the amount of capacitance is Q = C * V where:
Q = Charge in Coulombs
C = Capacitance in Farads
V = Voltage in Volts
There is also something else involved when there is 'charge', something stored called 'Energy'.
The formula to calculate the amount of energy is: W = V2 * C / 2 where:
W = Energy in Joules
V = Voltage in Volts
C = Capacitance in Farads
Is it difficult or complicated to 'charge' a capacitor? Not at all. Put proper voltage on the legs of the capacitor
and wait till current stops flowing. It goes very fast. Do NOT exceed the capacitor's voltage rating or it may explode.
And in case of a polarized capacitor, watch the orientation of the positive and negative poles. A healthy, good quality
capacitor (disconnected) can hold a charge for a long time. From seconds to several hours and some for several days
depending on its size.
An interesting experiment for a classroom. Try to build another capacitor than the Leyden Jar yourself too. Cut
two long strips of aluminum, say 1" wide by 48" long (25mm x 120mm).
Cut a strip of paper which is 1.5" by 50" (38mm x 125mm). Make sure the paper is dry. The paper is a bit wider and longer
then the foil to prevent the strips of foil from touching each other when you roll them up. Take two small metal
paperclips and 'unbend' them. One paperclip/strip aluminum foil is designated 'Positive' and other one 'Negative'.
Carefully roll up (all at once) the strips. First layer is tin foil, second one is paper (the insulator), and third
layer is tin foil again. Make sure the paper is dry or it won't work. Don't forget the paperclips (or wire) and make
sure the two strips don't touch each other. When you have the whole thing rolled up tightly as possible secure
it with tape or an elastic band or whatever.
Take a 9-volt battery and attach the negative (-) to one pole of the capacitor, and the other to your positive (+) pole.
It only takes a fraction of a second to charge it up. You can check the charge by hooking up a voltmeter or if that is
not available short the 'capacior' and you should see a spark.
Capacitor Codes
I guess you really like to know
how to read all those different codes. Not to worry, it is not as difficult as it appears to be. Except for the electrolytic and
large types of capacitors, which usually have the value printed on them like 470µF 25V or something, most of the
smaller caps have two or three numbers printed on them, some with one or two letters added to that value.
Check out the little table below.
Have a look at Fig. 2 and Fig. 3. As you can see
it all looks very simple. If a capacitor is marked like this 105, it just means 10+5zeros = 10 + 00000 =
100.000. And that's exactly the way you write it too. Value is in pF (PicoFarads). The letters added to the value
is the tolerance and in some cases a second letter is the temperature coeficient mostly only used in military
applications, so basically industrial stuff.
So, for example, it you have a ceramic capacitor with 474J printed on it it means: 47+4zeros = 470000 = 470.000pF,
J=5% tolerance. (470.000pF = 470nF = 0.47µF;) Pretty simple, huh? The only major thing to get used to is to
recognize if the code is µF; nF, or pF.
Other capacitors may just have 0.1 or 0.01 printed on them. If so, this means a value in µF. Thus 0.1 means just
0.1 µF. If you want this value in nanoFarads just move the comma three places to the right which makes it 100nF.
Easy huh?
But the average hobbyist uses only a couple types
like the common electrolytic and ceramic capacitors and depending on the application a more temperature stable type
like metal-film or polyester.
The larger the plate area and the smaller the area between the plates, the larger the capacitance. Which also
depends on the type of insulating material between the plates which is the smallest with air. (You see this type of
capacitor sometimes in high-voltage circuits and are called 'spark-caps'.) Replacing the air space with an insulator
will increase the capacitance many times over. The capacitance ratio using an insulator material is called
Dielectric Constant while the insulator material itself is called just Dielectric. Using
the table in Fig. 4, if a Polystyrene dielectric is used instead of air, the capacitance will be increased 2.60 times.
Look below for a more detailed explanation for the most commonly used caps.
Electrolytic - Made of electrolyte, which is basically a
conductive salt in solvent. Most common type capacitor. Applications: Filters, Timing circuits. Pro's: Cheap,
readily available, good for filters or storage of charge (energy). Con's: Not very accurate, marginal electrical
properties, leakage, drifting.
This type has come a long way and characteristics have constantly improved over the years. It is and always will be
an all-time favorite; unless something better comes along to replace it. But I don't think so for this decade; polarized
capacitors are very heavily used in most kind of equipment and consumer electronics.
Tantalum - Made of Tantalum Pentoxide. Pro's: Small
size fits anywhere, reliable, most common values readily available. Con's: Expensive, Easily damaged by spikes, Larger
value hard to obtain.
Polyester - and other types of polypropelene, etc.
Pro's: Temperature stable, readily available, widely used.
Con's: Can be quite large depending on capacity or rated voltage, may not fit everywhere.
Metalized - Made of Metal-Oxide. Pro's: Good quality,
low drift, temperature stable, all-round good capacitor.
Con's: Can be quite large depending on capacity or rated voltage, may not fit everywhere..
Epoxy - epoxy based polymers. Pro's: Widely availble,
stable, cheap. Con's: Can be quite large depending on capacity or rated voltage, may not fit everywhere.
Ceramic - Together with the electrolytics the most
available and used capacitor around. Pro's: Comes in very small size and value, very cheap, reliable. Con's: Subject
to drifting depending on ambient temperature. NPO types suppose to be temperature stable.
Silver-Mica - Highly stable, excellent for endurance, high
temperature or RF (military) electronic applications.
Combining Capacitors & Formula's:
Is it possible to combine capacitors to get to a certain value like we do with resistors? Certainly! Check below how
go about it.
Capacitors in Parallel
Capacitors connected in parallel, which is the most desirable,
have their capacitance added together, which is just the opposite of parallel resistors. It is an excellent way of
increasing the total storage capacity of an electric charge:
Ctotal = C1 + C2 + C3
Keep in mind that only the total capacitance changes, not the supplied voltage. Every single capacitor will
see the same voltage, no matter what. Be carefull not to exceed the specified voltage on the capacitors when
combining them all with different voltage ratings, or they may explode. Example: say you have three capacitors with
voltages of 16V, 25V, and 50V. The voltage must not exceed the lowest voltage, in this case the 16V one. As a matter
of fact, and a rule-of-thumb, always choose a capacitor wich is twice the supplied input voltage. Example: If the
input voltage is 12V you would select a 24V type (in real life 25V).
Capacitors in Series
Again, just the opposite way of calculating resistors. Multiple
capacitors connected in series with each other will have the total capacitance lower than the lowest single value
capacitor in that circuit. Not the prefered method but acceptable.
For a regular two capacitor series combo use this simple formula: 
If you have two identical capacitors in series the formula is simplicity itself: 
The Capacitor Future:
The future for capacitors looks good.
A constant search is going on by companies like Murata, Kemet, etc. Kemet in particular is researching a new type of a
dielectric substance called Niobium. Niobium Pentoxide (Nb2O5) offers a higher dielectric constant of 41 in comparison
to Tantalum Pentoxide (Ta2O5) at 26. It implies that approximately 1.5 more CV (Capacitance x Voltage rating) can be
obtained from the same amount of material, everything else being equal. What does this mean in plain english? Much
smaller capacitors with larger capacity, especially important in surface mount technology.
© Copyright & Credits:
"Leyden Jars" and portrait of "van Musschenbroek". Reprint with permission from
John D. Jenkins. More antique equipment and apparatus can be viewed at John's website called
The Spark Museum. This website contains a treasure of information and pictures, from vacuum tubes to radio transmitters. If it is
antique, John probably has it. I spend literally several weeks browsing and reading through his website. Amazing piece of work!
"Capacitor images on this page". Reprint with permission from Terence Noone,
President of The Capacitor Industries Companies which consists of Motor Capacitors Inc., Chicago Condenser Corp., and
SEI Capacitors Inc.
For detailed information please visit The Capacitor Industries Companies website.
Suggested Reading:
"The Radio Amateur Handbook" from the American Radio Relay Leaque (ARRL). Good resource.
"The Capacitor Book". by Cletus J. Kaiser., C.J. Publishing. ISBN: 0-9628525-3-8
Copyright © 2001, 2002 - Tony van Roon, ALL RIGHTS RESERVED
Last updated: March 25, 2002