# الموضوع: What Exactly Is A Capacitor?

1. ## What Exactly Is A Capacitor?

السلام عليكم ورحمة الله وبركاته
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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-static 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 ****llic 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 exciting laboratory demonstrations of capacitance, and exiting they are! Benjamin Franklin was acquainted 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 ****l tip. The kite was tied with wet conducting thin hemp cord and at the end he attached a ****l 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 opinion is: "Trembley, the editor, or the composer 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 ****l 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 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 equal to the battery or input voltage. Usually a capacitor has more than two plates depending on the capacitance or dielectric type.

The 'Charge' is called the amount of stored electricity 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
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

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 working breakdown voltage or, in case of an electrolytic capacitor, it will explode. The break down voltage is the voltage that when exceeded will cause the dielectric (insulator) inside the capacitor to break down and conduct. If that happens the results can be catastrophic. 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. A capacitor, in combination with other components, can be used as a filter that blocks DC or AC, being it current, frequency, etc.

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 ****l paperclips and 'unbend' them. One paperclip/strip aluminum foil is designated 'Positive' and other one 'Negative'. Carefully roll up (all at once) the strips. One paperclip (or wire) goes with it. First layer is tin foil, second one is paper (the insulator), and third layer is tin foil again. When you're almost at the end, don't forget the insert the other paperclip (or wire) with it. 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 'capacitor' 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 = 1.000.000pF = 1000 nF = 1 µF. 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 coefficient 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?

"NPO" is standard for temperature stability and 'low-noise', it does *not* mean non-polarized even though you might think so because the abbreviation looks similar. Polarized ceramic capacitors do not exist. The abreviation "NPO" stands for "Negative-Positive-Zero" (what is read as an 'O' is actually zero), and means that the negative and positive temperature coefficients of the device are zero--that is the capacitance does not vary with temperature. ONLY the black top indicates NPO qualification and the values are in the range from 1.8pF to 120pF, unless manufactured with different values for Military and/or industrial purposes on special request. They feature 2% tolerance which comes down to about 0.25pF variation, and all are 100V types. You may sometimes find NPO-type caps marked witht he EIA (Electronic Industrial Association) code "COG". The EIA has an established set of specifications for capacitor temperature characteristics (EIC 384/class 1B). Thus, a capacitor labeled "Y5P" would exhibit a plus/minus tolerance of 10% variation in capacitance over a temperature range of -30°C. to +85°C. Or it may say N12 which translates to 120pF. Or 2P2 (2.2pF). I'm sure you get the idea...

But the average hobbyist uses only a couple types like the common electrolytic and general purpose ceramic capacitors and depending on the application, a more temperature stable type like ****l-film or polypropylene.

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, basically conductive salt in solvent. Aluminum electrodes are used by using a thin oxidation membrane. Most common type, polarized capacitor. Applications: Ripple filters, timing circuits. Cheap, readily available, good for storage of charge (energy). Not very accurate, marginal electrical properties, leakage, drifting, not suitable for use in hf circuits, available in very small or very large values in µF. They WILL explode if the rated working voltage is exceeded or polarity is reversed, so be careful. When you use this type capacitor in one of your projects, the rule-of-thumb is to choose one which is twice the supply voltage. Example, if your supply power is 12 volt you would choose a 24volt (25V) type. 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 heavily used in almost every kind of equipment and consumer electronics.

Tantalum - Made of Tantalum Pentoxide. They are electrolytic capacitors but used with a material called tantalum for the electrodes. Superior to electrolytic capacitors, excellent temperature and frequency characteristics. When tantalum powder is baked in order to solidify it, a crack forms inside. An electric charge can be stored on this crack. Like electrolytics, tantalums are polarized so watch the '+' and '-' indicators. Mostly used in analog signal systems because of the lack of current-spike-noise. Small size fits anywhere, reliable, most common values readily available. Expensive, easily damaged by spikes, large values exists but may be hard to obtain. Largest in my own collection is 220µF/35V, beige color.

Super Capacitors - The Electric Double Layer capacitor is a real miracle piece of work. Capacitance is 0.47 Farad (470,000 µF). Despite the large capacitance value, its physical dimensions are relatively small. It has a diameter of 21 mm (almost an inch) and a height of 11 mm (1/2 inch). Like other electrolytics the super capacitor is also polarized so exercise caution in regards to the break-down voltage. Care must be taken when using this capacitor. It has such large capacitance that, without precautions, it would destroy part of a powersupply such as the bridge rectifier, volt regulators, or whatever because of the huge inrush current at charge. For a brief moment, this capacitor acts like a short circuit when the capacitor is charged. Protection circuitry is a must for this type.

Polyester Film - This capacitor uses a thin polyester film as a dielectric. Not as high a tolerance as polypropylene, but cheap, temperature stable, readily available, widely used. Tolerance is approx 5% to 10%. Can be quite large depending on capacity or rated voltage and so may not be suitable for all applications.

Polypropylene - Mainly used when a higher tolerance is needed then polyester caps can offer. This polypropylene film is the dielectric.
Very little change in capacitance when these capacitors are used in applications within frequency range 100KHz. Tolerance is about 1%.
Very small values are available.

Polystyrene - Is used as a dielectric. Constructed like a coil inside so not suitable for high frequency applications. Well used in filter circuits or timing applications using a couple hundred KHz or less. Electrodes may be reddish of color because of copper leaf used or silver when aluminum foil is used for electrodes.

****lized Polyester Film - Dielectric made of Polyester or DuPont trade name "Mylar". Good quality, low drift, temperature stable. Because the electrodes are thin they can be made very very small. Good all-round capacitor.

Epoxy - Manufactured using an epoxy dipped polymers as a protective coating. Widely available, stable, cheap. Can be quite large depending on capacity or rated voltage and so may not be suitable for all applications.

Ceramic - Constructed with materials such as titanium acid barium for dielectric. Internally these capacitors are not constructed as a coil, so they are well suited for use in high frequency applications. Typically used to by-pass high frequency signals to ground. They are shaped like a disk, available in very small capacitance values and very small sizes. Together with the electrolytics the most widely available and used capacitor around. Comes in very small size and value, very cheap, reliable. Subject to drifting depending on ambient temperature. NPO types are the temperature stable types. They are identified by a black stripe on top.

Multilayer Ceramic - Dielectric is made up of many layers. Small in size, very good temperature stability, excellent frequency stable characteristics. Used in applications to filter or bypass the high frequency to ground. They don't have a polarity. *Multilayer caps suffer from high-Q internal (parallel) resonances - generally in the VHF range. The CK05 style 0.1µF/50V caps for example resonate around 30MHz. The effect of this resonance is effectively no apparent capacitance near the resonant frequency.
As with all ceramic capacitors, be careful bending the legs or spreading them apart to close to the disc body or they may get damaged.

Silver-Mica - Mica is used as a dielectric. Used in resonance circuits, frequency filters, and military RF applications.
Highly stable, good temperature coefficient, excellent for endurance because of their frequency characteristics, no large values, high voltage types available, can be expensive but worth the extra dimes.

Adjustable Capacitors - Also called trimmer capacitors or variable capacitors. It uses ceramic or plastic as a dielectric.
Most of them are color coded to easily recognize their tunable size. The ceramic type has the value printed on them. Colors are: yellow (5pF), blue (7pF), white (10pF), green (30pF), brown (60pf). There are a couple more colors like red, beige, and purple which are not listed here. Anyways, you get the idea...

Tuning or 'air-core' capacitors.
They use the surrounding air as a dielectric. I have seen these variable capacitor types of incredible dimensions, especially the older ones. Amazing it all worked. Mostly used in radio and radar equipment. This type usually have more (air) capacitors combined (ganged) and so when the adjustment axel is turned, the capacitance of all of them changes simultaneously. The one on the right has a polyester film as a dielectric constant and combines two independent capacitors plus included is a trimmer cap, one for each side.

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 careful 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 which 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 preferred 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:

0.000001µF = 0.001nF = 1pF
0.00001µF = 0.01nF = 10pF
0.0001µF = 0.1nF = 100pF
0.001µF = 1nF = 1000pF
0.01µF = 10nF = 10,000pF
0.1µF = 100nF = 100,000pF
1µF = 1000nF = 1,000,000pF
10µF = 10,000nF = 10,000,000pF
100µF = 100,000nF = 100,000,000pF

Table 1. Capacitance ConversionThings Capacitors Don't Like:
Capacitors are very finicky devices. There are any number of conditions they don't like. Many types, for instance, lose a significant amount of their capacitance at high frequencies, making them unsuitable for RF applications. You also have to watch out for the inductance some may introduce in places where you don't want it. For that reason, some types of capacitors are indicated specifically as "non-inductive."
Temperature extremes are another thing to which capacitors, particularly electrolytics, are sensitive. Electrolytic capacitors, at elevated and at depressed temperatures, lose much of their capacitance. If you are going to operate electrolytics at extremes of temperature, make sure their tolerances extend to that temperature range.
Most capacitors do not care for Alternating Current (AC) either. It makes them overheat and--before they self-destruct--operate inefficiently. AC in particular is hard on capacitors because of the sinusoidal switching effect. For that reason it is of utmost importance to select a capacitor type with the correct dielectric, temperature coefficient, capacitance, and working voltage. As a rule-of-thumb in DC applications, select a capacitor of at least twice the supply power.

Polarized capacitors, as they already been pointed out, cannot tolerate reverse voltages. Not only does that make them heat up, it can cause them--especially tantalum types--to heat up so rapidly that a sudden and violent explosion can result (most polarized capacitors are encased in tightly sealed containers).
Explosions can also result from polarized capacitors being installed "backwards" in a circuit. The cathode (negative) side of a polarized capacitor should always connect to ground.

Uncommon Capacitors:
Capacitors vary in size from microscopic to the enormous. At the small end of the scale, there are the capacitors that are deposited on a substrate during the manufacture of integrated circuits. Hybrid integrated circuits such as those containing tuned circuits may require very precise capacitor values--with tolerances that are impossible to achieve using any economically feasible straight manufacturing process.
The precise capacitances required are obtained by intentionally making the capacitors oversize, and then trimming them with a laser until the circuit of which they are apart resonates at exactly the right frequency.
At the other end of the scale, the enormous energy requirements of the acceleration devices used in subatomic-particle research are also met by capacitors--rooms full of them! One of the largest such devices, a particle accelerator located outside of Chicago, is said to be able to store enough energy to meet the electrical demand of the entire world! Of course, that's only for an instant during the discharge cycle, but the figure involved is still big enough to boggle the mind.

Bypassing:
The phrase "bypass" or "decoupling" is referred to filtering noise off the power rails caused by switching of TTL IC's, MosFets, Transistors, etc. Especially TTL (Transistor-Transistor-Logic) IC's create a lot of noise and so this has to be cleaned up. Mounting a 100nF (0.1uF) ceramic bypass capacitor over the power rails and as close to the IC as possible and keeping the capacitor leads as short as you can, will clean up noise nicely. This has to be done on all IC's and power rails on a printed circuit board with this kind of digital logic. Noise can cause all sorts of problems such as false triggering, cross-talk, change to an undesirable logic state, etc. Decoupling is used where the supply voltage cannot be lowered, i.e., if one needed a noise free +12V on a PC bus, for example. You could get a "clean" +12 volts with a voltage regulator... if only there was +15 volts or higher to start with. But such is not the case. So you use a high "Q" inductor or RFC choke along with the proper bypass capacitor to effectively lowpass filter the +12 volt supply rail. For a real noisy supply you can use more than one inductor: a "pie" network for example.
As digital becomes faster and faster, it looks more like analog than digital. It would be an asset to have a good understanding of the analog/rf properties of high speed digital. Careful layout of a groundplane and proper decoupling and bypass requires close attention of a circuit design to maintain the integrity of the power distribution.
Excessive lead length will void the purpose of a bypass capacitor so keep the leads short.

Couple of Bypass Suggestions:
If needed, attempt the following as stated below.

Use 0.1uF (=100nanoF) or greater, ceramic capacitor, surface mount technology or SMT prefered.

One capacitor per SSI, MSI TTL, device.

The capacitors should be connected over the Vcc and Gnd with the shortest path possible.

Ceramic disc capacitors have the best high frequency characteristics and the least self inductance.

Only in RF applications it is appropriate to mix different type caps together to filter at different freq's.

Substituting a tantalum for a ceramic type will not work, but they can work together very well.

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. Recently, a new type capacitor with very high capacitance has been developed with capacitance designated in Farads! Yes, you read it well, Farads. This type of Electric Double Layer capacitor is known as a "Super Capacitor". I am sure we haven't heard the last of it about this type.

"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.

"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
"Williamson Labs". Website about bypassing, decoupling, shielding, layout, etc.: http://williamson-labs.com
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Last updated: January 8, 2006

2. ## رد: What Exactly Is A Capacitor?

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