Capacitors Blog

In a way, a capacitor is a little like a battery. Although they work in completely different ways, capacitors and batteries both store electrical energy. If you have read How Batteries Work, then you know that a battery has two terminals. Inside the battery, chemical reactions produce electrons on one terminal and absorb electrons on the other terminal. A capacitor is much simpler than a battery, as it can't produce new electrons -- it only stores them.

In this article, we'll learn exactly what a capacitor is, what it does and how it's used in electronics. We'll also look at the history of the capacitor and how several people helped shape its progress.

Inside the capacitor, the terminals connect to two metal plates separated by a non-conducting substance, or dielectric. You can easily make a capacitor from two pieces of aluminum foil and a piece of paper. It won't be a particularly good capacitor in terms of its storage capacity, but it will work.

In theory, the dielectric can be any non-conductive substance. However, for practical applications, specific materials are used that best suit the capacitor's function. Mica, ceramic, cellulose, porcelain, Mylar, Teflon and even air are some of the non-conductive materials used. The dielectric dictates what kind of capacitor it is and for what it is best suited. Depending on the size and type of dielectric, some capacitors are better for high frequency uses, while some are better for high voltage applications. Capacitors can be manufactured to serve any purpose, from the smallest plastic capacitor in your calculator, to an ultra capacitor that can power a commuter bus. NASA uses glass capacitors to help wake up the space shuttle's circuitry and help deploy space probes. Here are some of the various types of capacitors and how they are used.

Air - Often used in radio tuning circuits
Mylar - Most commonly used for timer circuits like clocks, alarms and counters
Glass - Good for high voltage applications
Ceramic - Used for high frequency purposes like antennas, X-ray and MRI machines
Super capacitor - Powers electric and hybrid cars

In the next section, we'll take a closer look at exactly how capacitors work.

Tantalum bead capacitors are polarised and have low voltage ratings like electrolytic capacitors. They are expensive but very small, so they are used where a large capacitance is needed in a small size.

Modern tantalum bead capacitors are printed with their capacitance, voltage and polarity in full. However older ones use a colour-code system which has two stripes (for the two digits) and a spot of colour for the number of zeros to give the value in µF. The standard colour code is used, but for the spot, grey is used to mean × 0.01 and white means × 0.1 so that values of less than 10µF can be shown. A third colour stripe near the leads shows the voltage (yellow 6.3V, black 10V, green 16V, blue 20V, grey 25V, white 30V, pink 35V). The positive (+) lead is to the right when the spot is facing you: 'when the spot is in sight, the positive is to the right'.

For example: blue, grey, black spot means 68µF
For example: blue, grey, white spot means 6.8µF
For example: blue, grey, grey spot means 0.68µF

This is a measure of a capacitor's ability to store charge. A large capacitance means that more charge can be stored. Capacitance is measured in farads, symbol F. However 1F is very large, so prefixes are used to show the smaller values.

Three prefixes (multipliers) are used, µ (micro), n (nano) and p (pico):

µ means 10-6 (millionth), so 1000000µF = 1F
n means 10-9 (thousand-millionth), so 1000nF = 1µF
p means 10-12 (million-millionth), so 1000pF = 1nF

Capacitor values can be very difficult to find because there are many types of capacitor with different labelling systems!

There are many types of capacitor but they can be split into two groups, polarised and unpolarised. Each group has its own circuit symbol.

Aluminum electrolytic capacitor maker Lelon Electronics has seen extra orders from China appliance makers due to the China government's program to promote sales of home appliances in rural areas. The extra orders have covered Lelon's reduced orders from the US and Europe, the company said.

The price of aluminum electrolytic capacitors has been stable since August 2008, while the price of aluminum foil continues to drop, lowering Lelon's cost pressure. However, order visibility for aluminum electrolytic capacitors is still low, the company noted.

Lelon has entered the wind power market with its large-size capacitors, and plans to apply the product to other applications. The company has already shipped a small volume of large-size capacitors to a motorcycle maker and expects to expand into the automotive segment.

A capacitor or condenser is a passive electrical component consisting of an insulating, or dielectric, layer between two conductors. When a voltage potential difference occurs between the conductors, an electric field occurs in the insulator. This field can be used to store energy, to resonate with a signal, or to link electrical and mechanical forces. Capacitors are manufactured as electronic components for use in electrical circuits, but any two conductors linked by an electric field also display this property. The effect is greatest between wide, flat, parallel, narrowly separated conductors.

An ideal capacitor is characterized by a single constant value, capacitance, the ratio of the amount of charge in each conductor to the potential difference between them. The unit of capacitance is thus coulombs per volt, or farads. Higher capacitance indicates that more charge may be stored at a given energy level, or voltage. In actual capacitors, the insulator allows a small amount of current through, called leakage current, the conductors add an additional series resistance, and the insulator has an electric field strength limit resulting in a breakdown voltage.

Smaller density, four times the capacity, instant recharge... awesomeness.

You smell that? It’s the silicon-enriched aroma of new technology, and the source is nothing short of amazing! A small company in Cedar Park Texas known as EEStor has claimed it has developed a battery that will solve all life’s troubles. Well, all tech enthusiasts’ troubles anyhow.

Anyone who owns, or has owned, a portable device is familiar with how irritating battery technology is. Not only does it take hours to recharge the battery, but the capacity is often mediocre at best. Furthermore, they degrade over time and their charge is lost at a rate of several percent each month. The likes of NiCad, Li-ion, NiMH and Li-Po (to name a few) all suffer these cruel flaws, and are commonly found in everything from MP3 players to notebook computers. Fortunately for consumers, a new battery technology is emerging. One that will make power issues a thing of the past – the super capacitor.

Super capacitors aren’t something new. They’ve been worked on, prodded, and enhanced by researchers and electrophiles (of the human variety) for many years with varying success. Nonetheless, it was only recently that such technology has become viable for industrial use, thanks to EEStor and its new patent. Its latest 127.7kg prototype dubbed as an EESU (Electrical Energy Storage Unit) claims to have a capacitance of 30.693 F, and retains a whopping 52,220 kWh of energy. Now that weight may seem excessive. Luckily this technology is completely scalable, meaning possible future development for small consumer devices. In actual fact, the prototype is made up of 31,353 smaller units arranged in parallel.

In addition to the high capacity and energy, there are other perks. How does unlimited recharge cycles sound? The EEStor prototype was charged and discharged over a million times; the result being no change in capacity whatsoever. Imagine that in your laptop or mobile phone! And to top it off, now imagine plugging your drained phone into its charger, having a short satisfying yawn, then unplugging it straight after with a full battery. They’ll charge as fast as you can pump power to them. Yes, capacitors are that cool.

So how does this miracle battery work, I hear you ask? Let’s start with the basics. A capacitor consists of two conducting parallel plates immersed in a non-conductive (dielectric) medium. The plates are directly connected to the terminals of the capacitor, which then connect to a circuit. When power is fed to the capacitor, the negatively charged plate gains electrons lost from the positively charged plate. The capacitor finishes charging when this process of losing/gaining electrons completes. Since the negative plate is negatively charged, it wants to give off electrons to restore it to its original state. Likewise for the positive plate, however it wants to gain electrons. When the capacitor is connected to a closed circuit, the extra electrons from the negative plate travel through the circuit back to the positive plate. This creates a current in the circuit. It’s that simple, two plates and a dielectric!

The capacitance of a capacitor is measured in Farads (F). The 30.693 F of the EEStor prototype may seem somewhat small - but what if I told you that the capacitors in your PC right now were about 30mF (micro farads). That makes this super capacitor one million times more effective in terms of capacitance. A Farad is calculated by multiplying current by time over voltage ((A*s)/V). Considering it runs at 3500v, 30F is quite large.

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Capacitors do not behave the same as resistors. Whereas resistors allow a flow of electrons through them directly proportional to the voltage drop, capacitors oppose changes in voltage by drawing or supplying current as they charge or discharge to the new voltage level. The flow of electrons “through” a capacitor is directly proportional to the rate of change of voltage across the capacitor. This opposition to voltage change is another form of reactance, but one that is precisely opposite to the kind exhibited by inductors.

The expression de/dt is one from calculus, meaning the rate of change of instantaneous voltage (e) over time, in volts per second. The capacitance (C) is in Farads, and the instantaneous current (i), of course, is in amps. Sometimes you will find the rate of instantaneous voltage change over time expressed as dv/dt instead of de/dt: using the lower-case letter “v” instead or “e” to represent voltage, but it means the exact same thing. To show what happens with alternating current, let's analyze a simple capacitor circuit below.

Suntan Technology Company Limited
---All Kinds of Capacitors

Dielectric absorption in a capacitor is difficult to characterize accurately because of the very wide range of the time constants involved and because of the high level of performance required in the measuring equipment. To get a good characterization, the capacitor response must be measured for a range of frequencies at least three decades higher and lower than 1/τ0. Frequency domain measurements must be made with vector impedance analyzers that can accurately resolve a small resistive component in a largely reactive impedance (they must be able to accurately measure large values of Q). In the frequency domain the resistive portion of the impedance gives the most information about dielectric absorption. In the time domain, which is usually used for measurement longer than 100 ms, an ammeter is needed with very low bias current and the ability to resolve very low currents. If these instruments are available, then an accurate and complete model can be made, but such a model is often not required. To completely model dielectric absorption would require, in most cases, a range of accuracy that spans ten decades of frequency or more. Generally, however, the application does not warrant such a model and one can get by with a model that is faithful to the behavior of the physical component over a much smaller range of frequencies.

Because of the special winding design the ESR values of the new capacitors have been lowered by 30 percent at 60 °C and the thermal resistance between base and windings has been reduced by 50 percent. The robust capacitors also feature a specially reinforced case for high long-term stability.

Thanks to the reduction of their longitudinal tolerance to ±0.2 mm, entire capacitor banks can be connected without problems via comparatively thin thermal pads to heat sinks. This ensures an optimal thermal connection to the heat sink and simultaneously lowers costs. Under suitable operating conditions, up to 50 percent higher ripple current capability can be achieved.