Capacitors Blog

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.

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---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.

Power supply designers are often concerned about the ripple current capabilities of capacitors on both the input and output sides of converters. The biggest concern is the internal temperature rise caused by the I2R power consumption of the capacitor. Since tantalums are a polar device, this ripple should always be accompanied by a DC bias. Since ESR is so dependent on frequency and temperature, the power ratings listed should be viewed as rules of thumb and not laws written in stone. The tantalum capacitors have a published data set for a 10°C rise above the ambient. The experimental set-up used to arrive at this number was emulated and done for a series of ceramic capacitor chips. It should be noted that different mounting techniques can alter the thermal conductivity greatly.

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Capacitor is an electromechanical device capable of storing electrical energy (electrons). It differs from a battery, as a battery produces electrical energy as a by-product of chemical activity. A capacitor can function in a circuit like a battery. Or a resistor, or an inductor, for that matter. It can also absorb energy, turning some into heat.

A farad of capacitance is a lot of capacitance. In high voltage, we generally deal in microfarads (mfd.) or picofarads (pf) which are one millionth of a farad and one millionth of 1 millionth of a farad, respectively. Infrequently used, a nanofarad is one thousandth of a microfarad.

Photo: Inside, an electrolytic capacitor is a bit like a Swiss roll. The "plates" are two very thin sheets of metal; the dielectric an oily plastic film in between them. The whole thing is wrapped up into a compact cylinder and coated in a protective metal case.

Warning: It can be dangerous to open up capacitors. First, they can hold very high voltages. Second, the dielectric is sometimes made of toxic or corrosive chemicals that can burn your skin.

A capacitor is a bit like a battery, but it has a different job to do. A battery uses chemicals to store electrical energy and release it very slowly through a circuit—sometimes (in the case of a quartz watch) it can take several years. A capacitor generally releases its energy much more rapidly—often in seconds or less. If you're taking a flash photograph, for example, you need your camera to produce a huge burst of light in a fraction of a second. A capacitor attached to the flash gun charges up for a few seconds using energy from your camera's batteries. (It takes time to charge a capacitor and that's why you typically have to wait a little while to take a flash photo.) Once the capacitor is fully charged, it can release all that energy in an instant through the xenon flash bulb. Zap!

Capacitors come in all shapes and sizes, but they usually have the same basic components. There are the two conductors (known as plates, largely for historic reasons) and there's the insulator in between them (called the dielectric). The two plates inside a capacitor are wired to two electrical connections on the outside called terminals, which are like thin metal legs you can hook into an electric circuit.

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----All Kinds of Capacitors

A vacuum variable capacitor uses a high vacuum as the dielectric instead of air or other insulating material. This allows for a higher voltage rating and/or capacitance value using a smaller total volume. In addition to the higher voltage rating a vacuum dielectric greatly reduces the chance of arcing between the plates. There are several different designs in vacuum variables, the most common geometry of the capacitor plates is usually inter-meshed concentric cylinders. The meshed cylinders are contained within a glass or ceramic vacuum envelope, similar to an electron tube. A metal bellows is used to maintain a vacuum seal while allowing positional control for the moving parts of the capacitor.

Common Applications

Vacuum variable capacitors are commonly used in high voltage applications 5000 volts(5kV) and above. They are used in equipment such as high powered broadcast transmitters, Amateur radio RF Amplifiers and large Antenna tuners.

Other Variations

A fixed value vacuum capacitor. Other variations of vacuum capacitors include fixed value capacitors. Fixed value capacitors are designed very much like the variable versions with the exception of an adjustment mechanism.

Comparison

When compared to other variable capacitors vacuum variables tend to be more precise and more stable. This is due to the vacuum itself. Because of the sealed chamber the dielectric constant remains the same over a wider range of operating conditions. With air Variable capacitors the air moving around the plates may change the value slightly; often it’s not much but in some applications is enough to cause undesirable effects. Another common problem with air variables is dust and insects. Dust collected on the plates or a curious insect can raise havoc. A wandering spider is bound to make fireworks with the open frame design used on most air variable capacitors.

Vacuum variable capacitors are generally more expensive than Air Variable Capacitors this is primarily due to their design and the materials used. Although most use copper and glass some may use other materials such as ceramics and metals such as gold and silver. Vacuum variables also vary in adjustment mechanisms which range from course to fine.

Suntan is a Hong Kong based manufacturer of Ceramic Capacitors. Including High Voltage Ceramic Capacitors, Multilayer Ceramic Capacitors, Ceramic Disc Capacitor, Mini Type Ceramic Capacitors etc.

Suntan Technology Company Limited
----All Kinds of Capacitors

Suntan pictures of Capacitors. Notice that all these capacitors have two wires coming out of them, one attached to each plate.

Suntan is well established and specializes in service and quality with an extensive export and import network. We provide quick turn around for our customers all over the world. Almost 30 years experience with all types of passive components using good materials, state of the art equipment and technology and ongoing QC coupled with extensive R & D assure our customers of the best quality and on time delivery.

Suntan offer many kinds of capacitors. Don’t hesitate to send us inquire to sales@suntan.com.hk .We will show you good quality and best price.