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Capacitor Construction

Topics covered in this presentation:

Capacitor Construction

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Introduction to Capacitors

A capacitor is a device that is able to store charge and acts like a temporary, rechargeable battery.

A capacitor can be charged by connecting it to a battery or other voltage source.

The amount of charge that a capacitor can store and the length of time this charge can be held is dependent on its construction.

If the charge is allowed to leak away then the capacitor will become discharged.

Capacitors are available with many different values that indicate the amount of charge that they can store.

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Capacitor Construction Types

Depending on their construction type, capacitors come in many different shapes and sizes.

Types of capacitors include polyester, ceramic, and electrolytic.

axial electrolytic radial electrolytic

ceramicpolyester

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Capacitor Construction

Any two conducting surfaces that face each other across a small distance will be able to store an electric charge.

The amount of charge that a capacitor can store is dependent upon the area of the two plates, the distance between the plates, and the material between the plates.

The circuit symbol for a capacitor comes from its construction.

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The capacitance (C) of a capacitor is defined as the amount of charge (Q) stored when one volt (V) is applied across its plates.

Capacitance

Capacitance is the ability of a device to store electrical charge.

VQ

C

The unit of capacitance is the farad (F). However, one farad is very large so capacitors are generally specified using sub-multiples.• microfarads (F) 1F = 1 x 10-6 F

• nanofarads (nF) 1nF = 1 x 10-9 F

• picofarads (pF) 1pF = 1 x 10-12 F

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Capacitor Construction

To keep the two plates apart and at a set distance an insulator, called a dielectric, is inserted between the plates.

In some capacitors an additional layer of dielectric is used so that the plates can be rolled to reduce the physical size.

The material used as the dielectric also affects the capacity of the capacitor.

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Capacitor Dielectrics

The type of dielectric used in a capacitor gives its type. For example, polyester, ceramic, mica, or electrolytic.

The type of dielectric also determines the range of values that a capacitor can have.

For example, ceramic capacitors will have small values in nanofarads and picofarads. Electrolytic capacitors will have larger values in microfarads.

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Capacitor Working Voltage

The thickness of the dielectric between the two plates is very small, and could easily be damaged if the voltage across the capacitor was high enough.

Capacitors are therefore given a working voltage that must not be exceeded.

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Capacitor Labelling

Capacitors are labelled in many different ways.

The casing of electrolytic capacitors is large enough to write the value in full, for example 4700F.

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Capacitor Labelling

Other types of capacitors use various code systems to indicate the capacitor value.

One system uses a 3 digit number to indicate the capacitor value in picofarads.

In another system a number on its own indicates a value in microfarads.

Note: If the first number is a zero, the value is in microfarads.

For example, 104 is one, zero, and 4 zeros (100,000pF).

A number followed by an n indicates nanofarads. For example 15n = 15nF

For example 10 = 10F

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Capacitor Labelling

Other coding on the label indicates the tolerance, indicated using a lettered code, and the working voltage, given in volts.

The tolerance code for capacitors is shown opposite.

As can be seen, the tolerance of capacitors can be quite wide.

Most capacitors have a 20% tolerance, and the tolerance for some ceramic capacitors is +80%/-20%.

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Polarised Capacitors

Although most capacitors can be connected into a circuit either way round, there are two types of capacitors that must only be inserted in a circuit one way.

These are electrolytic capacitors and tantalum capacitors.

Both of these capacitors must be connected so that their negative terminals are connected to the negative side of the circuit.

Labelling on the capacitor indicates the polarity of the terminals.

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Capacitor Data Sheet

The following are some of the criteria, found on the product data sheet, to consider when selecting a capacitor.

• Capacitor Value• Dielectric Type• Tolerance• Working Voltage• Leakage Current• Temperature Coefficient