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Capacitors... What are they?

Technical Bulletin No. 01

"What is a capacitor?" Webster's Collegiate Dic­tionary says:

"Capacitor: A device giving capacitance usu­ally consisting of conducting plates or foils separated by thin layers of dielectric with the plates on opposite sides of the dielectric layers oppositely charged by a source of volt­age and the electrical energy of the charged system stored in the polarized dielectric"

Now that the capacitor has been "defined," let's correlate this definition to a schematic drawing of the capacitor and the associated circuitry neces­sary for the capacitor to perform fully to its definition.

Figure 1
Figure 1

Figure 1 illustrates all of the elements necessary to the definition. The "two metallic conductors" and the "dielectric material'' are obvious. The addition of the plate connections, Switch S, and the ener­gy source allows the capacitor to fulfill its function of "storing electrical energy." The plate connec­tions, Switch s2, and the load, control the energy release factors to a "pre-determined rate" and "pre­-determined time".

Now that we know what a capacitor is and basically how it works, we must have some means of mea­suring or rating it. Since its function is to store energy, we measure or rate it by its ability to store this energy. The term used to describe this ability is "capacitance."

"Capacitance" then is a measure of the quantity of electrical charge that can be held per unit of voltage differential between the metallic conductors (electrodes). The basic unit of capacitance is the "farad" but, since the farad is a very large number, "microfarad" {one millionth of a farad) and "pico­farad" (one millionth of a microfarad) are in most common usage.

Figure 2
Figure 2

The mathematics associated with the conversion from the primary definition of capacitance equals quantity of electrical change per unit voltage differ­ential to the basic geometrical formula for capaci­tance is shown in Figure 2. With this geometrical formula, a capacitor engineer can design units to known values.

To fully understand just how the capacitance {C} measures the ability of a capacitor to store energy, Figure 3 illustrates the derivation of the formula concerned and shows the direct relationship be­tween energy and capacitance.

Figure 3
Figure 3

From the basic formula we note that C varies di­rectly with the dielectric constant (K) and area (A); and inversely with the distance between the plates (d). Both (A) and (d) are geometrically controlled figures, but what is this dielectric constant (K) and how is it determined?

The dielectric constant (K) of a material is a direct measure of its ability to store electrons when compared to air.

If we make a capacitor with given "A" and "d" di­mensions, and use just clean dry air as our dielect­ric, it will measure a certain value of capacitance. Now, if we substitute some other dielectric for the air and remeasure the capacitance, we will find that our capactance value has increased. If the capaci­tance figure doubled, for instance, this would mean that the second dielectric had a dielectric constant of 2 (twice that of air).

Figure 4 is a chart of various common dielectric materials and their approximate dielectric con­stants.

Dielectric Material K(Dlelectric Constant)
Vacuum 1.0 (exact)
Air 1.0001
Teflon 2.0
Polystyrene 2.5
Polypropylene 2.5
Polycarbonate 2.7
PolysuIfone 2.7
Mylar 3.0
Kapton 3.2
Polyethylene 3.3
Kraft Paper (impregnated) 2.0 to 6.0
Mica 6.8
Aluminum Oxide 7.0
Tantalum Oxide 11
Ceramics 35.0 to 6000
Figure 4
Figure 4

And that's what a capacitor is!

What is a capacitor? Why it's "something that an electronic circuit won't work without!"

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