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Types of Capacitors

(a) Guard Ring Capacitor.

In a parallel capacitor, the electric field between the plates is not uniform near the edges. This is called the “edge effect” or “fringing”. This is avoided by using a guard ring. The circular insulated plate P is surrounded by a circular coplanar ring G. The inner diameter of G is slightly larger than the diameter of P. The air gap between P and G is very small. The diameter of the plate Q is equal to the outer diameter of G. The field between P and Q is uniform throughout the common area between them. The irregularity in the field occurs at the outer edge of the guard ring. The effective area of the plate = A’ = Area of the plate P + ½ Area of the circular air gap between P and G. C = ε0A’/d. This is used as an absolute standard of capacitance.

(b) Mica Capacitors.

It consists of a number of parallel plate capacitors in parallel with the alternate metallic foils fixed to one end each. Mica is used as the dielectric. The capacitance of such is C = n εr εoA/d where n is the number of capacitors grouped in parallel; A is the surface is the area of the plate and d is the thickness of each mica sheet.
Metal foil

(c) Electrolytic Capacitor.

It consists of two aluminum electrodes A and C dipped in a solution of ammonium borate. On passing a direct current, a very thin film is an insulator. The arrangement can now be used as a capacitor with the anode as one plate, the solution as the other plate, and the aluminum oxide film as dielectric. Since the dielectric layer is very thin, the capacitance of this arrangement is very large. This capacitor must be placed only in a D.C. circuit. It cannot be used in an A. C. circuit.

(d) Variable Air Capacitor.

It consists of two sets of metal plates, one fixed and the other movable. The fixed set is semi-circular in shape. The movable set is like a cam and rotated with knobs. All the fixed plates are connected to one terminal. All the movable plates are connected to another terminal. Air is the dielectric. By rotating the knobs, the area of overlap between the two sets of plates is changed. Thus the capacitance of the capacitor changes. These capacitors are widely used in the tuning circuits of radio receivers.

Uses of Capacitors.

(i) They are used in the ignition system or automobile engines for elimination sparking.
(ii) They are used in radio circuits for tuning, to reduce voltage fluctuation in power supplies, and to increase the efficiency of alternating current power transmission.
(iii) They are used to generate and detect electromagnetic oscillation of high frequency.
(iv) They serve as useful devices for storing electric energy.

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