Basics of Capacitors

Basically capacitors are stores electrical energy in the form of charge. when a voltage source has been connected with a capacitor it gets charged with a potential level equal to the source.

If we connect it with a low potential circuit it starts discharging through the circuit. The rate of discharging is depends on total resistance of the discharging path.

And if we reverse the polarity of the voltage source than the polarity of the capacitor is also reversed. these are the basic functionality of a capacitor. This could be used for for various applications.

Types of Capacitors

Polarized Capacitors: 

Polarized capacitors are those in which the terminals are defined as positive (+ve ) & negative(-ve). Whenever use it in circuit we should keep in mind the terminal's polarity.

Types of Polarized capacitors are 

• Electrolytic Capacitor
• Tantalum Capacitor.

Aluminum Electrolytic Capacitors: 

Electrolytic Capacitors are generally used when very large capacitance values are required. Here a semi-liquid electrolyte solution in the form of a jelly or paste is used which serves as the second electrode (usually the cathode).

The dielectric is a very thin layer of oxide which is grown electro-chemically in production with the thickness of the film being less than ten microns. This insulating layer is so thin that it is possible to make capacitors with a large value of capacitance for a small physical size as the distance between the plates, d is very small.

                        

The majority of electrolytic types of capacitors are Polarized, that is the DC voltage applied to the capacitor terminals must be of the correct polarity, i.e. positive to the positive terminal and negative to the negative terminal as an incorrect polarization will break down the insulating oxide layer and permanent damage may result.

All polarized electrolytic capacitors have their polarity  clearly marked with a negative sign to indicate the negative terminal and this polarity must be followed.

Tantalum  Electrolytic Capacitor:

Tantalum Electrolytic Capacitors and Tantalum Beads, are available in both wet (foil) and dry (solid) electrolytic types with the dry or solid tantalum being the most common. Solid tantalum capacitors use manganese dioxide as their second terminal and are physically smaller than the equivalent aluminum capacitors.

The dielectric properties of tantalum oxide is also much better than those of aluminium oxide giving a lower leakage currents and better capacitance stability which makes them suitable for use in blocking, by-passing, decoupling, filtering and timing applications.

Electrolytic’s are widely used capacitors due to their low cost and small size but there are three easy ways to destroy an electrolytic capacitor:

• Over-voltage – excessive voltage will cause current to leak through the dielectric resulting in a short circuit condition.

• Reversed Polarity – reverse voltage will cause self-destruction of the oxide layer and failure.

• Over Temperature –  excessive heat dries out the electrolytic and shortens the life of an electrolytic capacitor

Non-Polarized Capacitors: 

Non-polarized capacitors are those in which terminals are not defined as positive (+ve ) & negative(-ve). It could be used in circuit by any direction.

Ceramic Capacitor:

These are made by coating two sides of a small porcelain or ceramic disc with silver and are then stacked together to make a capacitor. For very low capacitance values a single ceramic disc of about 3-6mm is used. Ceramic capacitors have a high dielectric constant (High-K) and are available so that relatively high capacitance’s can be obtained in a small physical size.

They exhibit large non-linear changes in capacitance against temperature and as a result are used as de-coupling or by-pass capacitors as they are also non-polarized devices. Ceramic capacitors have values ranging from a few picofarads to one or two microfarads, ( μF ) but their voltage ratings are generally quite low.

Application of Capacitors

Applications                         Suitable Types to be used for

Power Supply smoothing          # Electrolytic Capacitor: High                                                           capacitance & high ripple current.

Audio signal coupling               # Electrolytic Capacitor: High                                                            capacitance

                                                  # Tantalum Capacitor: High                                                                capacitance & Small size.

                                                  # Polyester/ Poly-carbonate:                                                                Cheap, but capacitance  is                                                                  comparatively low.

Radio Frequency coupling       # Ceramic: Small, cheap and low                                                           loss, but low capacitance.

                                                  # Polystyrene:  Very low loss, but                                                        large in size & expensive.

Radio Frequency Decoupling  # Ceramic: Small, cheap and low                                                          loss, but low capacitance.

                                                  # Silver Mica: Close tolerance, low                                                      loss but expensive.

Tuned Circuits                         # Ceramic: Small, cheap and low                                                           loss, but Silver Mica is better.