FACTORS TO BE OBSERVED WHILE CHOOSING A CAPACITOR

(1). WORKING VOLTAGE: The working voltage of capacitor should be 10 to 20 percent higher than the circuit voltage.

(2). CAPACITANCE: The capacitance of the capacitor selected should be exactly or nearly equal to the required value.

(3).TYPE OF DIELECTRIC: The selection of the dielectric used in the capacitor depends on the dielectric used in the capacitor depends on the circuit frequency:

(a) For filter circuit (50 Hz) – paper and electrolytic capacitors.

(b)A.F. circuits (up to 20 KHz) – paper, mica and ceramic capacitors.

(c) R.F. circuits (above 20 KHz) –mica, ceramic, air and vacuum dielectric capacitors.

(3)CIRCUITS REQUIREMENT: A fixed, variable or adjustable capacitor is selected depending on the circuit requirements.

(4)PRICE: Normally, the order of price of a capacitor lies in the following descending order – Electrolytic, ceramic, mica, paper etc. Hence, the selection of a capacitor depends on its price also.

Bypass capacitor

Caps in dc circuits are either used to stabilize voltage levels or prevent them from changing too fast or to obstruct out DC components of a signal.


A capacitor engaged to conduct an alternating current around a component or group of apparatus. Often the AC is removed from an AC/DC mixture, the DC being free to pass through the bypassed component. Noise caused by other circuit elements is shunted through the capacitor, sinking the effect they have on the rest of the circuit.

In practice, most digital circuits such as microcontroller circuits are designed as direct current (DC) circuits. It turns out that difference in the voltages of these circuits can cause problems. If the voltages dangle too much, the circuit may operate incorrectly.

For most practical purposes, a voltage that fluctuates is considered an AC component. The function of the bypass capacitor is to dampen the AC, or the noise. Another term used for the bypass capacitor is a filter cap.

Bypass capacitors supply power pulses to the components that they bypass. These both allows the component to function normally since the pulses no longer get dropped across the power bus impedance and defend the rest of the system from the power pulses that the component requires.

We often add another bypass capacitor since the different capacitors work for different frequencies. We usually need at least two in critical high frequency systems, say a 0.01uF and a 100pF. If the system sees low frequency pulses as well as would be the case if you were taking a microcontroller in and out of power down you should add a 1-10uF as well.

COUPLING CAPACITOR

A "coupling capacitor" where the capacitor is used to bond a signal from one part of a circuit to another but without allowing any direct current to flow.

Use of coupling capacitor in analog circuits is to connect two circuits such that only the Alternating Current signal from the first circuit can pass through to the next while Direct Current is blocked. This technique helps to separate the DC bias settings of the two coupled circuits. Capacitive coupling is also known as AC Coupling and the capacitor used for the purpose is known as a Coupling or DC blocking capacitor.

Capacitive coupling has the disadvantage of degrading the low frequency performance of a system containing capacitively coupled units. Each coupling capacitor along with the input electrical impedance of the next stage forms a high-pass filter and each successive filter results in a cumulative filter with a -3dB frequency that may be higher than each individual filter.

So for sufficient low frequency response the capacitors used must have high capacitance ratings. They should be high adequate that the reactance of each is at most a tenth of the input impedance of each stage, at the lowest frequency of interest. This disadvantage of capacitively coupling DC biased, transistor amplifier circuits is largely minimized in directly coupled designs.

AC coupling is also broadly used in digital circuits to spread digital signal with a zero DC constituent, known as DC-balanced signals. DC-balanced waveforms are useful in communications systems, since they can be used over AC-coupled electrical connections to stay away from voltage imbalance problems and charge accumulation between connected systems or apparatus.

Mostly coupling capacitors are used in an audio system the signals must pass through with little or no distortion. This is achieved if the time constant (RC) is larger than the time period (T) of the lowest frequency audio signals required.

CAPACITOR'S COLOUR CODE

Like resistors the capacitance value of modulated paper and mica and ceramic capacitors is expressed by means of colour dots or bands while the capacitance of other types of capacitors is printed on them. The method of expression of the value of a carbon resistor is recognized internationally and their exists only one method. But, there are following four methods for the expression of the capacitance value of a capacitor:

1. Three dots method of E.I.A.:
 
The Electronics Industrial Association method is used for the expression of the capacitance value of moulded mica capacitors. Their working voltage is 500 volts and their tolerance is +/-20% and the capacitance value is expressed in Pico farads.

Three dots method of E.I.A

2. Six Dots Method of E.I.A.:

This method is also used for the expression of the capacitance value of moulded mica capacitors. The working voltage and the tolerance of the capacitor may also be expressed by this method and the capacitance value is expressed in Pico farads.

Six dots method

3. Six Dots Method of J.A.N. Or A.W.S.:

The joint Army Navy or American War Standard method is used for the expression of the capacitance value of moulded paper and mica capacitors. In this method also, the capacitance is expressed in Pico farads.

Six dots method of J.A.N.

4. Colour Code of Ceramic Capacitors:

Five dots method

There are two methods:

Colour code of ceramic capacitors

(a)  Dot Method:

In this method, 5 dots of different colours are marked on the capacitor. Out of 5 dots, the first one is thicker than others and then there are second, third, fourth and fifth dots. In this method also, the capacitance is expressed in Pico farads.

(b) Band Method:

Band method for ceramic capacitors

In this method, 5 bands of different colours are marked on the capacitor in place of dots. The first band indicates temperature coefficient, second band indicates first digit of the capacitance value, third band indicates second digit of the capacitance value, fourth band indicates the no. of zeros to be put after two digits and the fifth band indicates tolerance value of the capacitor.

The colour code for this method is shown in figure. In this method also the capacitance value is expressed in Pico farads.

Examples