Capacitor

ITS ABOUT CAPACITOR

A capacitor is an electronic device for storing charge. Capacitors can be found in almost all but the most simple electronic circuits. There are many different types of capacitor but they all work in essentially the same way. A simplified view of a capacitor is a pair of metal plates separated by a gap in which there is an insulating material known as the dielectric.

1. FIXED CAPACITOR
1.1. Picture

Electrolytic Capacitor

Ceramic , mica , polyester Capacitor


 

SMD Capacitor





1.2.  Symbol

  Polarised Capacitor

Unpolarised Capacitor


1.3. Capacitor Marking

A. Polarised Capacitor
A.1. Electrolytic Capacitor

 

Electrolytic capacitors are polarised and they must be connected the correct way round, at least one of their leads will be marked + or -. They are not damaged by heat when soldering.
There are two designs of electrolytic capacitors; axial where the leads are attached to each end (220µF in picture) and radial where both leads are at the same end (10µF in picture). Radial capacitors tend to be a little smaller and they stand upright on the circuit board.
It is easy to find the value of electrolytic capacitors because they are clearly printed with their capacitance and voltage rating. The voltage rating can be quite low (6V for example) and it should always be checked when selecting an electrolytic capacitor. If the project parts list does not specify a voltage, choose a capacitor with a rating which is greater than the project's power supply voltage. 25V is a sensible minimum for most battery circuits.

A.2. Tantalum Bead Capacitors

 

Tantalum bead capacitors are polarised and have low voltage ratings like electrolytic capacitors. They are expensive but very small, so they are used where a large capacitance is needed in a small size.
Modern tantalum bead capacitors are printed with their capacitance, voltage and polarity in full. However older ones use a colour-code system which has two stripes (for the two digits) and a spot of colour for the number of zeros to give the value in µF. The standard colour code is used, but for the spot, grey is used to mean × 0.01 and white means × 0.1 so that values of less than 10µF can be shown. A third colour stripe near the leads shows the voltage (yellow 6.3V, black 10V, green 16V, blue 20V, grey 25V, white 30V, pink 35V). The positive (+) lead is to the right when the spot is facing you: 'when the spot is in sight, the positive is to the right'.
For example:   blue, grey, black spot   means 68µF
For example:   blue, grey, white spot   means 6.8µF
For example:   blue, grey, grey spot   means 0.68µF

B. Unpolarised Capacitor

Small value capacitors are unpolarised and may be connected either way round. They are not damaged by heat when soldering, except for one unusual type (polystyrene). They have high voltage ratings of at least 50V, usually 250V or so. It can be difficult to find the values of these small capacitors because there are many types of them and several different labelling systems!




Many small value capacitors have their value printed but without a multiplier, so you need to use experience to work out what the multiplier should be!
For example 0.1 means 0.1µF = 100nF.
Sometimes the multiplier is used in place of the decimal point:
For example:   4n7 means 4.7nF. 

Capacitor Number Code
A number code is often used on small capacitors where printing is difficult:
•    the 1st number is the 1st digit,
•    the 2nd number is the 2nd digit,
•    the 3rd number is the number of zeros to give the capacitance in pF.
•    Ignore any letters - they just indicate tolerance and voltage rating.
For example:   102   means 1000pF = 1nF   (not 102pF!)
For example:   472J means 4700pF = 4.7nF (J means 5% tolerance).

 

Capacitor Colour Code
A colour code was used on polyester capacitors for many years. It is now obsolete, but of course there are many still around. The colours should be read like the resistor code, the top three colour bands giving the value in pF. Ignore the 4th band (tolerance) and 5th band (voltage rating).


For example:


    brown, black, orange   means 10000pF = 10nF = 0.01µF.
Note that there are no gaps between the colour bands, so 2 identical bands actually appear as a wide band.
For example:
    wide red, yellow   means 220nF = 0.22µF

Polystyrene Capacitors
This type is rarely used now. Their value (in pF) is normally printed without units. Polystyrene capacitors can be damaged by heat when soldering (it melts the polystyrene!) so you should use a heat sink (such as a crocodile clip). Clip the heat sink to the lead between the capacitor and the joint.

1.4. Capacitor Networks

a. Series
b. Paralel

Series
Consider the series network of capacitors shown in Figure a.) where the positve plate is conected to the negative plate of the next.What is the equivalent capacitance of the network? Look at the plates in the middle, these plates are physically disconected from the circuit so the total charge on them must remain constant. It follows that when a voltage is applied across both of the capacitors, the charge +Q on the positive plate of capacitor C1 must be balanced by the charge -Q on the negative plate of capacitor C2. The net result is that both capacitors possess the same charge Q. The potential drops V1 and V2 across the two capacitors are in general, different. However, the sum of these drops equals the total potential drop V applied across the input and output wires. V=V1 + V2. The equivalent capacitance of the pair is again CT=Q/V. Thus, 1/CT = V/Q = (V1 + V2)/Q = V1/Q + V2/Q giving

In general, for N capacitors connected in series, is

By connecting capacitors in seires you store less charge so does ever make sense to connect capacitors in series? It is sometimes done because capacitors have maximum working voltages, and with two 900 volt maximum capacitors in series, you can increase the working voltage to 1800 volts.

Parallel
For a parallel circuit such as in Figure b.) the voltages are the same across each component. However the total charge is divided between the two capacitors since it must distribute itself such that the voltage across the two is the same. Also, since the capacitors may have different capacitances C1 and C2 the charges Q1 and Q2 must also be different. The equivalent capacitance CT of the pair of capacitors is simply the ratio Q/V where Q=Q1+Q2 is the total stored charge. It follows that CT = Q/V = (Q1+Q2)/V = Q1/V + Q2/V giving

It is fairly obvious from the previous discussion that for N capacitors in parallel, the total capacitance is

The overall capacitance increases by added together capacitors in parallel so we create larger capacitances than is possible using a single capacitor. High-energy physics labs often have large banks of capacitors which can store large quantities of energy to be released in a very short time.

2. VARIABLE CAPACITOR
2.1. Tuning Capacitor
A. Picture

 

 

A variable capacitor is a capacitor whose capacitance may be intentionally and repeatedly changed mechanically or electronically.
Variable capacitors are mostly used in radio tuning circuits and they are sometimes called 'tuning capacitors'. They have very small capacitance values, typically between 100pF and 500pF (100pF = 0.0001µF)..
Many variable capacitors have very short spindles which are not suitable for the standard knobs used for variable resistors and rotary switches. It would be wise to check that a suitable knob is available before ordering a variable capacitor.
Variable capacitors are not normally used in timing circuits because their capacitance is too small to be practical and the range of values available is very limited. Instead timing circuits use a fixed capacitor and a variable resistor if it is necessary to vary the time period.

B. Symbol

 

C. Tuning Capacitor marking
A Tuning Capacitor value typically between 100pF and 500pF. Its value is printed:
-    100, mean this capacitor can be trimmed from 0-100pf
-    220, mean this capacitor can be trimmed from 0-220 pf, etc

2.2. Trimmer Capacitor
A. Picture

Trimmer capacitors (trimmers) are miniature variable capacitors. They are designed to be mounted directly onto the circuit board and adjusted only when the circuit is built.
A small screwdriver or similar tool is required to adjust trimmers. The process of adjusting them requires patience because the presence of your hand and the tool will slightly change the capacitance of the circuit in the region of the trimmer!
Trimmer capacitors are only available with very small capacitances, normally less than 100pF. It is impossible to reduce their capacitance to zero, so they are usually specified by their minimum and maximum values, for example 2-10pF.

B. Symbol

C. Trimmer Capacitor marking
A Trimmer Capacitor value normally less than 100pF. Its value is printed:
-    10, mean this capacitor can be trimmed from 0-10pf
-    20, mean this capacitor can be trimmed from 0-20pf ,etc

References:
1. “Succes in Electronic”  by Tom Duncan
2. Wikipedia.org
3. kpsec.freeuk.com