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= Capacitors

Capacitors are common components of electronic circuits, used almost as frequently as resistors. Basic difference between the two is the fact that capacitor resistance (called reactance) depends on voltage frequency, not only on capacitors' features. Common mark for reactance is Xc and it can be calculated using the following formula:

f representing the frequency in Hz and C representing the capacity in Farads.

For example, 5nF capacitor's reactance at f  = 125kHz equals:

while, at = 1.25MHz, it equals:

Capacitor has infinitely high reactance for direct current, because f =0.

Capacitors are used in circuits for filtering signals of specified frequency. They are common components of electrical filters, oscillator circuits, etc.

Basic characteristic of capacitor is its capacity -  higher the capacity is, higher is the amount of electricity capacitor can accumulate. Capacity is measured in Farads (F). As one Farad represents fairly high capacity value, microfarad (F), nanoFarad (nF) and picoFarad (pF) are commonly used. As a reminder, relations between units are (1F= 106F = 109nF = 1012pF) that is 1F=1000nF and 1nF=1000pF. It is essential to remember this notation, as same values may be marked differently in different electrical schemes. For example, 1500pF may be used interchangeably with 1.5nF, 100nF may replace 0.1F, etc. Bear in mind that simpler notation system is used, as with resistors. If the mark by the capacitor in the scheme reads 120 (or 120E) capacity equals 120pF, 1n2 stands for 1.2nF, n22 stands for 0.22nF, while .1 (or .1u) stands for 0.1F capacity and so forth.

Capacitors come in various shapes and sizes, depending on their capacity, working voltage, insulator type, temperature coefficient and other factors. All capacitors can divided in two groups: those with changeable capacity values and those with fixed capacity values.

1. The Block-Capacitors

Commonly, capacitors are marked by a number representing the capacity value printed on the capacitor. Beside this value, number representing the maximal capacitor working voltage is mandatory, and sometimes tolerance, temperature coefficient and some other values are printed too. If, for example, capacitor mark in the scheme reads 5nF/40V, it means that capacitor with 5nF capacity value is used and that its maximal working voltage is 40v. Any other 5nF capacitor with higher maximal working voltage can be used instead, but they are as a rule larger and more expensive.

Sometimes, especially with capacitors of low capacity values, capacity may be represented with colors, similar to four-ring system used for resistors . The first two colors (A and B) represent the first two digits, third color (C) is the multiplier, fourth color (D) is the tolerance, and the fifth color (E) is the working voltage.

With disk-ceramic capacitors  and tubular capacitors  working voltage is not specified, because these are used in circuits with low or no DC voltage. If tubular capacitor does have five color rings on it, then the first color represents the temperature coefficient, while the other four specify its capacity value in the previously described way.

 Black 0  x 1 pF 20%  
 Brown 1  x 10 pF 1%  
 Red 2  x 100 pF 2% 250V
 Orange 3  x 1 nF 2.5%  
 Yellow 4  x 10 nF   400V
 Green 5  x 100 nF 5%  
 Blue 6  x 1 F    
 Violet 7  x 10 F    
 Grey 8  x 100 F    
 White 9  x 1000 F 10%  

Marking the capacity using colors

2. Electrolytic Capacitors

Electrolytic capacitors represent the special type of capacitors with fixed capacity value. Thanks to the special construction, they can have exceptionally high capacity, ranging from one to several thousand F. They are most frequently used in transformers for leveling the voltage, in various filters, etc.

Electrolytic capacitors are polarized components, meaning that they have positive and negative connector, which is of outmost importance when connecting the capacitor into a circuit. Positive connector has to be connected to the node with a high voltage than the node for connecting the negative connector. If done otherwise, electrolytic capacitor could be permanently damaged due to electrolysis and eventually destroyed.

Explosion may also occur if capacitor is connected to voltage that exceeds its working voltage. In order to prevent such instances, one of the capacitor's connectors is very clearly marked with a + or -, while working voltage is printed on capacitor body.

Several models of electrolytic capacitors, as well as their symbols, are shown on the picture below.

Electrolytic capacitors

Tantalum capacitors represent a special type of electrolytic capacitors. Their parasitic inductance is much lower then with standard aluminum electrolytic capacitors so that tantalum capacitor with significantly (even ten times) lower capacity can completely substitute an aluminum electrolytic capacitor.

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