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Capacitors

Capacitors store electrical energy by seperating positive and negative charges.  They store electrons by attracting them to a positive voltage.  When the voltage is reduced or removed the electrons move off aswell.  When the capacitor removes or adds electrons to the circuit it can work to smooth out voltage fluctuations.  The capacitor acts as a delay and can be combined with resistors.  If you increase the resistance it increases the delay in time.  A resistor and capacitor connected together can often be referred to as an RC circuit.  Any circuit I view that has a potential divider and features a capacitor will have something to do with time.

Power supplys that convert AC current to DC current often use capacitors to keep the voltage at a certain level. 

(AC current = alternating current and can run in different directions around the circuit, DC = direct current)

When a capacitor is connected in series with a signal source, such as a microphone, it can block the DC current but pass the AC current.  Most kind of amplifiers would use this function.

Capacitors can be used to make filters that reject AC signals above and below a desired frequency, by adjusting the value of the capacitor it can be possible to change the cut-off frequencies of the filter.

The capacitor itself is actually quite a simple device.  A typical capacitor has two metal plates inside it that are refrained from touching by a dielectric material which acts as an insulator seperating the two plates.  A common dielectric could be a material such as plastic or paper.

 A capacitor is measured by its capacitance and this is valued in Farads.  The higher the value the more electrons the capacitor can store at one time.  A single farad is a large unit of measurement however and so a large amount of capacitors are valued in micro (millionths) of a farad (uF), but there are even smaller measurements such as a nano (nF) and a pico farad (pF) which is a millionth of a millionth!

The working voltage or WV is the highest voltage a capacitor can withstand before the dielectric layers in the component become damaged.  If it is operating at high voltages a spark can develop within the capacitor that perforates the dielectric material and can short the unit.

A typical capacitor designed for a DC circuit rates at no more than 16-35 volts.  A higher voltage is not needed because 3.3 and 12 volts will power these circuits.  A general rule of thumb would be to select a capacitor with a working voltage of at least 10-15 percent more than the voltage in the circuit for safety.

Capacitors come in a variety of shapes.  The aluminium electrolytic and paper capacitors usually come in a cylindrical shape.  The tantalum electrolytic, ceramic, mica and polystyrene capacitors are typically dipped in an epoxy or plastic bath and gives them a more bulbous shape.

  Some capacitors will have their value in farads printed on them and this is quite common with the larger aluminium electrolytic types.  The smaller capacitors such as 0.1 or 0.01 uF mica disk capacitors use a three digit marking system that indicate it capacitance and tolerance.  This system is based on picofarads and a number using this marking system such as 103 will mean 10 followed by three zeros as in 10000 pico farads.  To convert pico to micro farads the decimal point is moved six places to the left, so if it is 10,000pF it would be converted to 0.01 Uf.

The markings on the individual capacitors would be as so:

 nn (a number from 01 to 99) = nn pF

101 = 0.0001 μF

102 =  0.001 μF

103 =  0.01 μF

104 = 0.1 μF

221 = 0.00022 μF

222 = 0.0022 μF

223 = 0.022 μF

224 = 0.22 μF

331 = 0.00033 μF

332 = 0.0033 μF

333 = 0.033 μF

334 = 0.33 μF

471 = 0.00047 μF

472 = 0.0047 μF

473 = 0.047 μF

474 = 0.47 μF

 Another system to look out for which is less common, uses numbers and letters and would be marked like this 4R1.  The R represents the position of the decimal point so it would mean 4.1.  This system does not indicate the units and so could be in micro or pico farads.  To test these capacitors I could use a multimeter or a capacitor meter.  It is worth noting that the capacitor should be plugged directly into the testing instrument as capacitance can increase with long leads making the reading less accurate.

 Capacitors in general can sometimes be less than exact and the value printed on the capacitor can differ from its actual value,  this is caused by manufacturing variations.  Like resistors, capacitors are rated by tolerance and this will come as a percentage.  On most capacitors a single code will indicate its tolerance.  This can either be found on its own on the capacitor itself or placed after the three number mark such as 103Z

With the three digit mark, such as 103Z.  The letter Z means a tolerance and in this case it would be of +80% to -20%.  These are the common code letters which will indicate the capacitors tolerance:

 B + 0.1 pF

D + 0.5 pF

F + 1%

G + 2%

J + 5%

K + 10%

M + 20%

Z + 80%, -20%C + 0.25 pF

 The value of a capacitor will change with temperature and this is known as temperature coefficient.  Again this should be indicated on the capacitor usually as a three digit code such as NP0 which indicates negative/positive zero.  A capacitor with this marking would indicate a high temperature tolerance.  Capacitor manufacturers have begun to use the EIA marking system which indicates temperature tolerance.  The three characters in each mark indicate its temperature tolerance and the maximum variation within the stated temperature range

 The table is as follows:

EIA CAPACITANCE CODE VS MICRO-PICO-NANOFARAD Go to:   Surface Mount Capacitor Kits
EIA CODE MF PF NF EIA CODE MF PF NF EIA CODE MF PF NF EIA CODE MF PF NF
0R5   0.5   300   30   621   620   104 0.1 100000 100
1R0   1   330   33   681   680   124 0.12   120
1R2   1.2   360   36   751   750   154 0.15   150
1R5   1.5   390   39   821   820   184 0.18   180
1R8   1.8   430   43   911   910   224 0.22   220
2R0   2   470   47   102 0.001 1000 1 474 0.47   470
2R2   2.2   510   51   112 0.0011 1100 1.1 105 1   1000
2R7   2.7   560   56   122 0.0012 1200 1.2        
3R0   3   620   62   132 0.0013 1300 1.3        
3R3   3.3   680   68   152 0.0015 1500 1.5        
EIA CODE MF PF NF EIA CODE MF PF NF EIA CODE MF PF NF EIA CODE MF PF NF
3R9   3.9   750   75   162 0.0016 1600 1.6        
4R0   4   820   82   182 0.0018 1800 1.8        
4R7   4.7   910   91   202 0.002 2000 2        
5R0   5   101   100   222 0.0022 2200 2.2        
5R6   5.6   111   110   242 0.0024 2400 2.4        
6R0   6   121   120   272 0.0027 2700 2.7        
6R8   6.8   131   130   332 0.0033 3300 3.3        
7R0   7   151   150   392 0.0039 3900 3.9        
8R0   8   161   160   472 0.0047 4700 4.7        
8R2   8.2   181   180   562 0.0056 5600 5.6        
9R0   9   201   200   682 0.0068 6800 6.8        
100   10   221   220   822 0.0082 8200 8.2        
EIA CODE MF PF NF EIA CODE MF PF NF EIA CODE MF PF NF EIA CODE MF PF NF
110   11   241   240   103 0.01 10000 10        
120   12   271   270   153 0.015 15000 15        
130   13   301   300   183 0.018 18000 18        
150   15   331   330   223 0.022 22000 22        
160   16   361   360   273 0.027 27000 27        
180   18   391   390   333 0.033 33000 33        
200   20   431   430   393 0.039 39000 39        
220   22   471   470   473 0.047 47000 47        
240   24   511   510   563 0.056 56000 56        
270   27   561   560   683 0.068 68000 68        

The final mark that can be found on a capacitor especially tantalum and aluminium electrolytic types is a polarity symbol.  Most capacitors will use the minus (-) for the negative terminal but the (+) will not be shown for the positive.  Only the larger value capacitors such as 1uF and upward will be polarized and therefore will feature a marking.  If the capacitor is polarized it is even more vital to ensure that it is in the circuit the right way round to avoid damaging it or the other components.

To calculate capacitance there are a number of different formulas that are basically the inverse of the formulas for resistors.

To calculate the value of numerous capacitors laid in parallel I simply have to add them up C1+C2+C3=total capacitance.

To calculate the capacitance of two capacitors wired in series would be: total capacitance =C1 x C2 divided by C1 + C2.

To calculate three or more capacitors wired in series it is a little more complicated:

total capacitance=                                 1

                                                                                                                                                                                   

              1  divided by C1        +             1  divided by C2              +      1 divided by C3

 

 

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