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Capacitors

 

Draft / ripped from various places.


This is a measure of a capacitor's ability to store charge. A large capacitance means that more charge can be stored. Capacitance is measured in farads, symbol F. However 1F is very large, so prefixes are used to show the smaller values.

Three prefixes (multipliers) are used, µ (micro), n (nano) and p (pico):

  • µ means 10-6 (millionth), so 1000000µF = 1F
  • n means 10-9 (thousand-millionth), so 1000nF = 1µF
  • p means 10-12 (million-millionth), so 1000pF = 1nF

Polarised capacitors (large values, 1µF +)

Examples:   

electrolytic capacitors
 
 
 
 

    Circuit symbol:   

electrolytic capacitor symbol
 
 
 

Electrolytic Capacitors

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. It 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.

Tantalum Bead Capacitors

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.

 


 

Unpolarised capacitors (small values, up to 1µF)

Examples:   

small value capacitors
 
 
 
 

    Circuit symbol:   

capacitor symbol

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!

100nF capacitor
 
 
 
 


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:

1nF capacitor
 
 
 
 

 

  • 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).

10nF and 220nF capacitors
 
 
 
 

 

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.

Some values are indicated with a colour code similar to resistors. There can be some confusion.
A 2200pf capacitor would have three red bands. These merge into one wide red band.

COLOR CODING OF CAPACITOR - Tolerance Diagram Some values are marked in picofarads using three digit numbers. The first two digits are the base number and the third digit is a multiplier.
For example, 102 is 1000 pF and 104 is 100,000 pF = 100 nF = 0.1 uF.


Polystyrene Capacitors

polystyrene capacitor
 
 
 
 

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.


Real capacitor values (the E3 and E6 series)

You may have noticed that capacitors are not available with every possible value, for example 22µF and 47µF are readily available, but 25µF and 50µF are not!

Why is this? Imagine that you decided to make capacitors every 10µF giving 10, 20, 30, 40, 50 and so on. That seems fine, but what happens when you reach 1000? It would be pointless to make 1000, 1010, 1020, 1030 and so on because for these values 10 is a very small difference, too small to be noticeable in most circuits and capacitors cannot be made with that accuracy.

To produce a sensible range of capacitor values you need to increase the size of the 'step' as the value increases. The standard capacitor values are based on this idea and they form a series which follows the same pattern for every multiple of ten.

The E3 series (3 values for each multiple of ten)
10, 22, 47, ... then it continues 100, 220, 470, 1000, 2200, 4700, 10000 etc.
Notice how the step size increases as the value increases (values roughly double each time).

The E6 series (6 values for each multiple of ten)
10, 15, 22, 33, 47, 68, ... then it continues 100, 150, 220, 330, 470, 680, 1000 etc.
Notice how this is the E3 series with an extra value in the gaps.

The E3 series is the one most frequently used for capacitors because many types cannot be made with very accurate values.


 

Combi

 

ning Capacitors & Formula's:

Is it possible to combine capacitors to get to a certain value like we do with resistors? Certainly! Check below how go about it.

Capacitors in Parallel

 

 
 
 

Capacitors connected in parallel, which is the most desirable, have their capacitance added together, which is just the opposite of parallel resistors. It is an excellent way of increasing the total storage capacity of an electric charge:
Ctotal = C1 + C2 + C3

Keep in mind that only the total capacitance changes, not the supplied voltage. Every single capacitor will see the same voltage, no matter what. Be careful not to exceed the specified voltage on the capacitors when combining them all with different voltage ratings, or they may explode. Example: say you have three capacitors with voltages of 16V, 25V, and 50V. The voltage must not exceed the lowest voltage, in this case the 16V one. As a matter of fact, and a rule-of-thumb, always choose a capacitor which is twice the supplied input voltage. Example: If the input voltage is 12V you would select a 24V type (in real life 25V).

Capacitors in Series

 
 
 

Again, just the opposite way of calculating resistors. Multiple capacitors connected in series with each other will have the total capacitance lower than the lowest single value capacitor in that circuit. Not the preferred method but acceptable.



For a regular two capacitor series combo use this simple formula:

 
 
 


If you have two identical capacitors in series the formula is simplicity itself:

 
 
 

asdf

Capacitor symbols used in schematics

Past electronic magazines such as Radio Electronics, Hands-on Electronics, and Popular Electronics used the symbols in fig. (a), (d), (c), and (g). While european magazines such as the Dutch Elektuur (Elektor) uses symbols as depicted in (a), (e), and (f).
Symbol in (c) is a variable capacitor like a trimmer cap, and (g) is a ganged variable capacitor such as a air-plate capacitor as used in radios.
Electrolytic capacitors are frequently indicated by a symbol with one straight and one curved line (d) or the european way of drawing this symbol in (e). A '+' sign is placed at the straight line to indicate the anode. Occasionally an electrolytic is drawn as two straight lines, but the plus sign is always included to indicate its polarity.
When a capacitor is shown as one straight line and one curved one, the curved line, which represents the outer case or electrode of the device, is assumed to be at a lesser potential than the straight one. Thus, since signal flow in a schematic diagram is usually from left to right, capacitors are drawn with their curved ends facing left or, if that is not possible, facing down, which is the direction usually used to represent ground. Electrolytics, especially, are depicted with the curved place facing downward.

 

In schematics, capacitor values are usually indicated in microFarads unles a note specifies that things are otherwise. Voltage ratings, if they are given, are usually indicated in microFarads (uF) unless a note specifies that things are otherwise. Voltage ratings, if they are given, are usually presented as part of a "fraction." A label of "4.7uF/35V" or "4.7/35" would indicate a capacitor with a value of 4.7uF with a a working voltage of 35 volts.
MicroFarads are indicated with the greek letter 'u' (µ).

Bypassing:

The phrase "bypass" or "decoupling" is referred to filtering noise off the power rails caused by switching of TTL IC's, MosFets, Transistors, etc. Especially TTL (Transistor-Transistor-Logic) IC's create a lot of noise and so this has to be cleaned up. Mounting a 100nF (0.1uF) ceramic bypass capacitor over the power rails and as close to the IC as possible and keeping the capacitor leads as short as you can, will clean up noise nicely. This has to be done on all IC's and power rails on a printed circuit board with this kind of digital logic. Noise can cause all sorts of problems such as false triggering, cross-talk, change to an undesirable logic state, etc. Decoupling is used where the supply voltage cannot be lowered, i.e., if one needed a noise free +12V on a PC bus, for example. You could get a "clean" +12 volts with a voltage regulator... if only there was +15 volts or higher to start with. But such is not the case. So you use a high "Q" inductor or RFC choke along with the proper bypass capacitor to effectively lowpass filter the +12 volt supply rail. For a real noisy supply you can use more than one inductor: a "pie" network for example.
As digital becomes faster and faster, it looks more like analog than digital. It would be an asset to have a good understanding of the analog/rf properties of high speed digital. Careful layout of a groundplane and proper decoupling and bypass requires close attention of a circuit design to maintain the integrity of the power distribution.
Excessive lead length will void the purpose of a bypass capacitor so keep the leads short.

 

Tolerances

Value tolerances used to be marked on the capacitor in percentage. An older cap might be marked as ".05 10%", meaning 0.05fF, q10%. Newer caps use a letter for the tolerance, and that will seem confusing right at first. A common value might be ".1M" which means 0.1fF, q20%. A little newer cap might be marked as "332K" and that drives some folks nuts. After all, "K" is a standard metric prefix multiplier and they automatically think they have a 332,000fF cap on their hands. In reality, they have a 3300pF (33 + 2 zeros, in pF) with a q10% tolerance. The letters on these caps correspond to the following list. I've marked the tolerances that you'll find to be the most common with an asterisk.

 

                    
B = q0.1pF
C = q 0.25pF
D = q0.5pF
E = q0.25%
F = q1.0%
G = q2%
H = q2.5%
J = q5% *
K = q10% *
L = q15%
M = q20% *
N = q30%
P = -0, +100%
S = -20, +50%
W = -0, +200%
X = -20, +40%

Voltage Ratings

In addition to value and tolerance, a capacitor is often marked with a voltage rating. These may simply be noted as
"50V" or "50VDC" or some such other voltage as appropriate. Voltage ratings are sometimes incorporated into a
capacitors "coded description". For instance, the value code "2A104K" has a "2A" prefix which translates to a voltage
rating of 100V. The "104K" part, as you now know, translates to 100,000pF or 0.1fF or 100nF with a tolerance of 10%.
Voltage prefixes include:

                                     1E       25V
1H 50V
2A 100V


Since this seems to be European in nature, these voltage markings are new territory for me. I would appreciate more information on this so that I can flesh out this article and make it more accurate. My e-mail address appears in the "Wrapup" section following in case you would like to contact me with some of this information. I try to be accurate, so please make sure that you include source material rather than depending upon hand-me-down folklore!

Temperature Coefficient

Capacitors, most notably ceramic capacitors, have temperature coefficients ("tempco" or TC). That is, their value will change with a change in temperature. Some "bulk" ceramic capacitors (those "M" tolerance things) can change over 10 or 20 percent with a 20 degree shift in temperature, so are unsuitable for use in circuits that are frequency-dependent, such as oscillators or filters. Capacitance changes are not necessarily linear or even directly proportional at all times for a particular type of capacitor.

You'll see markings on a cap such as "Z5U". That's a temperature coefficient. The "Z" part has nothing to do with the tolerance. I have no intention of going into tempcos here since it has nothing to do with reading the value. I just wanted you to be aware of them so that you don't confuse them with the tolerance.

Z = -20, +80% *

You'll find that the "Z" tolerance of -20, +80% to be common for aluminum electrolytic caps and for disc ceramic caps that are used for what is known as "bulk capacitance" in applications such as power supply bypassing or filtering. These kinds of capacitors are used where it's OK for the value to be a lot larger than nominal, but they don't want it to go very far below that value.

If you do a lot of analog circuit design and building where you attempt to get frequency-dependent circuits to be as accurate as possible, you'll see or want to find Mylar caps with a "G" or "F" tolerances of q2% or q1% respectively. They're harder to find in catalogs, but if you watch the electronics surplus catalogs, you can find then on a sporadic basis. The tolerances of "B" through "E" are in pF vs. percent and are normally used on small caps of around 10pF or less.