EMBEDDED ADVENTURES: CONNECT THE WORLD AROUND YOU
Tutorials in this section:
Capacitors are found in virtually every area of electronics, as they are used for a variety of different tasks.
Some capacitors are used for coupling, others for decoupling, while others may be used in filters, and some may be used for power supply smoothing. Some capacitors are designed for use in low frequency circuits, while others are designed for high frequency circuits.
In addition to the normal usage of capacitors intended to be part of a circuit design, there are also "parasitic" capacitances. These are the unwanted stray capacitances that are found in almost any type of component, such as resistors, transistors, IC's, and even inductors. As mentioned in the first article on capacitors, any two conductors with an insulating space in between can be considered a capacitor. Even two traces on a PC board can have a small capacitance. These capacitances are usually very small, however, and is usually in the range of picofarads (pF), where 1 picofarad = 1 micro-microfarad, and these small capacitances would only be a problem at very high frequencies.
The capacitor itself has many unwanted characteristics that will hinder the operation of a circuit. For example, many types will lose a significant amount of their capacitance at high frequencies, making them unsuitable for RF applications. Some types will have parasitic inductances that will also slow down the operation of a circuit. The capacitance of a capacitor can also change value with the circuit frequency (Hz) and/or with the ambient temperature.
Many types are very sensitive to temperature extremes.
Electrolytic capacitors tend to lose capacitance at both higher and lower temperatures outside of their range. Depending on the dielectric material, many capacitors will burn out if AC current is applied for long period at a high enough amplitude and frequency, even though it’s operating within the specified ranges.
The average hobbyist would probably not have to know all the details on the various types, as they normally use only common electrolytic and general purpose ceramic capacitors, but they may sometimes need a capacitor with better temperature stability, like metal-film or polypropylene. One of the first things to consider when choosing a capacitor is the range of operation, so a capacitor can be found that will work reliably in their circuit. They should also have some idea of the size and shape the capacitor that would to fit into their circuit, and whether it should be axial, with a lead coming out of each end like a resistor, or radial, with both leads coming out of the same end.
When deciding on what type of capacitor to use in a circuit, it's important to consider the various advantage and disadvantages of their operating parameters. Some of the parameters that are normally considered include: Nominal Capacitance (C), Working Voltage (WV), Tolerance (±%), Leakage Current, Working Temperature (T), Temperature Coefficient (TC), Polarization, and Equivalent Series Resistance (ESR).
Capacitance was already discussed, and values can range from 1 pF in a ceramic to about 1 Farad in an Electrolytic capacitor (or super-capacitor).
Working Voltage is the maximum continuous voltage either DC or AC (rms) that can be applied to the capacitor without failure while operating in its temperature range. Generally, the working voltage printed onto the side of a capacitor refers to its DC working voltage. Working voltage can be anywhere from 10V to 1000V, depending on type.
Leakage Current is determined by the insulation resistance as voltage is applied, and is the very small leakage current that flows through the dielectric or around its edges. A lower dielectric constant means a higher insulation resistance. Polypropylene and Polystyrene capacitors have low leakage currents and Electrolytic-type capacitors (tantalum and aluminum) have very high leakage currents.
Working Temperature is the range of temperature at which the value of capacitance can still remain within its tolerance level.
Temperature Coefficient is the maximum change in capacitance over a specified temperature range. The temperature coefficient of a capacitor is generally expressed as parts per million per degree centigrade (PPM/°C), or as a percent change over a particular range of temperatures.
Some capacitors (Class 2 capacitors) increase their value as the temperature rises, and these have a positive temperature coefficient (P). Some capacitors decrease their value as the temperature rises, and these have a negative temperature coefficient (N). For example "P150" means +150 ppm/°C and "N100" means -100 ppm/°C, etc. Some capacitors, such as Ceramic, Mica, or Polyester, have “NPO” marked on them, and this means Negative-Positive-Zero, which is the same as zero temperature coefficient.
Polarization generally refers only to electrolytic or tantalum type capacitors. These are usually polarized in that the voltage connected to the capacitor terminals must have the correct polarity, i.e. positive to positive and negative to negative. Incorrect polarization can cause the oxide layer inside the capacitor to break down. Polarized capacitors always have their negative terminal marked in a variety of ways, depending on type.
Equivalent Series Resistance is the AC impedance of the capacitor when used at high frequencies and is the resistance measured between the two leads at a particular frequency and temperature.
Normally, only the value of capacitance, working voltage, and sometimes tolerance is printed or coded onto a capacitor in a variety of ways. If the capacitor is polarized, this will be marked, also. Ceramic, Mica, and Polyester capacitors will sometimes have their temperature coefficient printed on them as explained above.
Each family or type of capacitor uses its own unique identification system with some sort of lettering scheme, color codes, or number codes. Once a certain type of capacitor has been chosen (ceramic, film, plastic or electrolytic, etc), the type’s coding scheme can be used to figure out what the capacitor label means.
There are four basic categories of uses that are common for capacitors):
Coupling capacitor - Allows AC signals to pass from one section of a circuit to another while blocking any DC components.
Decoupling capacitor - Removes any AC signals that may be on a DC bias point or power rail.
Smoothing capacitor - Essentially the same as a decoupling capacitor, but is normally used only in conjunction with a power supply.
Timing capacitor - Is used with a resistor and/or inductor in a resonant or time dependent circuit. The capacitor would provide the element needed for filtering, oscillating, tuning a circuit, etc. A timing capacitor could also be used for timing circuits where the time it takes to charge and discharge determines the operation of the circuit.
The list blow provides a more detailed explanation of the more common types of capacitors:
Ceramic Capacitors
A Ceramic disc capacitor is used where a low-cost, small, accurate capacitance is required with good temperature stability. Ceramic types normally perform very well at high frequencies, and have a very low loss factor. They are made by coating two sides of a small porcelain or ceramic disc with silver and are then are combined in many layers. Sometimes a single ceramic disc is used for very low capacitance. Ceramic dielectrics generally do not have as high a level of capacitance per unit volume as some types which results in lower capacitance values. Ceramic capacitors are used mostly for de-coupling or by-passing high frequency signals to ground. They are not used so much for other applications because they exhibit large non-linear changes in capacitance against temperature.
There is a variety of dielectric types for ceramic capacitors, the more common forms are:
COG: This type has low capacitance values, but gives a high level of stability because of a low dielectric constant.
X7R: This has higher capacitance values as it has a much higher dielectric constant than COG, but with lower stability.
Z5U: This is used for even higher values of capacitance, but has a lower stability than either COG or X7R.
Electrolytic Capacitors
Electrolytic capacitors are the most popular type for values greater than about 1 microfarad, having one of the highest levels of capacitance for a given volume.
This type usually uses aluminum foil as the plate material, although other types of foil are used here, also.
A very thin layer of oxide is grown electro-chemically or by using an anodizing process on one of the foil plates to form the dielectric. This film is less than ten microns thick, and makes it possible to make capacitors with a large value of capacitance in a small package. A semi-liquid electrolyte solution or paper soaked with electrolyte is sheet is placed between the two foils and then they are wound around on one another and placed into a can.
There is also an “etched foil” type differs from the plain foil type just described in that the aluminum oxide on the anode and cathode foils is chemically etched to increase its surface area and permittivity. Etched foil electrolytic capacitors are normally used in coupling, DC blocking and by-pass circuits while plain foil types are better suited as smoothing capacitors in power supplies.
Electrolytic capacitors are polarized, and if they are connected with a reverse voltage, it will cause the thin oxide layer to break down and damage the device. Because of the thin layer, the working voltage is much lower than other types of capacitors, so care must be used here also.
The tolerance range for electrolytic capacitors is quite large at around 20%. They do not operate well at high frequencies and are limited to about 100 kHz maximum. Typical values of capacitance for an aluminum electrolytic capacitor range from 1uF up to 47,000uF.
Tantalum Capacitors
Tantalum capacitors are similar to electrolytic capacitors, but instead of using a film of oxide on aluminum they us a film of oxide on tantalum.
Using tantalum oxide as a dielectric provides much lower leakage currents and better capacitance stability.
It also provides much higher levels of capacitance per unit volume than an electrolytic. This capacitor type is also polarized, and the positive lead is positive lead is usually identified by a polarity mark. Their working voltage is much less than an electrolytic, and it can range from about one volt to a maximum of 35 volts. Tantalum capacitors are more suitable for use in blocking, by-passing, decoupling, filtering and timing applications than electrolytic capacitors.
Typical values of capacitance range from 47nF to 470uF.
Film Capacitors
Film Capacitors are the most widely available of all types of capacitors, consisting of a large number of different types with the main difference between them being dielectric properties. A few types of film capacitors are listed as follows, with Polycarbonate, Polyester and Polystyrene being the most common type used:
Polycarbonate – These are the most often chosen for temperature stability. These capacitors feature a very high insulation resistance, a low temperature coefficient, and moderate levels of dielectric loss which can increase with frequency.
Polyester – These capacitors also have a very high insulation resistance and moderate levels of dielectric loss which can increase with frequency, but the tolerance is usually around 5% to 10%. Polyester film capacitors used where a low-cost medium capacitance is required at a moderately high voltage with moderately good temperature stability and tolerance.
Polystyrene – These have good temperature stability, low tolerance, and low loss, but tend to be bulky. Drawbacks include relatively high inductance and cost and susceptibility to cleaning solvents. They are mostly used where a small capacitance is needed and in tuned circuits (oscillators and filters) where frequency stability is important.
Polypropylene - These have very high tolerance (1%), very low dielectric losses and almost no change in capacitance over the frequency range. Polypropylene capacitors are used mostly in power electronics applications.
Film type capacitors are usually available in capacitance ranges from as small as 5pF to as large as 100uF is depending upon the actual type of capacitor and its voltage rating.
Normally they are non-polar. They tend to be used for a variety of purposes as a general-purpose capacitor, although their high frequency performance is not usually as good as that of the ceramic types.
The main advantage of plastic film capacitors compared to paper film types is that they operate well under conditions of high temperature, have smaller tolerances, a very long service life and high reliability. The paper types are rarely used because of susceptibility to moisture, unpredictable tolerance, and difficulty of manufacture compared with modern plastic dielectric types.
Silver Mica Capacitors
Silver mica capacitors use mica as the dielectric and have silver plates. They are made by plating silver electrodes directly on to the mica film dielectric. To achieve the required capacitance, several layers are used.
They work well at high frequencies and have a high temperature coefficient. They are ideal for applications such as filters, resonant circuits, and RF oscillators, although they are expensive. Because of this , they are used much less nowadays, as ceramic and some plastic film capacitor types have been developed with very good levels of performance, silver mica capacitors are used much less than previously. However, they are still used at times because of their known reliability, stability, and low loss.
Many specialized dielectric materials have recently been developed, such as PTFE (polytetrafluoroethylene), which are used for demanding applications where extremely high temperature stability is required. These types offer no advantage for ordinary circuits, as they are used only in aircraft/spacecraft, medical equipment, and other critical applications.
The most precise capacitors are actually those that are found in ICs. They are deposited on a substrate as the IC is manufactured.
These capacitors are made oversize intentionally, and then trimmed with a laser until they resonate at exactly the right frequency in a circuit. The resulting capacitor would then be used in a circuit that requires precise values of capacitance, such as a tuned circuit.
It would be very difficult, if not impossible to achieve this level of preciseness with other manufacturing processes.