This entry is part 21 of 24 in the series Electronics DC Revision

Practical Capacitors

There are a wide variety of capacitor types which use a number of different construction materials. Here is a broad selection of the ind of thing that is available.

Capacitor Types

Fixed Values

Ceramic Disc

Made by coating a ceramic disc with silver contacts on both sides. To make larger value capacitors they are constructed from multiple layers connected in parallel. They are NOT polarised.

Once the most prolific type used, with values ranging from a few pF up to 100µF and voltage ratings from just a few volts to several kV. Today they are mostly replaced by MLCC capacitors due to their smaller size.

Multi-Layer Ceramic (MLCC)

Made by mixing finely ground granules of para-electric and ferro-electric materials and alternatively layering the mix with metal contacts. Finally the mixture is sintered to form a solid mass. They consist of 500 or more layers and are essentially lots of small capacitors in parallel to form the desired value. They are NOT polarised.

Each layer is currently around 0.5 micron thick, but constantly reducing, yielding even smaller volumes for the same value.

They are the most abundant capacitor in use today. Available in both through hole and surface mount (SMD) formats.

Ceramic capacitors are manufactured in two grades: Class 1 and Class 2.

Class 1 are manufactured to very low tolerance nominal values (typically 1%) and a capacitance thermal stability of ±0.54% operating in the -55℃ to +125℃ range.

Class 2 tend to have a higher capacitance for a given volume, but only offer around 20% nominal value accuracy with thermal stability nearer to ±15%. As you may expect they are cheaper and far more common.

The value of capacitance may, or may not, be marked on the device. As devices get physically smaller it has become more difficult to provide any meaningful markings. If marked at all, it is a 1, 2, or 3 digit code giving the value in picofarads (pF). The first 1 or 2 digits give the value in pF. The 3rd digit (if present) is a power of 10 multiplier (often thought of as how many zero’s to add). For example:

Marking of “5” on it’s own means a nominal value of 5pF
Marking of “47” means 47pF
Marking “472” means 4700pF = 4.7nF
Marking “476” means 47,000,000pF = 47µF

Electrolytic

There are several types of electrolytic capacitor. However, when used in an unqualified way, people are usually talking about aluminium foil electrolytic capacitors, and that is what is being described here.

All electrolytic capacitors are polarised.

They are constructed from two sheets of aluminium foil sandwiching a sheet of paper soaked in an electrolyte, which is then rolled up in to a tight wad. One of the two aluminium sheets is coated with a layer of oxide, and that sheet acts as the anode. The other, uncoated, electrode acts as the cathode. During operation the anode must be kept more positive than the cathode.

They typically have values between 1µF and 47mF. However, the vast capacity comes with a cost…

They have a relatively poor tolerances for the nominal value (typically 20%), additionally they are not time stable. So as they age they get less and less accurate. they also tend to have high leakage currents, and a relatively high equivalent series resistance.

Values of devices are marked in one of two ways:

In the case of through hole (TH) mounted devices (radial or axial leads), the value is marked directly on the side of the can in micro-farads (µF) along with the voltage rating in volts. Polarity is typically marked against the cathode (negative) terminal (usually a stripe of ‘-‘ signs) on the side of the can.

In the case of Surface Mounted (SMD) devices there are two common schemes used.

The first marks the value in micro-farads (µF), and working voltage in volts directly. Unlike the TH style, the unit is not marked, it is just assumed to be in µF. For example “4.7 50V” would indicate a nominal value of 4.7µF with a working voltage of 50 volts. Polarity being marked on the top of the can above the cathode (negative) pad.

The other uses a letter followed by three digits. the letter indicates the working voltage according to the table below. The first two digits represent the value in picofarads (pF) and the third digit is the power of 10 multiplier (how many zeros to add to the first two digits).

Code	Voltage
e	2.5
G	4
J	6.3
A	10
C	16
D	20
E	25
V	35
H	50

Examples:

E475 = 4700,000pF = 4.7µF at 25V
H016 = 1,000,000pF = 1µF at 50V

Silvered Mica

Made by plating thin sheets of mica with silver. These silvered sheets are then layered to achieve the desired capacitance value. Electrodes are added and then the body encapsulated in either a ceramic or epoxy resin. they are NOT polarised.

Typically have nominal values in the range of a few pF to a few nF, though there are rare examples of up to 1µF. Working voltage is generally between 100V and 1000V, though 10kV examples do exist.

Tolerances on the nominal value are as low as 1%. They are also very stable over time, with a thermal temperature coefficient of around 50 ppm/℃.

Film Capacitors

Made by drawing out some kind of plastic film (which will be the dielectric) to around 1µm thick. It may then be metallised on one side before being then cut in to ribbons. The capacity of the device is trimmed by adjusting the width of the ribbon cut. Two ribbons are then rolled together and then pressed in to an oval shape making closer to a rectangular footprint which saves valuable PCB space.

Electrodes are added and the device inserted in to a case. A voltage is then applied to burn out any imperfections (taking advantage of the self-healing properties of film capacitors). The case is then sealed with a silicone oil to protect it from moisture, and the whole thing then dipped in plastic to hermitically seal the interior.

They can be made in capacitance values from below 1nF to values as high as 30µF with voltage ratings anywhere between 50V and 2kV. They are also very resilient to high vibration environments.

They are NOT polarised.

Tantalum

These are a sub-type of electrolytic capacitor and like aluminium foil electrolytic capacitors, they are polarity sensitive.

The anode is made from finely ground tantulum metal which is then sintered at high temperatures to form a solid, but porous pellet. the porocity gives the pellet an extremely large surface area which directly translates to higher values of capacitance.

The anode is then anodised forming a very thin layer oxide which acts as a dielectric. The anodisation process has to be very tightly controlled as the thickness of the oxide/dielectric affects the capacitance. Electrolyte is added to the anode by means of pyrolysis and then it is dipped in special solution and baked in an oven producing a manganese dioxide coating. The process is repeated until there is a thick coating on all internal and external surfaces. Next the pellet is dipped into graphite and silver to provide a good cathode connection.

Tantalum capacitors can have nominal values ranging from 1nF all the way up to 72mF. They are physically much smaller than aluminium foil electrolytic capacitors and are very stable over time. They are manufactured to a wide range of working voltages from 2V to 500V or more. They also have an ESR value one tenth that of the equivalent aluminium foil electrolytic which allows for use in higher current scenarios.

Through hole style tantalum bead capacitors normally have their value marked directly and unambiguously on their surface along with their working voltage.

SMD versions of tantalum capacitors are marked in two different ways (if at all). The first way is that they may be marked unambiguously in the same way as their through-hole counterparts. The second way is to use 3 digits and a letter. The first two digits represent the nominal value in picofarads (pF) and the third digit is the power of ten multiplier (how many zeros to add) with the letter representing the working voltage using the same table shown above for aluminium foil electrolytic capacitors.

Warning: Tantalum bead capacitors tend to have their anode (positive) end marked rather than the cathode as is customary for aluminium foil capacitors. In the case of the through hole variant a ‘+’ is normally printed beside the lead. In the case of the SMD version a bar and/or a letter ‘A’ are printed at the appropriate end.

Variable Values

There are two basic forms of mechanically variable capacitance devices. Larger ones that the end user directly interacts with (such as for tuning in a radio station); and smaller ones that are generally hidden inside the device and used by service engineers for fine tuning the circuit.

Variable Capacitor

These are commonly just two sets of semi-circular metal discs, separated by air gaps (though other solid dielectrics can be used). One set being fixed, and the other attached to a spindle. as the spindle is turned the discs go in and out of mesh thus adjusting the capacitance.

A reduction gear was also often used in between the spindle and the user adjustable knob to increase precision and allow finer tuning. Also, the plates could be shaped differently to provide a logarithmic progression as the spindle was turned instead of the linear progression of semi-circular plates.

In older radios the spindle was often fitted with a pulley to which a string was looped around having an indicator attached mid-loop. The user would then turn a knob which not only adjusted the capacitance, but also simultaneously moved a pointer along a tuning scale.

Trimmer Cap

These are the capacitance equivalents of trimmer pots. There are two types of trimmer capacitors: Air trimmers and ceramic trimmers. One uses air as the dielectric and the other uses other materials.

There are two techniques for varying the capacitance, both using a rotation (but to a very different end).

The first like it’s larger cousin the variable capacitor, is to use two sets of plates, one fixed while the other rotates in and out of mesh with it. Thus changing the effective surface area of the plates.

The second is to use a screw in an insulated core as one electrode while the other electrode is at the base of the threaded hole. As the screw is rotated it changes the distance between the two plates, thus altering the capacitance.

Speciality Capacitors

Varicap (a.k.a. Varactor)

Is a type of diode designed to exploit the voltage dependant capacitance of a reverse biased p-n junction. This is not the place to explain how they work, just to give a few examples of where they might be used.

Commonly used in:

Voltage controlled oscillators
Parametric amplifiers
Frequency multipliers

Digitally Tuned capacitor

These are integrated circuits based on a variety of technologies that allow a capacitance value to be dialled up simply by writing a digital command to it (usually over some serial protocol).

They are designed primarily for use in antenna impedance matching in multi-band cellular handsets.

Transducers based on Capacitance

Touch Sensor

Available as dedicated integrated circuits such as the Microchip CAP1203, but also commonly found as inputs directly to micro-controllers such as the Espressif ESP32.

Condenser Microphone

Although not thought of in this way. Condenser microphones are actually capacitors (hence the name – condenser is the old name for a capacitor).

The diaphragm is actually one plate of a capacitor. As the sound pressure vibrations cause the diaphragm to move in sympathy, so the gap between the two capacitor plates changes and hence the capacitance.