Types of Capacitors
There are numerous types of capacitors with various functions and applications. Capacitors range from small to large, and each has characteristics that make them unique. For example, some capacitors are small and delicate, such as the ones found in radio circuits. On the other hand, capacitors can be quite large such as those found in smoothing circuits.
When making comparisons between the various types of capacitors the dielectric used between the plates is what’s typically taken into consideration.
The range of capacitors is numerous. Take, for instance, variable type capacitors that give the user the ability to vary their capacitance value for use in “frequency tuning” type circuits. Some capacitors look tube-like due to metal foil plates that roll up into a cylinder. Dielectric material typically sits between the metal foil plates and the cylinder.
Then there are capacitors used for commercial purposes which are made from metallic foil interweaved with thin sheets of either Mylar or paraffin-impregnated paper.
Small capacitors are typically made from ceramic materials and then sealed with epoxy resin. No matter what type of capacitor is used they all play an integral role in electronic circuits. Let’s take a more in-depth look at many of the more common capacitor types currently available.
Film Capacitor Type
A Mallory 150 100nF 630 VDC polyester film capacitor
This is the most common type of capacitor (in terms of availability) that belongs to a relatively large family of capacitors. The primary difference between film capacitors and other forms of capacitors is their dielectric properties. These include polycarbonate, polypropylene, polyester (Mylar), polystyrene, Teflon, and metalized paper. Regarding capacitance range, film type capacitors are available in ranges starting from 5pF to 100uF.
Film capacitors come in a variety of case styles and shapes that include:
- Epoxy Case (Rectangular & Round) - the capacitor is enclosed within a molded plastic shell that's then filled with epoxy.
- Wrap and Fill (Oval and Round) - plastic tape is used to tightly wrap the capacitor, and the ends are sealed with epoxy.
- Metal Hermetically Sealed (Rectangular and Round) - the capacitor is enclosed within a metal can or tube and sealed with epoxy.
Film capacitors with dielectrics composed of Teflon, polystyrene, and polycarbonate are sometimes referred to as "Plastic capacitors." Plastic film capacitors share a similar construction with that of paper film capacitors. The primary difference between the two is that one uses paper while the other uses plastic.
Plastic film capacitors hold an advantage over impregnated-paper types in that they have smaller tolerances, high reliability, a long service life, and can continue operating sufficiently while in high temperatures.
Dielectric Capacitors
A capacitor with dielectric
Dielectric capacitors considered to belong to the "variable type" of capacitors in which a continuous variation of capacitance is needed for tuning transistor radios, transmitters, and receivers. Variable dielectric capacitors are unique in that they're multi-plate air-spaced types that have stator vanes (fixed plates) and rotor vanes (movable plates) that move between the fixed plates.
The capacitance value is ultimately determined by the position of the moving plates in relation to the fixed plates. Typically, when the two sets of plates mesh together fully the capacitance value will be at its maximum. Capacitors that have high voltages have relatively large air-gaps or spacing’s between the plates.
Besides variable type capacitors, there are also preset type variable capacitors called Trimmers. Trimmers are typically small and can be pre-set or adjusted to a specific capacitance value with the use of a screwdriver. Most Trimmers only hold a small capacitance of 500pF (or less) and are non-polarized.
Ceramic Capacitors
A ceramic capacitor
Ceramic Capacitors are typically referred to as "Disc Capacitors." They're made by taking a small ceramic or porcelain disc and coating both sides with silver before stacking them together to make a functioning capacitor.
Single ceramic discs of around 3 - 6mm are used when low capacitance values are needed. Ceramic capacitors have a high dielectric constant (High-K) and are typically available so that a high capacitance can be achieved from a smaller sized object.
Ceramic capacitors tend to display substantial non-linear changes in capacitance against temperature. As a result, ceramic capacitors are often used as by-pass or decoupling capacitors. Concerning values, ceramic capacitors range from a couple of picofarads to several microfarads (μF). Typically, however, ceramic capacitors have low voltage ratings.
A 3-digit code is typically printed onto the body of the ceramic type capacitors to determine their capacitance in pico-farads. The calculation is relatively simple once it has been calculated -- the first two digits represent the value of the capacitors while the third digit represents the number of zeros that need to be added.
Electrolytic Capacitors
An electrolytic capacitor
Electrolytic capacitors are typically reserved for situations where larger capacitance values are needed. Electrolytic capacitors are different in that instead of using a thin film layer (metallic) to act as one of the electrodes, an electrolyte solution in the form of a semi-liquid jelly or paste is instead used as the second electrode.
Most electrolytic types of capacitors are polarized, meaning that the correct polarity must be used for the DC voltage applied to the capacitor. In other words, positive polarity must pair with the positive terminal and negative polarity to the negative terminal. In the event of an incorrect polarization, the oxide layer acting as insulation may break down and may become permanently damaged as a result.
Due to their large capacitance and small size, electrolytic capacitors are used in DC power supply circuits. This is done for coupling and decoupling applications and to lessen the ripple voltage. Electrolytic capacitors come with a relatively low voltage rating (one of its main disadvantages). Because electrolytic capacitors polarize, they cannot (and must not) be used with AC supplies.
There are two forms of electrolytics you should be aware of – Tantalum Electrolytic Capacitors and Aluminum Electrolytic Capacitors.
1) Tantalum Electrolytic Capacitors
Tantalum Electrolytic Capacitors and Tantalum Beads come in two varieties – dry (solid) and wet (foil) electrolytic types. Dry tantalum capacitors are physically smaller than aluminium capacitors and use manganese dioxide as the second terminal.
2) Aluminium Electrolytic Capacitors
Aluminium Electrolytic Capacitors have two types – foil types and etched foil types. Due to the high breakdown voltage and the aluminium oxide film, Aluminium Electrolytic Capacitors have high capacitance values when compared to their size.
The capacitor has foil plates that are anodized with a DC current. During this process, the polarity of the plate material is set up, and the positive and negative sides are created.
The etched foil types differentiate themselves from the plain foil types in one primary way – the aluminium oxide on the cathode and anode has been etched chemically to increase its permittivity and surface area.
When it comes to etched foil electrolytics, they’re best used in DC blocking, by-pass circuits, and coupling. On the other hand, plain foil types are more geared towards smoothing capacitors in power supplies. Keep in mind that aluminiumelectrolytics are considered to be polarized devices. Thus, catastrophic consequences may occur when the applied voltage on the leads are reversed because the insulating layer located within the capacitor (as well as the capacitor itself) will be destroyed. Thankfully, if the damage is minimal, the electrolyte that’s used within the capacitor can help to repair the damage.
Electrolytes can do more than self-heal damaged plates. They can also re-anodize the foil plate. Because the anodizing process can be reversed, the electrolyte can remove the oxide coating from the foil (which would also occur if the capacitor were connected with reverse polarity). Be mindful that because the electrolyte can conduct electricity, catastrophic issues may occur if the aluminium oxide layer were removed from the equation or outright destroyed.
When it comes to dielectric properties, tantalum oxide is considered to be better than that of aluminium oxide because it gives better capacitance stability and lowers leakage currents which ultimately make them perfect for filtering, by-passing, applications, blocking, and decoupling.
Keep in mind that tantalum capacitors can tolerate reverse voltages much better than aluminium types (because they’re polarized), but are actually rated at lower working voltages. Typically, dry tantalum capacitors are utilized in circuits where the DC voltage is larger when compared to the AC voltage.
“Non-polarized” capacitors exist in which some tantalum types utilize two capacitors in one. In such a situation the connection is negative-to-negative (creating the non-polarized capacitor) which is often used in AC circuits with low voltage as a non-polarized device.
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