Capacitors are passive components used in electrical circuits, fundamental to their operations as inductors and resistors. A two-terminal component, capacitors exist in almost every electronic device, acting similarly to a battery by storing energy. They serve critical applications within electrical circuits, such as complex signal filtering, localized energy storage, and voltage spike suppression. For businesses that fabricate, use, or maintain electronic equipment, understanding the most common capacitors for various applications ensures that you have the necessary capacitors for any electrical undertaking.
Understanding Common Capacitors
Many capacitors are commonly used for electrical applications, each having its own characteristics. Common capacitors on the marketplace range from the most delicate trimming capacitors used in radio and oscillator circuits to robust and powerful capacitors for smoothing circuits and high voltage applications that look like metal cans. Comparing common capacitor types usually involves looking at the dielectric between the capacitor’s plates.
Common commercial capacitors generally consist of a metallic foil in which thin sheets of Mylar or paper infused with paraffin are intertwined. Certain capacitors are tube-like, with metal foil plates that roll into a cylinder, forming a small package containing a dielectric material that provides insulation between the plates. Smaller capacitors are generally made from ceramics dipped into epoxy resin, which seals them.
Common Capacitor Considerations
Due to their features, capacitor types generally work better for certain applications than others. While one type might have drawbacks for one application, it may be the best for others.
These considerations should be made when choosing from common capacitors:
- Equivalent series resistance: A capacitor’s terminals aren’t completely conductive, with a resistance of a hundredth of an ohm or less; this resistance only becomes a problem when a significant amount of current runs through the capacitor, which can cause power loss and produce heat.
- Leakage current: Capacitors are prone to leaking an almost insignificant amount of current as it passes through the dielectric between terminals. Usually only a few nanoamps at most, this leakage will slowly drain the capacitor’s energy.
- Maximum voltage: Capacitors are rated for the maximum voltage that can decrease electrical potential from the current’s path that flows through a circuit. This differs significantly from between 1.5V and 100V, with ratings that exceed this maximum voltage tending to destroy the capacitor.
- Size: This consideration deals not only with physical dimensions but also the size of the electrical charge a capacitor can store. Generally, the larger a capacitor is physical, the more charge it can hold.
- Tolerance: This figure isn’t precise, although capacitors are rated nominally for the amount of electrical charge they can collect and store. The actual value varies from 1 to 20 percent from the anticipated value.
Another key aspect of capacitors is capacitance, which relates to how much electricity a capacitor can store.
Types of Common Capacitors
Capacitors are categorized according to structure and whether they’re polarized or unpolarized. Structurally, they can be fixed, trimmer or variable capacitors. Polarized capacitors are also called electrolytic, referring to how it reacts when electricity passes through them. Unpolarized capacitors are generally preferred, however, as they aren’t damaged due to reverse voltage and work in purely alternating current circuits.
Ceramic Capacitors
These are the most common capacitors, called such due to the ceramic material that makes up its dielectric. They’re typically small physically and carry very little charge, though these capacitors work at high frequencies and are stable in hotter environments. Featuring two terminals that jut out, through-hole ceramic capacitors normally look like small bulbs that are commonly red or yellow in color. The most inexpensive and common of capacitors, they tend to be utilized for high-frequency applications that involve coupling and decoupling. However, their low capacitance limits the applications in which they can be used.
Aluminum and Tantalum Electrolytic Capacitors
These are common capacitors for applications in which significant capacitance values are necessary. They use a wafer-thin metallic layer as an electrode, with the secondary electrode formed from a pasty, jelly-like electrolyte solution. Another thin oxide layer of fewer than ten microns is produced electrochemically as the dielectric. As this insulating layer is so slim, it’s possible to make physically small capacitors yet have an outsized capacitance.
Electrolytic capacitors are usually polarized, meaning capacitor terminals must have the correct polarity. When DC voltage is applied, negative must touch negative, and positive must touch positive; otherwise, the insulating oxide layer will likely become damaged. Every polarized electrolytic capacitor has its negative terminal marked by a negative sign.
Generally used for circuits in DC power supplies, electrolytic capacitors’ larger capacitance, and smaller physical size allow them to decrease ripple voltage for coupling and decoupling applications. However, one disadvantage is their lower voltage rating. They’re polarized and can’t be used with AC power supplies. These common capacitors are most often made from either aluminum or tantalum.
Aluminum electrolytic capacitors come in two types, either etched or plain foiled. However, due to their high breakdown voltage and oxidation layer, these capacitors offer significantly higher capacitance values when compared to their size. The capacitor’s foil plates are anodized via DC current, setting the plate material’s polarity by determining which side is negative and which is positive. With the etched foils on the anode and cathode, the aluminum oxide is chemically etched to increase permittivity and surface area, which makes the etched foil better for coupling applications.
Tantalum electrolytic capacitors also come in wet foil and dry solid, with the latter being more common. Capacitors using solid tantalum are smaller than equivalent aluminum capacitors, with their second terminal coated with manganese dioxide. Tantalum oxide provides more reliable capacitance than aluminum, making them useful in timing, filtering, decoupling, bypassing, and blocking applications. Though polarized, they can tolerate reversed voltages better than aluminum, though their working voltages are much lower. Solid tantalum capacitors are normally used in circuits where the AC voltage is proportionately small compared to its DC voltage.
Dielectric Capacitors
These variable capacitors enable the continual capacitance variation required to tune receivers, transmitters, and transistor radios. Dielectric capacitors have multiple plates with air between fixed plates known as stator vanes, with moveable plates that move between them. Where the moving plates are positioned regarding the stationary plates determines an individual dielectric capacitor’s capacitance value. When fully meshed, capacitance tends to be at its maximum.
Trimmer Capacitors
Unlike those capacitors that are continually variable, these capacitors can be adjusted to specific capacitance values. Also known as preset variable capacitors, their capacitance value can be adjusted with a small screwdriver during manufacturing. End users don’t adjust them but rather serve to calibrate equipment during production or maintenance. Normally mounted directly to a printed circuit board (PCB), those with a very low capacitance are nonpolarized. Trimmer capacitors are less expensive than full-sized variable capacitors, as they’re used for rise and fall times, oscillator frequency values, latencies, and other circuit variations.
Film Capacitors
One of the most common capacitors available, this type belongs to a grouping with wide-ranging capacitances. Differing in their dielectric properties, they primarily utilize films made from plastics rather than impregnated paper, often called “plastic capacitors.”
The dielectrics for these common capacitors include:
- Polytetrafluoroethylene (PTFE aka Teflon)
- Polystyrene
- Polypropylene
- Polyester (aka Mylar)
- Polycarbonate
- Metalized paper
Film capacitors come in assorted case styles and shapes that include:
Wrap and fill: Oval, rectangular, or round in shape, these are the most common capacitors of the film variety, using plastic tape tightly wound around the capacitor, with its ends sealed with epoxy.
- Metal hermetically sealed: Though these use a metal can or tube in which the capacitor is encased, they utilize a plastic-like polyester as the dielectric and are rectangular or round.
- Epoxy case: These film capacitors come in rounded or rectangular shapes in a molded plastic or epoxy shell filled with epoxy.
The primary advantages of film capacitors include their tight tolerances, reliability, long service life, and ability to deal with high temperatures.
Supercapacitors
Known also as electric double-layer capacitors, electrochemical double-layer capacitors, super-condensers, or ultracapacitors, these electrochemical capacitors have hundreds of times greater energy densities than conventional electrolytic varieties. Designed for extremely high capacitances, these capacitors can store large amounts of energy, though they can’t handle high voltages. To mitigate this, supercapacitors are often linked in series to achieve higher voltage ratings, reducing the capacitance. Supercapacitors generally act like batteries; though unable to store as much as a battery of equal size, they release energy much more quickly and tend to last longer.
Solid State Inc. carries an array of common capacitors. For more information about the capacitors and other electronic components Solid State carries, we invite you to contact us today.