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## Description

Capacitors are passive electronic components that store electrical energy.

Image credit: Genteq | Mouser Electronics | Shenzhen Rich

## Basics

### Construction and Operation

Basic capacitors, formerly known as condensers, consist of two parallel plates - one positive and one negative - separated by a dielectric (nonconducting) material. The plates may be square, rectangular, cylindrical, or spherical, resulting in several possible designs and form factors. Common dielectric materials include air, paper, certain plastics, glass, oil, and Mylar.

A basic capacitor connected to a battery. Image credit: Electronics Tutorials

When a capacitor is connected to a voltage source such as a battery, current will flow from the battery's negative terminal to the plate closest to this terminal, which becomes the negatively-charged plate. Electrons on the other plate simultaneously move toward the battery's positive terminal due to Coulomb's Law; this plate therefore becomes the positive one. When the negative plate becomes saturated with electrons, the capacitor is said to be fully charged, and an electric field is developed across the plates.

The video below shows a simple circuit involving a capacitor. When the switch completes the lower circuit, the battery charges the capacitor. Note that, even when the switch is open and the capacitor is detached from its voltage source, the capacitor remains charged. When the upper circuit is closed by the switch, the capacitor discharges.

Video credit: Khurram Tanvir via YouTube

### Capacitance

Two related formulas can be used to calculate the quantity of charge on the plates or the device's maximum capacitance.

Capacitance is calculated as:

where:

C = capacitance

Q = charge

V = voltage

By rearranging this formula, we can also find the charge if the applied voltage and capacitance values are known:

Capacitance can also be calculated in relation to a capacitor's geometry:

where:

C = capacitance

Ε = permittivity of dielectric

A = plate area

d = distance between plates

This equation proves that capacitance is independent of the applied voltage and charge, and is only a function of the device's geometry and dielectric material.

Capacitance is measured in farads, which are extremely large units. Most capacitors are rated using smaller units, such as micro- or picofarads.

### Uses

Capacitors are versatile components used in a number of general and specialized applications. The table below illustrates the versatility of capacitors and the wide range of applications they are suited to.

 Use Capacitor actions Products / applications Energy storing Stores energy after disconnection from source; works like a temporary battery Charged devices, flashbulbs, volatile memory, automotive. Power conditioning Smooths rectifier output by passing AC power and storing DC, providing "clean" power Power supplies, automotive, widespread use in power circuits Signal coupling / decoupling Separation of AC and DC signals; filtering of noise and interference signals Signal conditioning, circuit protection Signal processing Stored energy used to select / represent information Radio tuners, random access memory (RAM), analog circuits Capacitive sensing Changes in capacitance - due to variable dielectric or plate distance - used to sense physical changes Level sensors, condenser microphones, pressure sensors, touch switches, accelerometers Pulsing Stored energy released in a controlled fashion to create a pulse Lasers, radar systems, particle accelerators, detonators, cold forming

Capacitor applications. Table credit: Wikipedia

## Specifications

### Fixed vs. Variable

Capacitors can feature either fixed or variable capacitance. Fixed capacitors simply have a fixed, nonadjustable capacitance value.

Variable capacitors can be adjusted by the user, using either mechanical or electronic means. These are also known as tuning capacitors due to their common applications in radio and antenna tuning. Mechanical variable capacitors are constructed with two sets of overlapping metal plates: a stationary set (stator) and a rotating set controlled by a shaft (rotor). Turning the knob changes the distance between plate sets, thus adjusting the device's capacitance according to the formula above.

Another type of mechanical variable capacitor, known as a trimmer capacitor, uses a mica dielectric between two plates and is adjusted using a screw.

A rotor-stator (left) and trimmer capacitor. Image credit: Integrated Publishing

Variable capacitors may also be produced in chip form, in which case they are digitally tuned.

### Dielectric Material

When selecting a capacitor, it is important to consider the dielectric material used. Various dielectric material groups feature different characteristics, advantages, and disadvantages. Though it is not mentioned in this table, air is sometimes used as a dielectric in high voltage applications.

 Type Examples Characteristics/advantages Disadvantages Paper Paper / oil-impregnated paper Largely obsolete; still used in some high voltage applications Large size, low moisture resistance, degradation due to moisture Film Polyester film / Mylar Generally replaced paper capacitors; small size, higher moisture resistance Low temperature stability, hazardous dielectric heating when used in RF applications Mica Mica Durable, excellent temperature stability Susceptible to moisture; high cost Glass Glass Similar to mica; reliable and stable; excellent radiation resistance High cost Polymer Polystyrene, polycarbonate, polyamide, PP, polyester General purpose construction; suitable for a variety of applications, including high voltage and RF Relatively low temperature resistance; large size;  may be easily damaged by pulses or transients Electrolytic Aluminum / tantalum electrolytic High capacitance to volume ratio; stable, reliable, durable; miniature construction High cost; may violently explode or burst when overloaded

Dielectric characteristics. Table credit: Wikipedia

### Standards and Compliance

Capacitors may be manufactured to comply with various standards, including:

• IEC 60384 (Fixed capacitors for use in electronic equipment; 26 parts)
• IEC 60418 (Variable capacitors; 4 parts)

• MIL 14409 (Variable capacitor performance; 19 parts)

### RoHS

The Restriction of Hazardous Substances Directive, or RoHS, is legislation adopted by the European Union in 2003. Although RoHS products are commonly known as "lead-free", the directive also restricts the use of mercury, cadmium, hexavalent chromium, polybrominated biphenyls, and polybrominated diphenyl ether in electrical and electronic products. The RoHS directive is designed to reduce medical hazards in third-world countries inundated with "high-tech trash."

While there is no standard RoHS mark, the one below is typical.

### Form Factor and Mounting

Capacitors are primarily manufactured as through hole (THT) or surface mount (SMT) devices.

#### Through Hole

Through hole components have long leads which are passed through holes on a printed circuit board (PCB) and soldered onto the opposite side. While THT mounting ensures a very strong mechanical connection, through hole products must be relatively large by necessity. THT has largely been replaced by surface mount technology, or SMT.

Through hole capacitors may feature one of several lead arrangements:

• Axial leads extend from the ends and along the axis of the capacitor, and not from the sides.

Image credit: Tactic-Tech | Amazon | Jamie's Pool and Spa

#### Surface Mount

Surface mount devices are mounted directly onto a PCB using short leads, flat contacts, BGAs, or other terminations. SMT devices can be manufactured to be much smaller than THT capacitors and are easier and cheaper to assemble due to the lack of necessary drilled holes.

Two surface mount capacitors. Image credit: Techpeeks

## References

Electronics Tutorials - Introduction to capacitors

Integrated Publishing - Fixed capacitors

Engineering Calculators Related to Capacitors

## Related Products & Services

• Ceramic Capacitors

Ceramic capacitors have a dielectric made of ceramic materials.

• Chip Capacitors

Chip capacitors or surface mount capacitors do not have leads.

• Film Capacitors

Film capacitors are insulated with polyester, polycarbonate, polypropylene, polystyrene, or other dielectric materials.

• High Voltage Capacitors

High voltage capacitors are used for storing charge and energy in high voltage applications.

• Power Capacitors

Power capacitors are used in electrical distribution systems to provide a static source of leading reactive power.

• Tantalum Capacitors

Tantalum capacitors are used in smaller electronic devices including portable telephones, pagers, personal computers, and automotive electronics.

• Ultracapacitors

Ultracapacitors store charges (energy) by physically separating positive and negative charges (unlike batteries which do so chemically).  Very high power densities can be achieved by this method.