Description

 

Capacitors are passive electronic components that store electrical energy.

 

 

capacitors selection guideq     capacitors selection guide      capacitors selection guide

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.

 

capacitors selection guide

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.

 

capacitors selection guide    capacitors selection guide

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.

 

capacitors selection guide

Image credit: Advanced Digital Cable

 

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.
  • Radial leads extend from the capacitor's sides instead of the ends.
  • Flying leads extend horizontally.

capacitors selection guide     capacitors selection guide     capacitors selection guide

(left to right) Axial, radial, and flying leads.

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. 

capacitors selection guide

Two surface mount capacitors. Image credit: Techpeeks

 

References

 

Electronics Tutorials - Introduction to capacitors 

 

Integrated Publishing - Fixed 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.