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Understanding capacitor

In electrical electronics, the component used to store electrical energy in an electric field is known as a “Capacitor”. It is a passive device that can store an electrical charge on its plates when connected to a voltage source. Capacitors contain two-terminal and their effect is known as capacitance. They can be found in all electrical appliances, which makes their applications so broad.

Today you’ll get to know the definition, characteristics, diagram, types, and working of a capacitor. You’ll also get to know the following:

  • Dielectric of a capacitor
  • Capacitance and charge
  • Standard units of capacitance
  • The capacitor in parallel and series
  • Energy in a capacitor, &
  • Capacitor color code.

capacitor

What is a capacitor?

A capacitor is a component that has the ability or capacity of storing energy in form of an electrical charge producing a potential difference (static voltage) across its plates. The electrical component serves much like a small rechargeable battery. In common words, a capacitor is a device that stores electrical energy in an electric field.

The result of a capacitor is called capacitance, which can exist between any two electrical conductors in proximity to a circuit. The device is designed to add capacitance to a circuit. Capacitors are originally known as a condenser. There are various kinds of capacitors available today, from the very small capacitor beads used in resonance circuits to large power factor correction capacitors. However, they all perform the same task which is to store charge.

Furthermore, a capacitor consists of two or more parallel conductive (metal) plates that are not connected or in contact with each other. However, they are electrically separated by either air or by some form of good insulating material like ceramic, waxed paper, mica, plastic, or some form of liquid gel. The insulating layer between the capacitor’s plates is known as a Dielectric.

Characteristics of a capacitor

The characteristics of a capacitor can be determined by its temperature, voltage rating, capacitance range, and its use in a particular application. Capacitors are of different types and have their own unique set of characteristics and identification systems. Although some are easy to recognize, some can still be misleading through letters, colors, or symbols.

The best way to know the characteristics of a capacitor is to figure out the family the capacitor belongs to whether ceramic, film, plastic, or electrolytic. Most capacitors have the same capacitance value, they may have different voltage ratings. So, if a smaller rated voltage capacitor is substituted in place of a higher-rated one, the increased voltage may damage the smaller capacitor.

Capacitor with any other electronic component can be designed with its series of characteristics. These features can be found in the data sheets that the capacitor manufacturer provides. Below are the important ones to know.

Read more: Understanding the charge in a capacitor

Nominal capacitance, (c)

The nominal value of the capacitance is measured in pico-farads (pF), nano-Farads (nF), or micro-Farad (μF). It is marked onto the body of the capacitor as numbers, letters, or colored bands. The capacitance of a capacitor can change value with the circuit frequency (Hz) y with the ambient temperature. Smaller ceramic capacitors can be designed to have a nominal value as low as one pico-Farad, (1pF) while larger electrolytic’s can have a nominal capacitance value of up to one Farad, (1F).

Working Voltage, (WV)

The working voltage is another important characteristic to consider in a capacitor. It defines the maximum continuous voltage either DC or AC which can be applied to the capacitor without failure during its working life. Generally, the working voltage is printed onto the body of the capacitor indicating its DC working voltage, (WVDC).

Tolerance, (±%)

Just as resistors, capacitors also have a tolerance rating expressed as a plus-or-minus value either in picofarad’s (±pF) for low-value capacitors. It is generally less than 100pF or as a percentage (±%) for higher value capacitors generally higher than 100pF. The tolerance value is the extent to which the actual capacitance is allowed to vary from its nominal value and can range anywhere from -20% to +80%. Thus, a 100µF capacitor with a ±20% tolerance will legitimately vary from 80μF to 120μF and remain within tolerance.

Leakage current

The dielectric contained in a capacitor to separate the conductive plates is not a perfect insulator. This results in a very small current flowing or “leaking” through the dielectric due to the influence of the powerful electric fields built up by the charge on the plates when applied to a constant supply voltage. The small DC flow in the region of nano-amps (nA) is known as the capacitors, Leakage Current. This leakage current is due to the electrons physically finding their way through the dielectric medium, around its edges, or across its leads and will over fully discharge the capacitor over time.

Working temperature, (T)

Changes in the working temperature around the capacitor can affect the value of the capacitance because of changes in the dielectric properties. Too hot and too cold of air or surrounding temperature will affect the capacitance value of the capacitor which may change the correct operation of the circuit. The normal working range for most capacitors is 30oC to +125oC with nominal voltage ratings. The working temperature should not be more than +70oC, especially for the plastic capacitor types.

Temperature coefficient, (TC)

This is the maximum change of a capacitor in its capacitance over a specified temperature range. The temperature coefficient of a capacitor can be generally expressed linearly as parts per million per degree centigrade (PPM/C), or as a percent change over a particular range of temperatures. Although some capacitors are non-linear (class 2 capacitors), their value increases as the temperature rise giving them a temperature coefficient that is expressed as a positive “P”. Some capacitors decrease their value as the temperature rises to give them a temperature coefficient that is expressed as a negative “N”.

Polarization

The polarization of a capacitor is generally referring to the electrolytic type but mainly the Aluminum Electrolytic, with regards to their electrical connection. Most electrolytic capacitors are polarized types, that is, the voltage connected to the capacitor terminals must have the correct polarity, I.e., positive to positive and negative to negative. Incorrect polarization can cause the oxide layer inside the capacitor to break down leading to very large currents flowing through the device. Thus, resulting in destruction.

Equivalent series resistance, (ESR)

The equivalent series resistance of a capacitor is the AC impedance of the capacitor when used at high frequencies and includes the resistance of the dielectric material. Also, the DC resistance of the terminal leads, the DC resistance of the connections to the dielectric, and the capacitor plate resistance are all measured at a particular frequency and temperature.

In some ways, ESR is the opposite of the insulation resistance which is presented as a pure resistance (no capacitive or inductive reactance) in parallel with the capacitor. A perfect capacitor would only have capacitance but ESR is presented as a pure resistance (less than 0.1 Ω) in series with the capacitor (hence the name Equivalent Series Resistance), and which is frequency-dependent making it a DYNAMIC quantity.

Diagram of a capacitor:

diagram of capacitor

Types of capacitors

There are various types of capacitors out there, they range from very small delicate trimming types used in oscillator or radio circuits up to large power metal-can type capacitors used in high voltage power correction and smoothing circuits. Below are the different types of capacitors used in different applications.

Dielectric Capacitor

These types of capacitors are usually the variable type where a continuous variation of capacitance is needed for tuning transmitters, receivers, and transistor radios. Variable dielectric capacitors are multi-plate air-spaced types having a set of fixed plates (the stator vanes) and a set of moving plates (the rotor vanes). These vanes move in between the fixed plates.

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The position of the moving plates to fixed plates determines the overall capacitance value. The capacitance is generally at maximum when the two sets of plates fully mesh together.

Variable capacitor symbol

Apart from the continuously variable types, preset type variable capacitors are also called trimmers. They are generally small devices that can be adjusted or “pre-set” to a particular capacitance value with the aid of a small screwdriver and are available in very small capacitance’s of 500pF or less and are non-polarized.

Film capacitor type

The film capacitors are the most common types available. They consist of a relatively large family of capacitors with differences being in their dielectric properties such as polyester (mylar), polystyrene, polypropylene, polycarbonate, metalized paper, Teflon, etc. These types of capacitors are available in capacitance ranges from as small as 5pF to as large as 100uF depending upon the actual type of capacitor and its voltage rating. They also come in an assortment of shapes and case styles which include wrap & fill (oval & round), epoxy case (rectangular & round), metal hermetically sealed (rectangular & round).

Film capacitors that employ polystyrene, polycarbonate, or Teflon as their dielectric are sometimes called “Plastic capacitors”.

Ceramic capacitors

Ceramic capacitors are generally called DISC capacitors. They are made by coating two sides of small porcelain or ceramic disc with silver and are stacked together to make a capacitor. When a very low capacitance value is required, a single ceramic disc of about 3-6mm should be used. Ceramic capacitors have a high dielectric constant (High-K) and are available so that relatively high capacitance can be obtained in a small physical size.

These types of capacitors can exhibit large non-linear changes in capacitance against temperature and as a result are used as de-coupling or by-pass capacitors as they are also non-polarized devices.

Electrolytic capacitors

Electrolytic capacitors are generally used when very large capacitance values are needed. A semi-liquid electrolyte solution in form of jelly or paste is used rather than using a very thin metallic film layer for one of the electrodes. The semi-liquid electrolyte solution serves as the second electrode (usually the cathode).

The dielectric is a very thin layer of oxide that is grown electro-chemical in production with the thickness of the film being less than ten microns. The insulating layer is so thin that it is possible to make capacitors with a large value of capacitance for small physical size as the distance between the plates, d is very small.

The majority of electrolytic types of capacitors are polarized, that is, the DC voltage applied to the capacitor terminals must be of correct polarity, i.e., positive to the positive terminal and negative to the negative terminal.

Working principle of capacitor

The working of a capacitor is less complex and can be easily understood. The physical form and construction of practical capacitors vary widely and there are many types available. Most capacitors have at least two electrical conductors often in form of metallic plates or surfaces separated by a dielectric medium. The conductor may be foil, thin film, sintered bead of metal, or an electrolyte. The nonconducting dielectric acts to increase the capacitor’s charge capacity.

Capacitors are widely used as parts of electrical circuits in many common electrical devices. An ideal capacitor does not dissipate energy like a resistor. Although real-life capacitors dissipate a small amount when an electric potential difference (a voltage) is applied across the terminals of a capacitor. For instance, when a capacitor is connected to a battery, an electric field develops across the dielectric, causing a net positive charge to collect on one plate and a net negative charge to collect on the other plate.

Watch the video below to learn more on the working of a capacitor:

The dielectric of a capacitor

Apart from the overall size of the conductive plates and their distance or spacing apart from each other, the type of dielectric material used in a capacitor is another factor that can affect the overall capacitance. This is also known as Permittivity (ε) of the dielectric. The conductive plates of a capacitor are generally made of a metal foil or a metal film allowing for the flow of electrons and charge, but insulator material is always used as a dielectric. Different types of insulating materials can be used as the dielectric in a capacitor. They differ in their ability to block or pass an electrical charge.

Just as earlier mentioned, the dielectric material can be made from several insulating materials or combinations of these materials. The most common types used are air, paper, polyester, polypropylene, Mylar, ceramic, glass, oil, or a variety of other materials.

The process in which the dielectric material or insulator increases the capacitance of the capacitor compared to air is known as the Dielectric constant, K. A dielectric with a high dielectric constant is a way better insulator than a dielectric material with a lower dielectric constant. The dielectric constant is a dimensionless quantity since it is relative to free space.

Conclusion

That is all for this article, where the definition, characteristics, diagram, types, and working of a capacitor were discussed. I hope you gain a lot from the reading, if so, kindly share with other students. Thanks for reading, see you around!

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