Basic Instrumentation and Measurement - Capacitor

Capacitor is a passive electronic component which can store electrical charge and at the same time store the energy. It is actually made of 2 flat conductors which are separated at a very minimum distance. Between the 2 electrodes there is non - conductive substance which is called dielectric.

Capacitor is used in many electronic devices and nowadays we can find capacitors in almost all electronics devices. They are used especially for electrical energy storage , reactance and also as frequency resistor and also sometimes as sensors. For special purpose sometimes they are configured as none linear capacitor.

Charge separation in a parallel-plate capacitor causes an internal electric field. A dielectric (orange) reduces the field and increases the capacitance.

Theory

If there is a constant voltage supplied to a uncharged capacitor, the current will flow across the capacitor and charged the capacitor with charges. Charges will be stored in the capacitor if the voltage is not supplied anymore. Now the capacitor contain potential different between its 2 poles. If the capacitor is discharged, the potential difference will reduce.

The charges which have been stored inside the capacitor is proportional to the voltage across those 2 electrodes of the capacitor. The constant of of the proportionality is the capacitance. If the capacitance is bigger than more charges can be stored in that particular capacitor. The equation is :

Q = C V

which is Q is the charges in Coulomb (C) or Amperesecond (As), C is the capacitance in Farad (F) and U is the voltage in Volt (V).

Applications

Energy and Charge Storage

The energy which have been stored depends on capacitance (C) and the voltage (V) and can be shown in this formula below :

E = ½ C V²

This characteristic is applied in the field of power electronics as a voltage source invertors  which is used in switched-mode power supply and also in invertors.

Information Storage

The charge in the capacitor can represent the information in digital or analogue form . To store a large quantity of information , a lot of capacitor are combined together in a integrated circuit i.e dynamic RAM (DRAM) , Erasable Programmable Read Only Memory (EPROM) , Flash Storage and also Ferroelectric Random Access Memory.

Analogue information can be stored in capacitors which can be normally found in Sample – and – Hold Circuit. During sampling a capacitor is connected to the voltage input signal whereby during the hold phase the voltage input signal will be separated. Then the result of the sampling will transferred to the Analogue – Digital Converter (ADC) to get the digital signal. Another way to store the analogue information is by using the Bucket Brigade Device (BBD).

Frequency-dependent Resistors

For the application as frequency-dependent resistor (reactance) the value of capacitor is known whereby the reactance value is depend on the value of the capacitor. Reactance is normally used in electrical energy technique , communication technique , analogue and digital technique.

Energy technique

In the field of energy technique capacitors are widely used to change the phase between current and voltage in the AC current. To reduce the idle current from the electromotor , capacitor is used to compensate the idle current by shifting the phase. For this purpose the capacitor and the inductivity should have the same impedances.

Filter Applications

The frequency-dependent of the AC current resistors are used to filter the frequency-dependent of the AC current. A simple circuit which is normally used is RC- circuit which act as either low-pass filter or high-pass filter.

Types of Capacitors

Fix Capacitors

Fix capacitor has fix value of capacitance. It depends on applications , size , voltage and etc and also type of dielectrics , electrodes and type of geometric form.

Except electrolytic capacitor the capacitance is in the range of Pico farad until Microfarad. Electrolytic capacitor normally has capacitance range until farad. For double layer capacitor the range is normally until Kilo farad .

Ceramic Capacitors

It has ceramic as the dielectrics with high electric strength . They have high different dielectric constant (Permittivity). These capacitors have big range of capacitance between 0.5pF until 100uF. The type of ceramics used are Titandioxid TiO₂ (paraelectric material ) , Bariumtitanat BaTiO₃ (ferroelectrical material) . These stuffs are granulated in high temperature between 1200^oC until 1400^oC. Extra stuff like Aluminium silicate or magnesium silicate or aluminium oxide are mixed together with the ceramics to get the permittivity  εr between 6 to 14.

Plastic Film capacitors

This capacitor used plastic or the mixing of plastics as dielectric and are made by these 2 methods :

• ·         Plastic films with metal sheet are rolled together.
  
• ·         plastic films are metallised with aluminium and rolled together.
  

These types of capacitors normally are used for high voltage applications.

Metal – Paper Capacitors

It is produced from 2 isolator paper and metal foil. They are rolled together. Papers act as the mechanical separator of the electrode and fix the distances between 2 foils. This type of capacitor is normally used in the field of power electronic.

Electrolytic Capacitors

There are 3 types of electrolytic capacitors which are :

• ·         Aluminium electrolytic capacitors – aluminiumoxid as the dielectric
  
• ·         Tantal electrolytic capacitors – Tantal pentoxid as the dielectric
  
• ·         Niob electrolytic capacitors – Niob pentoxid as the dielectric.
  

How it is made ? It is made by the electrolysis process whereby the non – conductive material are formed at the anode electrode (anode oxidization) . At cathode side the electrolyte is formed as electrolytic capacitors.

Double Layer Capacitor

It has very high capacitance which come from the dissociation of ion in the electrolyte. It is actually a combination with an electrode with a big surface area. Double layer capacitor is like electrolytic capacitor which have their own pole.

Vacuum Capacitor

It is normally applied for high frequency current and voltage in Kilovolt range which can be found usually at the transmitter.

Glass Dielectric

It can be used in high temperature range from 75^oC to 200^oC . Typical value is between 300pF to 100nF.

Symbol

Parallel Circuits

Capacitors are in parallel circuits when the potential across each capacitors are the same . In this case the total capacitance is :

C_total = C₁ + C₂ + C₃ +...... C_n

 

In the siries circuits , the current which flow across each capacitor is the same so the total capacitance is :

1/C_total = 1/C₁ + 1/C₂ + .....1/C_n