Dielectric Constant - The dielectric constant of a dielectric material can be defined as the ratio of the capacitance using that material as the dielectric in a capacitor to the capacitance using a vacuum as the dielectric. Dielectric constant is often indicated by the symbol K and always has a value greater than 1.0. Typical values of K for dielectrics are :
The values shown are for the raw materials, practical application has shown the dielectric constant to be lower. In wound capacitors there is a general decrease due to a very thin layer of air that exists between the two electrodes. Metalized dielectrics however have a higher K due to the air that is eliminated on one side of the film by the bond of the metalized layer and the film.
The dielectric constant is not a stable constant in that the frequency, temperature, and sometimes thickness can change the dielectric constant. Dielectric films are classed as being either polar, where the molecular dipoles rotate and align themselves with the applied field, or as non-polar, where the materials do not rotate and align themselves. As the temperature and frequency increase this alignment is impeded by molecular collisions and drag resulting in a loss of energy and a drop in the dielectric constant. The effects of frequency and temperature upon dielectric constant must be considered when selecting a dielectric for your application.
Temperature coefficient is a measure of capacitance change at discrete temperatures when referred to the initial capacitance value and temperature. This is usually expressed as “parts per million per °C” (PPM/°C) over a specific temperature range for linear dielectrics and “%” change for nonlinear dielectrics.
Stability is a measure of a capacitor to return to its initial value after being submitted to any environmental changes such as temperature.
Drift is any difference between two readings taken at a specific temperature and occurring during one of more temperature cycles.
Retrace is the ability of a capacitor to follow the same pattern of capacitance variations when submitted to repeat temperature cycles.
Dielectric Absorption is a critical factor in devices such as computers, welding, telemetering, or servomechanisms where time is very sensitive.
Dielectric Absorption occurs when a charge is applied to a dielectric and it does not want to give up its electrons when discharged due to the polarization effect that takes place on the dipoles. These electrons become trapped in the dielectric during the discharge period and when the shorting mechanism is removed the electrons become free and will move to the electrode surface. This results in a difference between the electrodes and you have a recovery voltage. Given a finite amount of time the dielectric will charge to some level on its own resulting in a malfunction when the application is expecting a discharged capacitor. The recovery voltage divided by the charging voltage is expressed in percentage as “% Dielectric Absorption”.
The absorption rate varies with the dielectric material and any impregnation or contamination that enters a part. It is very important that this is taken into account and used when designed so the occurance of Dielectric Absorption is minimized. Any measured values of Dielectric Absorption are also affected by other factors such as the charging time, discharge time, temperature, time from discharge, and the amplitude of the charging voltage.
Dissipation Factor (DF) – a measurement in percent, the smaller the better, to express the efficiency of a capacitor operating under AC or pulse voltages. DF is important where AC currents are found as this can lead to capacitor heating. The ratio of power in to power out of a capacitor.
Insulation Resistance (IR) – A measurement of the insulation properties of a dielectric expressed in megohms.
Equivalent Series Resistance (ESR) – A measurement of the combined resistance of all the components and all metallic terminations of a capacitor. Also referred to as a loss, which is expressed in ohms at a given frequency. The frequency is usually 100Khz except for circuit analysis, which is done at the actual operating frequency.
Corona – Electrical discharges, resulting from the ionization of air or any other gas from exposure to high voltages, which can cause electrical noise in the capacitor and lead to its eventual failure.
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