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The specifications and parameters or characteristics of a capacitor need to be known and understood before a choice is made for a capacitor in a given circuit.
Capacitor specifications can vary considerably between different capacitors of the same value because of the different properties or characteristics of the different types of capacitor.
Accordingly it is necessary to have a good understanding of the different capacitor specifications and parameters.
Key capacitor specifications
Some of the key capacitor specifications and characteristics include:
- Capacitance value: The nominal capacitance is probably the most important capacitor specifications. The basic unit of capacitance is the Farad, although most capacitors have values well below a Farad - the submultiples below being the most common.:
- microfarads, µF, a millionth of a Farad, 10-6
- naonofarads, nF a 1000 millionth of a Farad, 10-9
- picoFarads, pF a million millionth of a Farad, 10-12
The nominal capacitance may also be quoted at a certain frequency as the capacitance for some forms of capacitor, typically electrolytic will vary slightly with frequency.
- Working voltage: The working voltage capacitor specification is of great importance in many instances. If exceeded it can result in failure of the capacitor.
The working voltage capacitor characteristic defines the maximum continuous voltage that may be applied across the capacitor. This is normally printed on the case and will be mentioned in the datasheet. The voltage normally refers to the largest DC voltage that can be applied. Also be aware that when a capacitor is operating on a circuit with an AC waveform superimposed on a DC voltage, then the voltages experienced may be well above the quiescent DC value.
For some capacitors used in AC applications, an AC value may be quoted. Be aware that this refers to the RMS voltage and not the peak value which is √2, or 1.414 times greater.
It is always good practice to run capacitors well within their rated voltage. Often commercial design guidelines stipulate that capacitors should not be run above 50% of their rated values. This ensures high levels of reliability are achieved.
- Dielectric: The dielectric is one of the key items that governs many of the capacitor characteristics. As a result capacitors are often referred to by their dielectrics: electrolytic; tantalum, ceramic; plastic film; silver mica; and the like. As the characteristics of these capacitors and the capacitance ranges available vary, it is important to select the required dielectric, looking carefully at the performance and overall capacitor specification in the datasheet.
- Working temperature: All capacitors have a limited working temperature range. This specification details the limits within which the capacitor will work satisfactorily and over which it is designed to operate. Some aspects that limit the working range of a capacitor are the voltage - this falls with increasing temperature; the ripple current - again lower with increasing temperature. The lower temperature specification can be governed by a number of factors. One is the electrolyte operation in components such as electrolytic capacitors.
- Leakage current: The leakage current specification indicates the amount of current that flows through the capacitor. Leakage current occurs as a result of the fact that capacitors are not perfect insulators.
- ESR: The Equivalent Series Resistance or ESR, is an important specification in many instances.. It is the impedance of the capacitor to alternating current and it is particularly important at high frequencies. The ESR specification includes the resistance of the dielectric material, the DC resistance of the terminal leads, the DC resistance of the connections to the dielectric and the capacitor plate resistance all measured at a particular frequency.
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