Parametric characteristics of capacitors

Dec 11,2019

1. Nominal capacity: The nominal capacity is the capacity marked on the capacitor.


2. Allowable deviation: the maximum allowable deviation range between the actual capacitance and the nominal capacitance. Generally divided into three levels: level I ± 5%, level II ± 10%, level III ± 20%. In some cases, there is a level 0 with an error of ± 20%. The tolerance of precision capacitors is small, while the error of electrolytic capacitors is large, they use different error levels. Commonly used capacitors have the same accuracy level as resistors. Expressed by letters: D——005 grade—— ± 0.5%; F——01 grade—— ± 1%; G——02 grade—— ± 2%; J——I grade—— ± 5%; K— — Class II— ± 10%; M—Class III— ± 20%.


3. Rated working voltage: The rated working voltage refers to the maximum DC voltage or AC voltage that can be continuously applied to the capacitor within a specified temperature range without damaging the capacitor. It can also be regarded as the maximum DC voltage that the capacitor can work stably and reliably in the circuit for a long time. For capacitors with the same structure, dielectric and capacity, the higher the withstand voltage, the larger the volume.


4. Surge Voltage: Generally, the voltage is 1.3 times the rated voltage of the capacitor itself. It needs to work normally without abnormalities.


5. Leakage current: It is a specific specification of the electrolytic capacitor. Generally, after the capacitor's rated voltage is pressurized for 3 Min, the ammeter is connected in series. The leakage current must be 0.01CV (the product of uF capacitance and rated voltage) or 3uA (whichever is greater).


6. Temperature coefficient: the relative change in capacitance for each 1 ° C change in temperature within a certain temperature range. The smaller the temperature coefficient, the better. There are positive and negative temperature coefficients. Positive temperature coefficient capacitors indicate that the capacitor increases with temperature, while negative temperature coefficients do the opposite.


7. Operating temperature range: The operating temperature range of general electrolytic capacitors is -25 ℃ to + 85 ℃. Specific high-temperature or low-leakage currents range from -40 ° C to + 105 ° C. Plastic film capacitors are -40 ° C to + 85 ° C. Ceramic capacitor T / C type is -40 ° C to + 85 ° C. Hi-K type and S / C type are -25 ° C to + 85 ° C.


8. Insulation resistance: Also called leakage resistance. Because the dielectric between the two poles of the capacitor is not an absolute insulator, his resistance is not infinite. The DC voltage is added to the capacitor and generates a leakage current. The ratio between the two is called the insulation resistance. When the capacitance is small, the insulation resistance mainly depends on the surface state of the capacitor. When the capacity is> 0.1uf, the insulation resistance mainly depends on the performance of the medium. Generally, a small-capacity capacitor has a large insulation resistance, in the hundreds of megaohms or several gigaohms. The insulation resistance of electrolytic capacitors is generally small.


9. Dielectric loss: The energy consumed by a capacitor in a unit of time due to the electric field. These losses mainly come from dielectric losses and metal losses.


10. Frequency characteristic: The property that the electric parameter of the capacitor changes with the frequency of the electric field. Capacitors that operate under high-frequency conditions have a lower capacitance because the dielectric constant is smaller at high frequencies than at low frequencies. Losses also increase with increasing frequency. In addition, during high-frequency operation, the distribution parameters of the capacitor, such as the resistance of the pole piece, the resistance between the lead and the pole piece, the inductance of the pole piece, and the lead inductance, will affect the performance of the capacitor. All these make the use frequency of the capacitor limited. Different capacitors have different maximum operating frequencies. Small mica capacitors are within 250MHZ; wafer type ceramic dielectric capacitors are 300MHZ; round tube ceramic dielectric capacitors are 200MHZ; disc type ceramic dielectric capacitors can reach 3000MHZ; small paper dielectric capacitors are 80MHZ; medium paper capacitors are only 8MHZ.


11. Capacitor time constant: A time constant has been introduced to properly evaluate the insulation of bulk capacitors. It is equal to the product of the capacitor's insulation resistance and capacity.


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