DINGFENG CAPACITOR - - Capacitor selection guide

Jun 2,2020

General characteristics parameter selection requirements


(1) Model


Generally used in the circuit for low frequency coupling, bypass decoupling, etc. When the performance requirements are not strict, paper dielectric capacitors and electrolytic capacitors can be used. For the coupling capacitor of the low-frequency amplifier, use an electrolytic capacitor of 1-22μF;


The bypass capacitor is selected according to the circuit operating rate. For example, in low-frequency circuits, the bypass capacitor of the emitter is an electrolytic capacitor with a capacity of 10-220μF;


In the intermediate frequency circuit, 0.01-0.1μF paper media, metalized paper media, organic film capacitors, etc. can be used;


In high-frequency circuits, mica capacitors and ceramic capacitors should be used. In power supply filtering and decoupling circuits, electrolytic capacitors can be used.


(2) Accuracy


In bypass, decoupling, and low-frequency coupling circuits, there is generally no strict requirement on the accuracy of the capacitor. When selecting, you can choose a capacitor with a similar capacity or a slightly larger capacity according to the design value.


But in other circuits, such as the oscillation circuit, delay circuit, and tone control circuit, the capacity of the capacitor should be as consistent as possible with the calculated value.


In various filters and various networks, there are higher requirements for the accuracy of capacitance, and high-precision capacitors should be selected to meet the requirements of the circuit.


(3) Rated working voltage


The rated working voltage of the capacitor should be higher than the actual working voltage with sufficient margin to prevent damage due to voltage fluctuations. Generally speaking, the working voltage should be lower than 10%-20% of the rated working voltage of the capacitor.


In some circuits, the amplitude of voltage fluctuation is large, and a larger margin can be left.


Capacitors with polarity cannot be used in AC circuits.


The temperature resistance of electrolytic capacitors is very poor. If the operating voltage exceeds the allowable value, the dielectric loss will increase, which can easily lead to excessive temperature rise and eventually damage.


Generally speaking, only lower temperature rise is allowed when the capacitor is working, otherwise it is abnormal. Therefore, when the equipment is installed, it should be as far as possible away from heating elements (such as high-power tubes, transformers, etc.). If the working environment temperature is high, the working voltage should be reduced.


Generally, small-capacity capacitors have low dielectric loss and good temperature resistance and stability, but the circuit requirements for them are also relatively high. Therefore, when selecting the rated operating voltage, there should still be a certain margin, and pay attention to the environment. The effect of operating temperature.


(4) Insulation resistance


The smaller the insulation resistance, the greater the leakage current. The leakage current not only consumes power in the circuit, it is important that it will cause the circuit to malfunction or reduce the performance of the circuit. The power loss caused by the leakage current will cause the capacitor to heat up, and its temperature will increase, and it will generate a larger leakage current. This cycle will easily damage the capacitor. Therefore, when choosing a capacitor, choose a capacitor with a sufficiently high insulation resistance, especially a capacitor used under high temperature and high pressure conditions.


In the case of bridge arms and computing elements in bridge circuits, the level of insulation resistance will affect the accuracy of measurements and calculations. Capacitors with high insulation resistance values ​​must be used.


The loss of the capacitor also directly affects the performance of the circuit in many occasions. In filters, intermediate frequency circuits, oscillation circuits and other circuits, the loss is required to be as small as possible, which can improve the quality factor of the circuit and improve the performance of the circuit.


(5) Temperature coefficient


The larger the temperature coefficient of the capacitor, the greater the change in its capacity with temperature, which causes the circuit to drift, causing unstable circuit operation. This is not allowed in many circuits, such as oscillating circuit components and filters in oscillating circuits. In these cases, capacitors with small temperature coefficients should be selected to ensure stable operation.


(6) Frequency characteristics


In high-frequency applications, the performance of the capacitor will deteriorate due to the influence of the capacitor's own inductance, lead inductance, and high-frequency loss. Capacitors with poor frequency characteristics can not only play their due role, but also cause a lot of trouble. For example, the distributed inductance of paper dielectric capacitors will cause high-frequency amplifiers to generate ultra-high frequency parasitic feedback, making the circuit inoperable. Therefore, when choosing capacitors for high-frequency circuits, one should pay attention to the frequency parameters of the capacitor, and the other should pay attention to that the leads of the capacitor should not be left too long, so as to reduce the adverse influence of the lead inductance on the circuit.


(7) Derating


The use of derating can improve the reliability, and the components working in the worst conditions are an important factor for the actual life not reaching the rated life. The capacitance derating standard is as follows:


(8) Use environment


The quality of the use environment directly affects the performance and life of the capacitor. In environments with high operating temperatures, capacitors are prone to leakage and accelerated aging. Therefore, when designing and installing, use capacitors with a small temperature coefficient as far as possible, and stay away from heat sources and improve ventilation and heat dissipation in the machine. If necessary, forced air cooling should be used. In cold conditions, due to the low temperature, ordinary electrolytic capacitors will fail due to the freezing of the electrolyte, which will make the equipment work abnormally. Therefore, cold-resistant electrolytic capacitors must be used.


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