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Problems needing attention in the process of using tantalum capacitors

Source:未來(lái)電子 Author:Admin Visit:2 Pubtime:2016-09-10 11:54:20

When the tantalum capacitor is installed on the PCB board, under the mechanical stress, when the tantalum capacitor installation is not at that time, it is easy to lead the lead to bear large stress or resonance.

The following should be taken into account when selecting and using a tantalum capacitor:

A) circuit designers in order to design a circuit that can be satisfied with the work within the time required, the capacity of the capacitor to be used must be considered. Dielectric absorption; capacitance and pressure, vibration and shock; capacitance in the circuit aging and storage conditions.

B) to take into account the capacitance between the terminal and the capacitor, if this capacitance will produce stray capacitance and leakage current.

C) can be used in combination with a variety of capacitors to obtain the required amount of capacitance, and thus the amount of compensation capacity - temperature characteristics, etc..

D) the peak voltage applied to the capacitor can not exceed the specified value of the corresponding specification. Usually, the same peak voltage may be decreased due to the following conditions: aging; temperature rise; medium voltage region increases; high frequency; moisture into the capacitor. One point to be stressed is that don't ignore the short term transient voltage in the application.

E) when the capacitor is higher than the ground potential under high pressure condition, and to adopt additional insulation insulation, a capacitor terminal connected to the shell, because the capacitance and voltage distribution depends on between the capacitor core and shell and the capacitance between the casing and the chassis.

F) the peak current of charge and discharge must be considered according to the time constant of the circuit.

G) must consider internal heat and ambient temperature.

H) must consider the influence of humidity, pressure, corrosion resistance, mold, vibration and shock and other environmental factors.

I) must consider the insulation resistance, especially in the high temperature insulation resistance.

J) when working in series in a DC circuit, a balanced resistor must be considered.

K) the effective inductance of a large capacitance capacitor can be reduced by a parallel capacitor.

L) because the capacitor has an inductance, it is possible to generate instantaneous oscillation in parallel in each work or instantaneous operation of the circuit.

Poor electrical contact in low voltage M may open or produce noise.

N) the energy stored in the capacitor is dangerous to people and equipment, and the appropriate precautions should be taken.

O) filled with liquid capacitors can not be inverted, because it will lead to internal corona.

P) non - air sealed capacitors may be affected by "breathing".

Tantalum capacitor dielectric oxide film has a unidirectional conductivity and rectification characteristics, when the reverse voltage is applied, there will be a lot of current through, and even cause a short circuit failure. Therefore, the reverse voltage should be strictly controlled in the application. Solid electrolyte tantalum capacitors are generally not allowed to add reverse voltage, and can not be used in pure AC circuit for a long time. If in no circumstances, allow the reverse voltage applied amount in a short period of time, its value is below 25 DEG C: less than 10%UR or 1V (for small) 85 degrees less than or equal to 5%UR or 0.5V (for small) 125 degrees less than or equal to 1%UR or 0.1V (for small). If the capacitor is used in a circuit with reverse voltage for a long time, the use of bipolar tantalum capacitors, but also can only be in the polarity of the DC or pulse circuit frequency is not too high.

Non solid electrolyte tantalum capacitors are in the factory before the test, there is no problem of welding, the machine does not need to pre processing. If necessary (if stored for more than two years, or damp, or by acid gas pollution, etc.) can be used for tin treatment. Fully sealed solid tantalum capacitors, whether it is welding, or dip tin treatment, processing distance should be controlled in the technical specifications of the 3.2mm outside the sealing tin package, the temperature is not higher than 260 degrees, the time is not more than 5 seconds. Because the sealing material sealed solid tantalum capacitor is solder, if the time is too long, the temperature is too high or too close, body welding distance is less than 3.2mm, are likely to cause the sealing tin melting, lead capacitor humidity, seal, affect the electrical performance and reliability; serious, electric heated container internal negative pressure, the the seal cavity is caused by inhalation of solder, redundances and short circuit. For tantalum capacitor tin dipping treatment, at rated voltage and 85 DEG C aging 4 ~ 8 hours, and then measure the electrical properties (bipolar product per hour commutation time, leakage current should also be two directions were measured).

Tantalum capacitors can be stored for 14 years (except for welding), but storage for more than 2 years or dip tin tantalum capacitors, rated voltage is applied, the internal resistance of the power supply before the use of less than 3 ohm.

Tantalum capacitors should be used to avoid over temperature. High temperature will make the material properties change, due to a variety of thermal products with different coefficients of expansion, may produce internal stress due to product failure; long time storage at high temperature, the product may be produced due to internal stress failure. Therefore, the tantalum capacitor must be used in the temperature range of the standard.

The failure rate of the tantalum capacitor is the difference of the DC rating (85 C, rated voltage), and due to the different conditions (ambient temperature, applied voltage, circuit resistance, etc.). In the actual circuit, the voltage or current is often the peak impulse and ripple current, or other unexpected electric shock, so the actual use of the drop design is necessary. Proposed general down to 65%UR below, so as to ensure that the product and the complete line. When the ambient temperature is greater than 85, the amount of the reduction should be considered in the third article.

Non solid electrolyte tantalum capacitors in wet PH test before the leak should fully discharge, otherwise it will be due to incomplete discharge of the capacitor to the contact strip and the capacitor anode is red; the contact strip and the capacitor cathode is blue LED