Electrolytic capacitanceelectrolytic capacitance is a kind of capacitance with oxide layer formed on the electrode by the action of electrolyte as the insulating layer, which usually has a large capacity. Electrolyte is a liquid, gel like substance rich in ions. Most electrolytic capacitors have polarity, that is to say, the positive voltage of the capacitor is always higher than the negative voltage.
The high capacity of electrolytic capacitor also sacrifices many other characteristics, such as large leakage current, large equivalent series inductance and resistance, large capacitance error, short life and so on.
In addition to the electrolytic capacitors with polarity, there are also electrolytic capacitors without polarity. In the figure below, there are two kinds of 1000uf, 16V electrolytic capacitors, the larger one is non-polar, the smaller one is polar.
▲ non polarity and polarity electrolytic capacitor the inside of electrolytic capacitor may be liquid electrolyte or solid polymer, and the electrode material is commonly aluminum or tantalum. The following figure shows the internal structure of a common aluminum electrolytic capacitor with polarity. Between the two electrodes is a layer of fiber paper impregnated with electrolyte, and then an insulating paper is added to turn it into a cylinder, which is sealed in an aluminum shell.
▲ internal structure of electrolytic capacitor ▲ dissect the electrolytic capacitor, and you can clearly see its basic structure. In order to prevent the evaporation and leakage of electrolyte, sealed rubber is used to fix the capacitance pin.
The figure also shows the difference between the internal volume of the electrolytic capacitor with and without polarity. Under the same capacity and voltage withstand level, the electrolytic capacitor without polarity is about twice as large as that with polarity.
The difference between the internal structure of non-polar and polar electrolytic capacitors is mainly due to the large difference of electrode area between the two capacitors. The left side of the figure below is a non-polar capacitor electrode, and the right side is a polar electrode. In addition to the area difference, the thickness of the two electrodes is also different. The thickness of the electrode with polarity is thin.
When the voltage applied by the capacitor exceeds its withstand voltage, or when the polarity of the voltage applied by the polar electrolytic capacitor is reversed, the leakage current of the capacitor will rise sharply, resulting in the increase of the internal heat of the capacitor, and the electrolyte will produce a lot of gas.
In order to prevent the capacitor from exploding, three grooves are pressed on the top of the capacitor shell, so that the top of the capacitor breaks first under high voltage and releases the internal pressure.
However, during the manufacturing process of some capacitors, the groove at the top of the electrolytic capacitor is pressed unqualified. The pressure inside the capacitor will make the sealing rubber at the bottom of the capacitor pop out. At this time, the pressure inside the capacitor will suddenly release, which will form an explosion.
1. The figure below shows a non-polar electrolytic capacitor at hand, with a capacity of 1000uf and withstand voltage of 16V. When the applied voltage exceeds 18V, the leakage current suddenly increases, and the temperature and pressure inside the capacitor increase. Finally, the rubber sealing ring at the bottom of the capacitor exploded, and the internal electrode was smashed loose like popcorn.
By binding a thermocouple on the capacitor, the temperature of the capacitor can be measured as the applied voltage increases. The following figure shows the process that the internal temperature of the nonpolar capacitor continues to increase when the applied voltage exceeds the withstand voltage.
The relationship between voltage and temperature is shown in the figure below, which shows the current change through the capacitor during the same process. It can be seen that the increase of current is the main cause of the internal temperature rise. In this process, the voltage increases linearly. With the rapid increase of the current, the voltage in the power supply decreases. Finally, when the current exceeds 6a, the capacitor explodes with a loud noise.
The relationship between voltage and current is due to the large internal volume of non-polar electrolytic capacitor and the large number of electrolyte, so the pressure generated after overcurrent is huge. As a result, the pressure relief groove at the top of the shell is not broken, and the sealing rubber at the bottom of the capacitor is exploded.
2. The explosion of the polar electrolytic capacitor applies voltage to the polar electrolytic capacitor. When the voltage exceeds the withstand voltage of the capacitor, the leakage current will also rise sharply, causing the capacitor to overheat and explode.
The figure below shows the limiting electrolytic capacity, 1000uf, 16V. The process of releasing internal air pressure through the top relief tank after overpressure. Therefore, the capacitor explosion process is avoided.
The figure below shows how the temperature of the capacitor changes with the increase of applied voltage. When the voltage is close to the withstand voltage of the capacitor, the residual current of the capacitor increases and the internal temperature continues to rise.
The relationship between voltage and temperature is shown in the figure below as the change of leakage current of capacitor. During the test, when the voltage is more than 15V, the leakage of the capacitor starts to rise sharply.
The relationship between voltage and current can also be seen for the withstand voltage limit of the 1000uf ordinary electrolytic capacitor through the experience of the previous two electrolytic capacitors. In order to avoid capacitor breakdown by high voltage, therefore. When using electrolytic capacitor, it is necessary to leave enough margin according to the actual voltage fluctuation.
When the electrolytic capacitors are connected in series, larger capacitance capacity and larger capacitance withstand voltage can be obtained by parallel connection and series connection respectively.
In some applications, the voltage applied to the capacitor is AC voltage, such as the coupling capacitance of the loudspeaker, AC phase compensation, motor phase-shifting capacitance, etc., which requires the use of non-polar electrolytic capacitor.
In the user manual given by some capacitor manufacturers, it is also given that the effect of non-polar capacitor can be obtained by using the traditional polar capacitor through back-to-back series connection, that is, the two capacitors are connected in series, but the polarity is opposite.
Under the condition of applying forward voltage, reverse voltage and two electrolytic capacitors connected in series back to back to form nonpolar capacitor, the leakage current changes with the increase of applied voltage.
1. The forward voltage and leakage current measure the current flowing through the capacitor through a resistor in series. Within the withstand voltage range of the electrolytic capacitor (1000uf, 16V), gradually increase the applied voltage from 0V, and measure the relationship between the corresponding leakage current and voltage.
The figure below shows the relation between the leakage current and the voltage of the aluminum electrolytic capacitor with polarity. This is a nonlinear relationship. The leakage current is less than 0.5mA.
The relationship between voltage and current after forward series connection 2. Reverse voltage and leakage current use the same current to measure the relationship between applied direction voltage and leakage current of electrolytic capacitor. As can be seen in the figure below, when the applied reverse voltage exceeds 4V, the leakage current starts to increase rapidly. According to the slope of the curve, the reverse electrolytic capacitance is equivalent to a resistance of 1 ohm.
The relationship between reverse voltage voltage and current 3. The back-to-back series capacitor connects two same electrolytic capacitors (1000uf, 16V) in series, forming an equivalent electrolytic capacitor without polarity. Measure the relation curve between their voltage and leakage current.
The figure below shows the relationship between the capacitance voltage and the leakage current. It can be seen that when the applied voltage exceeds 4V, the leakage current will increase and the current amplitude is less than 1.5mA.
But the measurement is a little surprising. You can see that the leakage current of these two back-to-back series capacitors is actually greater than that of a single capacitor when the voltage is applied forward. It's really strange.
The relationship between the voltage and the current after the positive and negative series connection is not tested repeatedly because of time. Maybe one of the capacitors used is the capacitor just tested for reverse voltage. There is damage inside. So the above test curve is generated.