Permalloy iron core: detailed explanation of circuit board magnetic component mapublish time：2019-11-26 18:36:03
The components related to "magnetism" in the circuit board include permalloy core, inductance coil, transformer, electromagnetic relay, contactor, Hall sensor, etc. In this paper, Jinxin magnetic material explains the maintenance methods of magnetic components of circuit board for you.
(1) inductance coil
The inductance coil is to wrap the wires around the insulation framework, which can be hollow, iron core or magnetic core. In the application of industrial control circuit board, the most common is to do the filtering or energy storage in switching power supply. Inductance appearance. The unit of inductance is Henry, which is abbreviated to Heng. It is indicated by the letter h. In addition, there are MH, uh and NH. Their relationship is as follows:
1H = 1000mH 1mH = 1000uH 1uH = 1000nH
Direct marking method is used for inductance coil, 22uh, 100 for 10uh, 4r7 for 4.7uh, R10 for 0.1uh, 22n for 22nh; color ring marking method is used for other inductors, and its inductance is the same as that of color ring resistance, such as color ring brown, black, brown, silver for 100uh ± 10%
There is a kind of inductor used to absorb interference of ultra-high frequency (above 50MHz), which is called magnetic bead. There is also a common type of inductance called common mode inductance, also known as common mode choke. It is a four terminal device which is led out by two identical windings wound on a ferrite core (permalloy core). Each group of coils is connected in series in the circuit. If there is a differential mode signal, the flux generated by the two coils will cancel each other, so the coil has no blocking effect on the differential mode signal When there is a common mode signal, the magnetic flux produced by the two coils is strengthened, and the coil has a stronger blocking effect on the signal. This blocking effect is bidirectional, which can not only prevent the interference signal of the front stage from entering the rear stage, but also prevent the interference signal of the rear stage from entering the front stage.
In the maintenance of the industrial circuit board, the inductive coil is a kind of component which is not easy to be damaged. Occasionally, it is broken due to corrosion, too much current and short circuit between turns of the coil. Open circuit damage can be measured by multimeter resistance. Inductance can be measured by inductance tester. It is recommended to use digital bridge to test inductance. Because most of the power circuit energy storage inductors work at a higher frequency, all of them are above 10kHz, so when using the bridge test, the frequency is 10kHz. In addition to focusing on the inductance, the test focuses on the D value. The normal D value should be less than 0.1, and if the D value is greater than 0.2, it is determined that there is a coil turn to turn short circuit.
The transformer is a device that changes the voltage by using the electromagnetic induction principle. The common transformers on the industrial control circuit board are the power frequency transformer with iron core and the switch transformer with ferrite core (permalloy core).
The basic characteristic of an ideal transformer is that the ratio of the input and output AC voltage is the same as the ratio of the number of turns of the input and output coils, so theoretically, the AC voltage can be increased or decreased arbitrarily.
The transformer with silicon steel sheet core is generally used in 50 ~ 400Hz power frequency occasions. The magnetic flux density of silicon steel sheet iron core is large. Although there is insulation paint insulation between the superimposed silicon steel sheets, there is eddy current loss in the single silicon steel sheet, which is not suitable for such iron core in high frequency occasions.
The resistivity of ferrite core is much higher than that of metal and alloy magnetic materials, so the eddy current loss is very small. The transformer made of ferrite core is used to compare the energy storage inductor and switching transformer of switching power supply.
In addition, there are different kinds of new core materials, such as permalloy and amorphous nanocrystalline materials, which can take into account the permeability and eddy current loss.
Transformer failure detection method
The damage of transformer is often caused by open circuit of coil burning or internal overheating, which causes coil turn to turn short circuit. Coil open circuit is better to judge, just measure the resistance, while turn to turn short circuit is more troublesome to judge, because the resistance of coil itself is small, which is not easy to distinguish through resistance test.
Generally speaking, the transformer with internal turn to turn short circuit has a large heat output, which will scorch the covering material of transformer coil and have more or less scorching smell. This situation can be clearly distinguished by observing the appearance of the transformer.
Some transformers have internal turn to turn short circuit, which is not obvious from the appearance. Friends who often repair the switching power supply may have such experience, that is, they have detected or even replaced almost all suspected components of the switching power supply except the transformer, including permalloy iron core. The power source has not been repaired, and finally they suspect that the switching transformer is damaged. If there's a way to detect Transformer Damage in the first place, wouldn't it be easy? In fact, this is entirely possible. The instrument for testing is still a digital bridge. The method is to put the digital bridge at 10kHz to test the D value of inductance loss, online test can be done without removing the transformer, the bridge signal voltage is 0.3V, test the D value of the main winding coil of the transformer, the normal D value of the transformer should be less than 0.1, if the D value is greater than 0.2, then judge the transformer damage.
In addition to switching transformer, digital bridge is also applicable to judge whether other types of transformer are damaged, but attention shall be paid to the frequency close to the actual working frequency of transformer when selecting the frequency.
(3) electromagnetic relay and contactor
Electromagnetic relay and contactor are a kind of devices that use the electromagnetic force generated by electromagnetic coil and spring and mechanical lever to control the on-off of contact. Generally, the relay has a sealed packaging space, so as to minimize the impact of external bad environment on the contact. Compared with the contactor, the contact current it controls is smaller; the contact current of the contactor is larger. In addition, the principle of dry spring relay is similar to that of electromagnetic relay, but the contact current is relatively smaller