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Components related to "magnetism" in circuit boards include Permalloy cores, inductors, transformers, electromagnetic relays, contactors, Hall sensors, etc. This article explains the inspection and maintenance methods of magnetic components in circuit boards.   (1) Inductor Inductor coils are wires wound around an insulating frame, which can be hollow, iron core or magnetic core. In the application of industrial control circuit boards, the most common use is for filtering or energy storage in switching power supplies. Various appearances of inductors. The unit of inductance is Henry, abbreviated as Henry, represented by the letter H. There are also millihenry (mH), microhenry (uH), and nanohenry (nH). The relationship between them is: 1H = 1000mH 1mH = 1000uH 1uH = 1000nH Inductor coils use direct marking method, 22uH, 100 represents 10uH, 4R7 represents 4.7uH, R10 represents 0.1uH, and 22n represents 22nH; some inductors use color ring marking method, and their inductance is the same as the color ring resistor, such as color ring brown, black, brown, and silver represent inductance 100uH ±10% There is a type of inductor used to absorb ultra-high frequency (above 50MHZ) interference, this type of inductor is called magnetic beads. There is another type of commonly used inductor called common mode inductor, also called common mode choke. It is a 4-terminal device led out by two identical windings wound on a ferrite core (Permalloy core). Each set of coils is connected in series in the circuit. If there is a differential mode signal, the magnetic flux generated by the signal through the two coils cancels each other, and the coil has no blocking effect on the differential mode signal. When there is a common mode signal, the magnetic flux generated by the two coils is enhanced, and the coil's blocking effect on the signal is enhanced. This blocking effect is bidirectional, which can not only prevent the front-stage interference signal from entering the rear stage, but also prevent the rear-stage interference signal from entering the front stage. In the maintenance of industrial circuit boards, inductor coils are components that are not easy to damage. Occasionally, they are broken due to corrosion, burned due to excessive current, and short circuits between coil turns. Open circuit damage can be measured with the resistance range of a multimeter. The inductance can be measured with an inductance tester. It is recommended to use a digital bridge to test the inductance. Because most power circuit energy storage inductors operate at higher frequencies, all above 10kHz, the frequency is selected at 10kHz when using a bridge test. In addition to paying attention to the inductance, the test focuses on the D value. The normal D value should be less than 0.1. If the D value is greater than 0.2, it is determined that there is a short circuit between the coil turns.   (2) Transformer The transformer is a device that uses the principle of electromagnetic induction to change the voltage. Common transformers in industrial control circuit boards are power frequency transformers using iron cores and switching transformers using ferrite cores (Permalloy cores). The basic characteristics of an ideal transformer are: the ratio of the input and output AC voltages is the same as the ratio of the number of turns of the input and output coils, so in theory, the AC voltage can be arbitrarily stepped up or down. Transformers with silicon steel sheet cores are generally used in industrial frequency applications of 50~400HZ. The magnetic flux density of the silicon steel sheet core is large. Although there is insulation paint between the stacked silicon steel sheets, there is still eddy current loss in a single silicon steel sheet. This type of core is not suitable for high-frequency applications. The resistivity of ferrite core is much greater than that of metal and alloy magnetic materials, so the eddy current loss is very small. Transformers made of ferrite core are used in relatively high-frequency occasions such as energy storage inductors and switching transformers of switching power supplies. In addition, different new transformer core materials have appeared, such as Permalloy and amorphous nanocrystalline materials, which can take into account both magnetic permeability and eddy current loss. Transformer failure detection method Common transformer damages include coil burnout or internal overheating that burns the coil insulation and causes a short circuit between coil turns. It is easier to judge the coil open circuit by measuring the resistance, while the short circuit between turns is more troublesome to judge because the coil itself has a small resistance and is not easy to distinguish through resistance testing. Generally speaking, transformers with serious internal short circuits between turns generate more heat, which will burn the covering material of the transformer coil and have more or less burnt smell. This situation can be clearly distinguished by observing the appearance of the transformer. Some transformers have internal turn-to-turn short circuits, which are not so obvious from the appearance. Friends who often repair switching power supplies may have such an experience, that is, they have tested and even replaced almost all suspected components of the switching power supply except the transformer, including the Permalloy core, but the power supply has not been repaired yet, and finally they suspect that the switching transformer is damaged. If there is a way to detect transformer damage at the beginning, wouldn’t it be easy? In fact, this is completely possible, and the instrument for detection is still a digital bridge. The method is to put the digital bridge in the 10KHZ test inductance loss D value state, and test it online without removing the transformer. The bridge signal voltage is selected as 0.3V, and the D value of the transformer main winding coil is tested. The normal transformer D value should be <0.1. If the D value is >0.2, the transformer is judged to be damaged. In addition to switching transformers, digital bridges are also applicable to determine whether other types of transformers are damaged, but it should be noted that when selecting the frequency, a frequency close to the actual operating frequency of the transformer should be used.   (3) Electromagnetic relays and contactors Electromagnetic relays and contactors are devices that use the electromagnetic force generated by electromagnetic coils in conjunction with springs and mechanical levers to control the on and off of contacts. Relays usually have a sealed packaging space to minimize the impact of external adverse environments on contacts. Relative to contactors, the contact current they control is smaller; the contact current of contactors is larger. There are also reed relays, whose principles are similar to those of electromagnetic relays, but the contact current is relatively smaller, the contacts are sealed, and are not polluted by dust, moisture and harmful gases, and the response speed and reliability are greatly improved. Common faults of relays and contactors are large contact resistance, burnt contacts, and open circuit when contacts are closed. When testing, the rated voltage can be applied to the coil to detect the conduction and closure of the contacts. Both the coil and the non-powered conditions must be tested. The ohm range of a multimeter can be used to measure the resistance of the contacts when they are on. If there is no abnormality, it is basically close to 0Ω. If it is above 10Ω, it is considered a fault. If the contacts are visible, emergency maintenance can be performed by filing off the ablated and oxidized parts of the contacts to reveal the metallic luster. The relay or contactor can be put back into use. For safety reasons, it is recommended to replace new parts. After the relay coil is energized, the energy is transmitted to the coil and the armature is attracted. After the power is cut off, if no measures are taken, the electromagnetic energy of the coil will inevitably generate a high self-induced electromotive force at both ends of the coil during the transition from power on to power off. There will be a high voltage, which may damage other components. Therefore, a diode should be connected in reverse parallel to the coil to provide a release circuit for the electromagnetic energy of the coil. When testing the quality of the relay online, a reverse coil rated voltage can be applied to the diode end according to the direction of the diode for detection without removing the relay to detect the Permalloy core. In high current occasions such as inverters and servo drives, many current detections require Hall sensors. The working principle of Hall current sensors is based on the Hall effect. A certain current is passed through a conductive sheet in the x direction, and the magnetic field in the z direction passes through the sheet vertically. Then, the electrons in the conductive sheet are acted upon by the Lorentz force in the process of moving toward the negative pole, and gather in the y+ direction, making one end in the y+ direction negatively charged and the other end in the y- direction positively charged. If the voltage VH at both ends is measured, its magnitude is proportional to the current I and the magnetic induction intensity B. If the current I is constant, then the magnitude of VH directly reflects the magnitude of the magnetic induction intensity B, so as long as VH is measured, the magnitude of B can be known. The principle of the Hall current sensor is that the through-core conductor generates a circumferential magnetic field proportional to the current. The magnetic field passes vertically through the Hall sensor in the middle of the iron core. The Hall voltage Vh induced by the sensor is proportional to the measured current of the conductor, so the current of the measured conductor can be measured contactlessly. The actual Hall sensor has three wires, two positive and negative power lines, and one current lead line. A sampling resistor is connected in series between the current lead line M and 0V. The direction and magnitude of the current flowing through the sampling resistor are proportional to the magnitude and direction of the current passing through the sensor wire. Therefore, the magnitude and positive and negative of the voltage at both ends of the sampling Permalloy core resistor reflect the magnitude and direction of the wire current. Detection method of Hall current sensor It is common for the current sensor to be damaged. The most convenient test method is to test the voltage of the output end to 0V after power is turned on. If there is no current in the core wire, the voltage at the sensor signal output end should be 0V. If the measured voltage offset is more than ±1V, it is judged that there is a problem with the Hall current sensor and the Permalloy core. Some Hall sensor Permalloy cores are connected to a single power supply. When there is no current in the corresponding core wire, the signal output voltage is half of the power supply voltage. If a 5V voltage is connected, the output voltage is 2.5V.