Determination of the saturation current of inductors with magnetic circuits. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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In the development and manufacture of inductors, pulse transformers, the question arises of their suitability for operation in specific conditions. This is due to the fact that the parameters of the magnetic circuits used are often not exactly known. As a result, a situation is possible when the material of the transformer magnetic circuit enters saturation, which reduces the efficiency of the power supply or disables it. For inductors (chokes) this leads to a significant reduction in inductance with the ensuing consequences. The authors propose a device that allows checking such elements for the possibility of their operation in specific conditions.

The device is designed to determine the current of inductors (chokes) or windings of pulse transformers with ferromagnetic, alsifer cores, at which saturation of the magnetic circuit material occurs. Although there are various recommendations for the calculation and manufacture of such elements, but without knowing the actual parameters of the magnetic circuit (especially with a non-magnetic gap), it is difficult to obtain the desired result or determine the possibility of their application in a particular device.

Fig. 1

The scheme of the device is shown in fig. 1. It consists of a pulse generator on logic elements DD1.1-DD1.6, a buffer stage on transistors VT1, VT2, a powerful field-effect switching transistor VT3 and a current sensor on resistor R8. The buffer stage provides fast charging and discharging of the gate-source capacitance of the transistor VT3, the VD4 diode serves to limit voltage surges on the tested inductor. The pulse generator implements separate adjustment by resistors R4 and R5 of the pulse duration and their repetition period, respectively. The duration of the pulses is changed within 6 ... 60 μs in one range and 60 ... 600 μs in the other. The repetition period can be changed within 0,2...2 ms and 2...20 ms, respectively. The ranges are switched by switch SA1. The supply voltage is supplied to the pulse generator through the diode VD3 and smoothed by the capacitor C3, which reduces the impact on its operation of interference that occurs in the power supply circuit of the device during the flow of pulsed currents. A low-resistance resistor R3 is installed in the source circuit of the transistor VT8, the voltage drop across which is proportional to the current flowing through this transistor and the checked inductor "Lx". The voltage is applied to the input of the oscilloscope, on the screen of which its shape is controlled.

Fig. 2

Initially, in the first range, the minimum pulse duration is set at the maximum duty cycle (maximum repetition period). A large duty cycle allows you to reduce the average power dissipation on the VT3 transistor, as well as use a less powerful power source, since the pulsed current is provided by capacitors C4, C5. An oscilloscope is connected to the XS2 sockets, the tested inductor is connected to the XS1 sockets and the supply voltage (10 ... 15 V) is applied. On the oscilloscope screen, it is necessary to obtain an oscillogram corresponding to Fig. 2. If the brightness of the image on the oscilloscope screen is insufficient, resistor R5 should reduce the pulse repetition period. But you should not get carried away with this, since this will lead to an increase in the current consumption and heating of the VT3 transistor.

Fig. 3

Then, the pulse duration should be gradually increased until the linear increase in voltage turns into a nonlinear one (Fig. 3), and the point Un determines the current at which the material of the magnetic circuit is saturated: Ihac = Un/0,2. If it was not possible to reach the Un point on the first range, the second range of the generator is switched on.

It should be noted that the maximum allowable duration of the voltage pulse on the inductor tn at the point Un is inversely proportional to the voltage of this pulse. For example, if a pulse transformer is checked in a device at a supply voltage of 15 V and saturation occurs at a pulse duration tn = 300 μs, then in a network switching power supply at a supply voltage of 300 V, the pulse duration should be 20 times less: tn <15 μs.

Fig. 4

Construction and details. All parts are mounted on a board made of one-sided foil fiberglass, its drawing is shown in fig. 4. The board is placed in a case made of insulating material, on the walls of which there are sockets for connecting an oscilloscope, inductors (crocodile clips can be used), a switch and variable resistors. The device uses variable resistors SP, SPO, SP-4, resistor R8 - C5-16MV-2W, the rest - MLT, C2-33. Capacitors C4, C5 - K50-24, C3 - K50-35 or similar imported ones, C1, C2 - K73-9, K73-24, K10-17. Diodes KD510A are replaceable by pulse low-power series KD503, KD521, KD522 with any letter indices, diode FR801 can be replaced by FR802, FR803, HER801, transistor IRFZ44N - by IRFZ48N, transistors KT3117A, KT313A - respectively by KT698 and KT6127 with any letter indexes.

To power the device, a stabilized power supply with current protection and an output voltage of 10 ... 15 V at a current of up to 1 A is used. The adjustment comes down to checking the generator's performance and, if desired, grading the scales of variable resistors. The practical benefit of the measurements carried out is that it is possible to simplify calculations that give approximate results and require experimental verification, and obtain specific results that are more compatible with the problem being solved.

Author: Yu. Gumerov, A. Zuev, Ulyanovsk; Publication: radioradar.net

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