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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Device for limiting the starting current of an electrical appliance. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network The limiter circuit is shown in fig. 1. It is a reworking of a device previously developed and described in [1]. The use of a more modern element base and a slightly different approach to the problem made it possible to increase the power of the protected load, significantly reduce energy losses, increase reliability and reduce the dimensions of the device.
When the contacts of the SA1 switch are closed, the capacitor C2 is quickly charged through the resistors R1, R2 and the diodes VD1, VD2. The voltage across this capacitor is limited by the Zener diode VD3 to 15 V. The field-effect transistor VT1 opens. As soon as the voltage drop across the resistors R4 and R5, proportional to the load current, reaches (taking into account the smoothing effect of the capacitor C4, the R6C3 circuit and the position of the trimmer resistor R7 engine) a value sufficient to open the trinistor VS1, the latter will open. This will lead to a sharp decrease in the voltage at the gate of the field-effect transistor VT1, it will close, de-energizing the load. However, at the end of the current and the beginning of the next half-cycle of the mains voltage, the current through the trinistor will stop and it will close, allowing the capacitor C2 to charge again, and the transistor VT1 to open. Then the process is repeated, however, in each next half-cycle, the resistance of the heating or accelerating load becomes greater than in the previous one, and the time required to reach the SCR opening threshold increases. In the end, the amplitude of the voltage pulses across the resistors R4, R5 becomes insufficient to open the trinistor, and it remains closed all the time. This is the steady state of operation of the limiter, in which the transistor VT1 is open all the time, and the load operates in the nominal mode. Varistor RU1 protects the transistor from damage by high voltage pulses, the source of which can be either a power supply or an inductive load, for example, a transformer winding. Unlike some other devices [2], the proposed one cannot be included in the gap of one of the load power wires. I do not consider this a disadvantage, because instead of installing a protective device near the switch, where access to the second mains wire is difficult, it can easily be mounted where both wires are present: at the base of a chandelier, in the body of a lamp or other protected electrical appliance. Since there are no inertial elements in the limiter (time-setting capacitors, thermistors), it is ready for repeated smooth switching on of the load immediately after switching off. Another feature is the operation of the field-effect transistor VT1 in the key mode, both during start-up and in the steady state of the load. Therefore, the power dissipated by this transistor is small, which greatly increases the reliability of the device. With the values of resistors R4, R5 indicated in the diagram, the limiter works with incandescent lamps with a total power of 25 ... 120 W as a load.
All parts are hinged mounted on a round board with a diameter of 50 mm (Fig. 2). It can be easily placed in most pendant and wall lights. The IRF840 field effect transistor can be replaced, for example, with BUZ40B, IRFP4S2, IRF450, TSD2M450V or other n-channel field effect transistors with a drain-source voltage limit of at least 500 V and an open channel resistance of no more than 1 Ohm. Between the board and the transistor case located parallel to it, an air gap of 2 ... 3 mm is required for air circulation. Instead of the KU112A trinistor, another low-power one from the KU107, MCR100 series is suitable, and instead of the 1N4006 diodes, any for a current of at least 1 A and a voltage of more than 400 V, for example, KD243Zh, KD247G, KD258V. The zener diode can be not only 1N4744A, but also KS215Zh, KS515G, TZMC-15, BZX / BZV55C15 or another 15 V. As C1, the author used a small-sized imported capacitor for a voltage of 250 V AC. Oxide capacitor C4 is small-sized for surface mounting, but it is permissible to install an oxide capacitor of a conventional design here. The rest are small-sized film or ceramic with a small TKE. Trimmer resistor R7 - imported closed design. The SDR-38 tuning resistors often used by radio amateurs are unsuitable, their reliability is too low. The TNR10G561 varistor can be replaced by another with a classification voltage of 560 V -FNR-10K561, FNR-14K561. If it is not supposed to work with loads with power less than 75 W, it is desirable to reduce the values of resistors R4 and R5 to 1 ohm. You can install one twice the power instead of two resistors. Resistors of even smaller rating and higher power will have to be installed to work with a load of more than 120 watts. In this case, it is also necessary to replace the diodes VD4-VD7 and the field effect transistor VT1 with more powerful ones. Several field-effect transistors of the same type can be connected in parallel, be sure to install them on a common heat sink. To work with a powerful load, the installation of the device should be made less dense, and the board should be placed in a case with good ventilation. It is necessary to adjust the limiter with the electrical appliance for the protection of which it is supposed to be used in the future, and at a rated or slightly increased voltage in the network. If the load is an incandescent lamp, it must be new, not subject to prolonged use. Before turning on for the first time, the engine of the tuning resistor R7 is set to the right position according to the diagram. After turning on the power, the slider is moved very slowly until the lamp starts to glow. When properly adjusted, the lamp reaches full brightness 2 to 3 seconds after being turned on. Moreover, for more than half of this time, its glow will not be visible. It should be noted that the more powerful the lamp, the longer and smoother it lights up. If the limiter is set to work with a lamp with a power of, for example, 100 W, and then connected in parallel with it another power of only 15 W, then when turned on, both lamps will not light up. This feature can be used to prevent damage to the luminaire if a lamp is accidentally installed in it with a power exceeding the allowable one. For example, many table lamps are designed only for incandescent lamps with a power of no more than 60 watts. Lamps of the same size with a power of 100 ... 150 W, when installed in a similar lamp, overheat its plastic parts up to melting and deformation. Literature
Author: A. Butov, p. Kurba, Yaroslavl region; Publication: radioradar.net
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