Electronic mains fuse. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network
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The fuse circuit that can instantly turn off the load when the current consumed by it increases above the set limit is shown in fig. 7.30.
The power part of the device consists of a diode bridge VD1...VD4, the diagonal of which includes a thyristor VS1. If the photodinistor in optocoupler U1 is illuminated by an LED, then at the beginning of each half-cycle, a voltage appears on the control electrode of the thyristor sufficient to turn it on, and the "diode bridge - thyristor" chain will be conductive. If the photodinistor in the optocoupler remains unlit, the thyristor VS1 will be closed and a break will form in the circuit connecting the electrical network with the load.
The fuse is turned on with the SB1 button. At the moment of its closing, the thyristor VS2 closes, but the fuse remains off (the power supply circuit of the optocoupler LED remains shunted), and only when SB1 opens, the mains voltage will be supplied to the load. This avoids damage to the fuse when trying to turn it on during a short circuit in the load circuit. To turn off the load manually, you just need to press the SB2 button. Thyristor VS1 and diodes VD1 ... VD4 are mounted on aluminum plates - heat sinks 50x80x5 mm.
Resistor R10 - a piece of wire PEV-1-0,6 mm about two meters long (or shorter if you take a wire of high resistivity - manganin, constantan, etc.). Capacitor C1 - K73-17; C2 and C3 - K50-6. Diodes VD1.VD4 can be replaced with D232, D233, D247, KD203, KD206 and others with a reverse voltage of at least 400 V and an operating current of at least 10 A.
The D814D zener diode can be replaced with D814G, D813, D811, KS213 and others with a stabilization voltage of 10 ... 12 V. The KU101 (VS2) thyristor can have any letter index, and the KU202 (VS1) thyristor - indices K or N. Transistor VT1 - any low-power silicon pn-p structure - from KT361, KT209, KT201, KT502, KT3107, etc. The fuse threshold (10 A) is changed by selecting the resistor R10.
When operating an electronic fuse, it must be borne in mind that when some devices are connected to the network, an initial current surge occurs (the starting current of the electric motor, the current of a cold incandescent lamp, etc.), which can cause it to operate. In such cases, either increase the current threshold of the fuse, or in one way or another increase its inertia (for example, by increasing the capacitance of capacitor C3). Both have their downsides. It is better to take measures to reduce the current surges themselves, which are harmful primarily to the load itself and lead to its accelerated wear.
Author: Semyan A.P.
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