Automatic AC limiter. 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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This device (Fig. 7.21) is designed to automatically disconnect the load if the current flowing through it exceeds the allowable one.
The current flowing through the load connected to connector X1 creates a voltage drop across resistor R3. Part of this voltage, taken from the engine of the variable resistor R2, is fed into the base circuit of the transistor V3. In the collector circuit of this transistor, an electromagnetic relay K1 is switched on. If the load current exceeds the specified value, then the relay K1 will work and its contacts K1.1, K1.2 will disconnect the load from the network and block. The device remains in this state until the button S1 "Reset" is pressed.
Resistor R1, diode V2, zener diode V1 and capacitor C1 form a stabilized power supply. Diode V4 protects the emitter junction of transistor V3 from being exposed to reverse polarity voltage. The current limit is set by a variable resistor R2. The minimum current limit is determined by the resistance of the resistor R3. With the rating indicated on the diagram, it is 0,2 ... 0,3 A. To protect the network from short circuits in the load, fuse F1 is used. Contacts K1.1, K1.2 of the relay are connected in parallel to increase the possible maximum load current.
Transistor V3 may be from series. MP25, MP26 with any letter index, diode V4 - series. D7, D9, D311. The D816G zener diode can be replaced by three D814D zener diodes connected in series. Relay K1 - RES9 (passport RS4.524.205). Button S1 - MT1-1 or P2K. The maximum load current limited by the device should not exceed 1,5 A - otherwise the contacts of relay K1 may burn.
Author: Semyan A.P.
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