Electronic fuse. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer
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As you know, there are many different current sources that do not provide protection against emergency current overloads - these are almost all galvanic cells and batteries, most batteries and batteries from them, network power supplies assembled according to the simplest scheme, etc. no less often, such sources are used to power the load for a long time without the supervision of an operator.
If for one reason or another there is a significant increase in the current consumed by the load, this will naturally lead to overheating of such a source and its failure, sometimes with very serious consequences. The device described below is intended for automatic disconnection of the load from the DC source in the event of an overload in its circuit and for light indication of an emergency condition. This two-pole, like a fuse, is included in the break in the positive wire of the load circuit.
The electronic fuse (Fig. 1) consists of a powerful composite switching element based on VT4VT3 transistors, a current-measuring resistor R2, a transistor analog of the VT1VT2 dinistor and a shunt transistor VT5.
When the power source is turned on, the composite transistor VT4VT3 opens with a current flowing through the resistor R1 and the emitter junction of the transistor VT4. The remaining transistors remain closed. The rated voltage is supplied to the load, the rated current flows through it.
When an overload occurs, the voltage drop across the current-measuring resistor becomes sufficient to open the analog of the dinistor. Following it, the transistor VT5 opens and shunts the emitter junction of the transistor VT4. As a result, transistors VT4 and VT3 are closed, disconnecting the load from the power source. The load current decreases sharply, but the analog of the dinistor remains open.
The fuse can remain in this state indefinitely. A residual current flows through the load, determined by the resistance of the resistor R1, i.e., ten times less than the nominal one. The voltage drop across the closed transistor VT3 turns on the LED HL1 "Crash".
In order to resume the operation of the load in the nominal mode after the elimination of the cause that caused the overload, it is necessary to either turn off the power source for a short time or disconnect the load.
With the ratings of parts indicated in the diagram, the fuse has the following Features:
- Rated supply voltage. AT 12
- Rated load current, A ...... 1
- Operation current, A ...... 1,2
- Residual voltage on the load after the fuse tripped, V......1,2
- Voltage drop across the fuse in nominal mode, mV......750
The advantage of this fuse, in comparison with that described in [1], is a more reliable closing of the switching element (since the voltage across the open and saturated transistor VT5 is significantly less than on the open trinistor VS1 in [1]. In addition, the voltage drop across the switching element of the considered the fuse is much smaller than that of the compared one; this is achieved by using transistors of various structures in it [2].
The device is easily placed on a 45x45 mm printed circuit board (Fig. 2). Transistor VT3 is best used as indicated in the diagram. Attempts to replace it with other powerful transistors led to an increase in the voltage drop across the fuse.
It is necessary to include a fuse in the protected circuit in a strictly defined polarity. This requires the application of appropriate marking of its conclusions.
Literature
- Esaulov N. Adjustable electronic fuse. - Radio, 1988, No. 5, p. 31,32.
- Golovatsky V. A. et al. Sources of secondary power supply. - M.: Radio and communication, 1990, p. 86.
Author: O. Sidorovich, Lviv, Ukraine
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