Adjustable electronic fuse, 0,1-1,5 amps. Scheme, description

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Adjustable electronic fuse, 0,1-1,5 amps. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network

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This device is designed to protect DC circuits from overcurrent and load circuit short circuits. It is connected between the power supply and the load. The fuse (Fig. 7.18) is made in the form of a two-terminal circuit and can work in conjunction with a power supply unit with an adjustable output voltage within 3 ... 35 V.

The maximum total voltage drop across the fuse does not exceed 1,9 V at maximum load current. The tripping current of the protective device can be continuously adjusted in the range from 0,1 to 1,5 A, regardless of the voltage at the load. The electronic fuse has good thermal stability and speed, and is reliable in operation.

(click to enlarge)

In operating mode, the trinistor VS1 is closed, and the electronic key on transistors VT1, VT2 is opened by the current flowing into the base of the transistor VT1. In this case, the load current flows through the electronic key, a set of resistors R3 ... R6, a variable resistor R8 and the contacts of the SB1 button.

When overloaded, the voltage drop in the circuit of resistors R3.R6, R8 reaches a value sufficient to open the trinistor VS1 through the control electrode circuit. The opened trinistor closes the base circuit of the transistor VT1, which leads to the closing of the electronic key. The current in the load circuit drops sharply; there remains a slight residual current, equal at 9 V - 12 mA, and at 35 V - 47 mA. In order to restore the operating mode after eliminating the cause of the overload, you need to briefly press the SB1 button and release it, while the trinistor will close, and the transistors VT1 and VT2 will reopen.

The residual current can be reduced by increasing the resistance of the resistor R1,5 by 2,5 ... 1 times and using transistors VT1 and VT2 with a large static current transfer coefficient. However, an excessive increase in the resistance of the resistor R1 leads to an increase in the voltage drop across the transistor VT2, i.e. an increase in the voltage drop across the fuse during operation.

It should be borne in mind that with a supply voltage that has significant ripple, the electronic fuse trips at voltage peaks, so the average current through the load will be slightly lower than when using a well-smoothed voltage. Transistor VT2 must be installed on a small heat sink, for example, on a duralumin plate measuring 90x35x2 mm with bent edges. The device can also use transistors in a metal case, you only need to change the design and dimensions of the heat sink. The KT817B transistor can be replaced with KT815B KT815G, KT817V, KT817G, KT801A, KT801B, and KT805AM - with KT802A, KT805A, KT805B, KT808A, KT819B ... KT819G. The static current transfer coefficient of the transistors must be at least 45 V; it is better to use the KU103A trinistors with an opening voltage of 0,4 ... 0,6 V.

Author: Semyan A.P.

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