Adjustable electronic 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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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 is made in the form of a two-terminal circuit and can work together with a power supply unit with an adjustable output voltage in the range of 3 ... 35 V. The maximum total voltage drop across the fuse does not exceed 1,9 V at the 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 (3 ... 5 µs), reliable in operation.

The circuit diagram of the electronic fuse is shown in fig. 1. In the operating mode, the trinistor VS1 is closed, and the electronic key on the transistors VT1, VT2 is opened by the current flowing through the resistor R1 to 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 across the circuit of resistors R3-R6, R8 reaches a value sufficient to open the trinistor VS1 along 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 to Ires = Upit / R1. At Upit=9 V Ires=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. In this case, 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 in the operating mode.

The residual current can be significantly reduced (up to 2 ... 4 mA) at any supply voltage by using a current source based on a KP1A or KP303B field-effect transistor with an initial drain current of 303 ... 1 mA to bias the transistor VT2,5. In this case, the resistor R1 is excluded. The gate and source of the field effect transistor must be connected together and connected to the base of the transistor VT1, and the drain to its collector. It should be borne in mind that in this case the device is operable in circuits with a voltage of not more than 25 V.

Fig. 1

On fig. 2 shows the dependence of the fuse operation current on the resistance of the resistor R8. The type of this characteristic strongly depends on the opening voltage of the trinistor.

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 somewhat lower than when using a well-smoothed voltage.

The fuse operating current can be determined from the expression: I_srab=U_openVS1/ (R_eq+R8), where U_openVS1- SCR opening voltage, and R_eq - equivalent resistance of the circuit of resistors R3-R6. As the graph in Fig. 2, the regulation of the operation current by the resistor R8 in the range of limit values ​​is rather rough, therefore it is advisable either to reduce the regulation limits by reducing the resistance of the resistor R8 by 1,5 ... 2 times, or to introduce multi-stage regulation by a switch with a set of precisely matched resistors.

The fuse is mounted on a printed circuit board made of fiberglass with a thickness of 1,5 mm (Fig. 3). The board contains all the details, except for the transistor VT2, the resistor R8 and the button SB1. The VT2 transistor 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. Fixed resistors - MLT, MT and MON; variable resistor - any wire; button SB1 - P2K without latch.

In the fuse, it is better to use KU103A trinistors with an opening voltage of 0,4 ... 0,6 V.

The assembled fuse for adjustment, as a rule, does not require. In some cases, it is required to select the resistance Req by adding another resistor to set the maximum operating current. The board provides space for four resistors R3-R6.

It is easy to calculate the fuse for a larger trip current (up to 3 ... 5 A). This will require more powerful transistors.

Fig. 2

Fig. 3

Author: N. Esaulov, pos. Ivanovka, Voroshilovgrad region; Publication: cxem.net

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