Electronic fuse, 5-25 volts. Scheme, description

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Electronic fuse, 5-25 volts. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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When establishing various electronic equipment, it is advisable to use a power supply with built-in and adjustable electronic protection for the load current. If the block at your disposal does not have such protection, it can be made in the form of a set-top box that is connected between the output sockets of the block and the load. Thus, the fuse attachment, in case of exceeding the specified maximum load current, will instantly disconnect it from the power supply.

The electronic fuse (see figure) contains a powerful transistor VT2, which is included in the negative power wire, two current regulators on field-effect transistors - one adjustable (on VT1) and the other unregulated (on VT3). and sensing element - trinistor VS1. The control voltage to the trinistor comes from the current sensor, which is the resistor R1 of a very low resistance (0.1 Ohm), and from the resistor R2. This type of trinistor turns on when the voltage on the control electrode (relative to the cathode) is 0.5 ... 0.6 V.

Electronic fuse, 5-25 volts

The fuse works like this. In the initial state, a current of approximately 3 ... 8 mA flows through the transistor VT15, which remains almost unchanged when the output voltage of the power supply changes. This current flows through the HL2 LED (it lights up, signaling the passage of load current through the device) and the base circuit of the transistor VT2, which opens. Since the static transfer coefficient of the transistor is several thousand, it is able to pass a current of several amperes into the load. In this case, the voltage drop across the transistor will not exceed 1 V.

The load current creates a voltage drop across the resistor R1, which is opening for the trinistor. In addition, the current flowing through the transistor VT1 (it can be changed by a variable resistor R3) creates a voltage drop across the resistor R2, which will also be opening for the trinistor. When the sum of these voltages reaches a certain value, the trinistor will open, the voltage on it will decrease to 0,7 ... 0,8 V. The HL1 LED will light up and signal an accident. At the same time, the voltage on the HL2 LED will decrease so much that it will turn off. Transistor VT2 will close, and the load will be disconnected from the power supply.

The load current at which the fuse will operate can be set with a variable resistor R3 ranging from a few tens of milliamps to about 5 A.

After the fault in the load has been eliminated, the electronic fuse is reset by the SB1 button, which, when its contacts are closed, de-energizes the trinistor, and it closes. Transistor VT2 opens, current flows to the load.

In the device, it is permissible to use, in addition to those indicated in the diagram, field-effect transistors KP307A or similar with an initial drain current of 10 ... 15 mA and a maximum allowable voltage of at least the output voltage of the power supply. Transistor VT2 can be KT829A-KT829G, KT827A-KT827V. With a load current of more than 1 A, the transistor must be installed on a radiator. LEDs - any low-power (AL307, AL341), but in place of HL1 it is better to install a red LED, and in place of HL2 - green. Trinistor - 2U107A-2U107V. Variable resistor - SPO, SP, SP4, constant - MLT, S2-33, resistor R1 is made from a piece of high-resistance wire.

Establishing the device is reduced to setting the maximum tripping current by selecting the resistance of the resistor R1 when the power supply of the drain of the transistor VT1 is disconnected from the plus. The minimum operating current is selected by connecting a resistor R3 of a different rating. In this case, it is allowed to connect a constant resistor in series with it or in parallel with it.

If, when the fuse is triggered, a residual current still flows through the transistor VT2 (the transistor does not close), it is recommended to use the HL2 LED with a high operating voltage or connect the KD102B, KD103B, KD105B, KD522B diode in series with it.

If the power supply has a voltage stabilizer, the fuse should be turned on in front of it, and not at the output of the unit.

Author: I.Aleksandrov, Kursk

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