Voltage stabilizer 35-70 volts. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Surge Protectors

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One of the tasks that radio amateurs have to solve in their practical work is the construction of a high-current (5 A or more) stabilizer with an output voltage of 50 V or more. The range of microcircuit stabilizers with such parameters is limited, and they are not always available. You can assemble such a stabilizer on the basis of powerful field-effect switching transistors, which work quite satisfactorily in a linear mode.

The stabilizer circuit is shown in fig. 1. The device uses a powerful switching field-effect transistor IRF840 (VT1) with a permissible drain-source voltage of 500 V, a maximum drain current of up to 5 A at an ambient temperature of 70 ° C and 8 A at a temperature of 25 ° C, a slope of 4 ... 5 A / V, an open channel resistance of 0,85 ohms and an allowable power dissipation of 125 watts.

The transistor is controlled by the DA1 chip - a parallel voltage regulator. The output voltage in the range from 35 to 70 V is set with a tuned resistor R6. The maximum allowable voltage for this microcircuit is 30 V, therefore, a VT2 transistor is provided to ensure its normal operation. It reduces the voltage on the microcircuit due to the fact that only a part of the output voltage is supplied to its base. Capacitors C1, C3, C4 ensure stable operation of the stabilizer. Capacitor C2 and diode VD1 contribute to an increase in the stabilization factor with a decrease in input voltage, since the capacitor is charged almost to the amplitude value of the input voltage.

After the input voltage is applied, the field-effect transistor begins to open and the voltage at the output of the stabilizer increases. When it reaches a value at which the control input (1) of the DA1 microcircuit becomes about 2,5 V, the current through the microcircuit and the transistor VT2 will increase and the voltage at the gate of the transistor VT1, and therefore at the output of the stabilizer will decrease. The minimum voltage drop across the transistor at a current of 5 A is about 5 V.

In the device, along with those indicated in the diagram, it is permissible to use the following elements: a regulating transistor VT1 - suitable from the list, which is given in the article "Powerful field-effect switching transistors by INTERNATIONAL RECTIFIER", published in the journal "Radio" No. 5 for 2001 on p. 45; microcircuit - TL431I; diode - KD105B; transistor VT2 - KT815G, KT817G, KT630A-KT630G; capacitor C3 - K10-17, the rest - any oxide; fixed resistors MLT, S2-33, R1-4, tuning SPZ-19, SPZ-3. The device is assembled on a printed circuit board made of one-sided foil fiberglass, the drawing of which is shown in fig. 2. The appearance of the assembled device is shown in fig. 3. The board, together with the control transistor, is placed on a heat sink, which must be isolated from other elements of the device.

Establishment comes down to establishing the required output voltage. It is calculated by the formula Uout = 2,5[1+(R3+R4)/(R5+R6)]. It is imperative to check whether the device is self-excited in the entire operating current range. If it occurs, in parallel with the terminals of the gate-source transistor VT1, a capacitor with a capacity of 0,01 ... 0,1 μF should be installed with the terminals of the minimum length. The values ​​of the elements indicated in the diagram provide an output voltage regulation range from 35 to 70 V. The resistor R3 is selected so that the voltage at the base of the transistor VT2 does not exceed 30 V in the entire output voltage range. For the elements indicated in the diagram, the input voltage should not exceed 95 V, and the output voltage can reach 90 V. For large values, elements designed for the corresponding voltage should be used.

Author: I. Nechaev, Kursk

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