Laboratory power supply, 220/3-24 volts 2,7 amps. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The power supply generates seven fixed values ​​​​of the output voltage: 3, 5, 7, 9, 12, 18 and 24 V or others that can be set at will during the adjustment process. Its basis (Fig. 1) is a switching step-down voltage regulator, assembled on a DA1 chip and a powerful field-effect switching transistor VT3. A microchip is used in the control unit. K174KPZ (DA2), designed for use in TV program selectors of the 4USCT series. It is an 8-channel switch, and when the supply voltage is applied to one of its inputs X1-X8 using the SB1-SB8 buttons, the corresponding output (Y1-Y8) is connected to a common wire. The state of the outputs of the microcircuit displays the inclusion of one of the LEDs HL1-HL8.

Chip DA1 generates control pulses supplied to the gate of the transistor VT3, the duration of which depends on the voltage level at the inputs IN1. These inputs are used to control the output voltage of the stabilizer. The voltage at the non-inverting input IN1 comes from a divider formed by resistors R23-R27 and R29-R31. Depending on the selected output voltage, the control device on the DA2 chip connects one of the resistors R23-R27, R30, R31 to the common wire, providing the required voltage at the output of the power supply. The inverting inputs IN1 and IN2 receive voltage from a resistive divider R14R16 connected to the output of the built-in reference voltage source (VREF).

The switching regulator is powered by a power supply unit assembled on a step-down transformer T1, a diode bridge VD1-VD4 and smoothing capacitors C3, C4, and the DA2 switch chip is powered by a parametric voltage regulator R11VD5C5.

(click to enlarge)

The signals received at the IN2 inputs control the turning on and off of the stabilizer. When you press the button SB1 "Off." DA2 chip connects the output of resistor R18, right according to the scheme, to a common wire. Transistor VT1 opens and supplies voltage from the VREF output of the DA1 chip to its non-inverting input IN2. The voltage on it will become greater than on the inverting one, the operation of the switching regulator will stop and the transistor VT3 will close.

The DA2 chip together with the VT2 transistor protects the power supply from output current overload. When it occurs, the voltage at the output of the rectifier VD1-VD4 begins to drop, while the voltage at the base of the transistor VT2 decreases. When it falls below the voltage on the charged capacitor C7, the transistor VT2 will open, the voltage from the capacitor C7 will go to the input. X1 of the DA2 chip and will put the stabilizer in the "Off" state. Diode VD6 eliminates the discharge of capacitor C7 through resistor R17, and diode VD7 protects the base-emitter junction of transistor VT2 from reverse polarity voltage breakdown.

MLT resistors are used in the power supply, oxide capacitors are imported, the rest are K73-17. Diodes D243 can be replaced by any of the D243-D247 series, the KA7500V microcircuit can be replaced by its complete analogue TL494. Buttons - any small-sized with self-return. The choke is wound on two MP 140-4 permalloy rings folded together, size K24x13x7. The winding contains 36 turns of wire. PEV-2 1,0. Before winding the wire, the rings are folded together and wrapped with a layer of varnished cloth.

The VD10 diode and the VT3 transistor are installed using heat-conducting insulating spacers on a common ribbed heat sink with dimensions of 65x35x20 mm. The step-down transformer must provide an alternating voltage on the secondary winding of 25 ... 27 V at a load current of up to 2,7 A. A transformer from the series can be used in the device. CCI, for example, CCI276-127/220. When manufacturing or choosing a ready-made transformer, it should be remembered that in any mode of operation, the voltage at the output of the rectifier should not exceed 40 V - the maximum allowable supply voltage of the DA1 stabilizer.

Most of the parts and the heat sink are installed on a printed circuit board made of one-sided foil-coated fiberglass, the drawing of which is shown in fig. 2 and 3. It is mounted on racks in a metal case, and a transformer is also mounted there (Fig. 4). Buttons, LEDs, power switch, fuse holder and output sockets are installed on the front panel.

Fig. 2

Fig. 3

Fig. 4

Establishing the device comes down to a selection of resistors R23-R27, R30, R31 to accurately set the output voltage values. The operation of the protection against current overloads is checked by briefly closing the output of the power supply, while it should switch to the "Off" state. A short press on one of the SB2-SB8 buttons should restore the device.

Author: Abramovich A.

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