Low-power laboratory power supply with charger function, 220/1,25…14 volts 150…400 mA. Scheme, description

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Low-power laboratory power supply with charger function, 220/1,25…14 volts 150…400 mA. Encyclopedia of radio electronics and electrical engineering

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In amateur radio practice, undoubtedly, a low-power laboratory power supply with adjustable output voltage and output current limitation in the range from several to several hundred milliamps will be in demand. It can be used to power the equipment being adjusted, assembled on elements sensitive to overcurrent, as well as to charge individual batteries or batteries.

A diagram of such a device is shown in Fig. 1. An adjustable stabilizer with an output voltage of 2 ... 1,25 V is assembled on the DA14 chip. The output voltage is set with a variable resistor R7. On the DA1 chip - a parallel voltage regulator, a transistor VT1 and a current sensor - resistors R5, R6 - a current limiter-stabilizer is assembled. Its value in the range of 6 ... 190 mA is set by a variable resistor R5.

Rice. 1. Diagram of the device (click to enlarge)

The mains voltage is supplied to the primary winding of the transformer through the pushbutton switch SB1 and the fusible link FU1. The voltage of the secondary winding of the transformer rectifies the bridge rectifier on Schottky diodes VD1-VD4. Capacitor C3 - smoothing, LED HL1 indicates the presence of a rectified voltage.

When the current protection is disabled, the movable contact of the SB2 pushbutton switch is in the lower position according to the diagram, the current sensor is closed and a small current flows through the DA1 chip (no more than 0,3 mA). At pin 3 of this microcircuit, the voltage is close to rectified (about 17 V). This voltage is supplied to the gate of the transistor VT1, so it is open, the resistance of its channel does not exceed hundredths of an ohm, and all the voltage stabilized by the DA2 microcircuit is supplied to the output sockets XS1, XS2. In this mode, with a transformer TP-112-3, the output current at a voltage of up to 5 V should not exceed 600 mA, up to 10 V - 400 mA, up to 14 V - 150 mA.

In the "Protection" mode, the movable contact of the switch SB2 is in the upper position according to the scheme and the HL3 LED signals the inclusion of this mode. In this case, the control input (pin 1) of the DA1 microcircuit receives voltage from the current sensor. When this voltage exceeds 2,5 V, at pin 3 of this microcircuit and the gate of transistor VT1, the voltage will decrease and the transistor will close. As a result, the device enters the current limitation (stabilization) mode, the value of which depends on the resistance of the resistor R6 and the input part of the resistor R5: Ilim. min = 2,5/(R5 + R6), Ilim. max = 2,5/R6. In this case, the HL2 LED turns on, signaling that the device is operating in the current stabilization mode.

For the device, a case from the watch "Electronics 12-41A" was used (Fig. 2), therefore, a one-sided printed circuit board was developed for it, the drawing of which is shown in Fig. 3. This case already has a fuse holder. The device uses fixed resistors C2-33, R1-4, variables - SP3-4aM, polar capacitors - imported, the rest - K10-17, K73, LEDs can be any with a case diameter of 3 mm, preferably different colors of glow: HL1 - green , HL2 - red, HL3 - yellow, switches - P2K. The IRFZ44N field effect transistor can be replaced with an IRFZ34N transistor or similar. Capacitor C8 is installed on the terminals of sockets XS1 and XS2. The field-effect transistor and the KR142EN12 chip are mounted on ribbed heat sinks 25x16x8 mm in size. Variable resistors are glued to the board with epoxy glue on the side of the printed conductors, LEDs are soldered on the same side.

Low power laboratory power supply with charger function, 220/1,25…14 volts 150…400 mA
Rice. 2. Case from the watch "Electronics 12-41A"

Low power laboratory power supply with charger function, 220/1,25…14 volts 150…400 mA
Rice. 3. Drawing of a single-sided printed circuit board and the arrangement of elements on it

The axis of the variable resistors protrude from the holes in the front panel. Handles with risks are put on the axis, and two scales are made on the false panel, graduated in milliamps and volts. The scale of the output voltage regulator is calibrated using a voltmeter connected to the output of the device, and the limiting current regulator is carried out by connecting an adjustable load and a milliammeter to the output.

To charge the battery (battery), the device is switched to the "Protection" mode, the required voltage is set to which it must be charged, then the charging current is set and the battery is connected. In this case, the LED HL2 "Current" should light up. As it charges, the brightness of the glow of this LED will decrease until it goes out completely. The output voltage is set based on the calculation of 1,4 ... 1,45 V per one Ni-Cd or Ni-MH battery, and the charging current (in milliamps) - Icharge \u0,1d 3 Ca, where Ca is the battery capacity in mA h. For the convenience of measuring the output voltage on the rear or one of the side walls of the device, you can install additional sockets XS4 and XSXNUMX "Control", to which a multimeter is connected. If you plan to operate the device at maximum current and for a long time, it is advisable to make several dozen ventilation holes on the side and rear walls of the case.

If you use a different case, the elements can be installed on the board, the drawing of which is shown in fig. 4. In this case, the LEDs, variable resistors, sockets, switch and switch may be of other types, they are installed directly on the case. In addition, it is better to increase the size of the heat sinks.

Low power laboratory power supply with charger function, 220/1,25…14 volts 150…400 mA
Rice. 4. Drawing of a printed circuit board and the location of elements on it

The TL431CLP chip can be replaced with a KT817 series transistor (it has a maximum base current of 1 A): pin 1 is the base, pin 2 is the emitter, pin 3 is the collector. In this case, the limitation current interval will change (Ilimit min = 0,7 / (R5 + R6), Ilimit max = 0,7 / R6) and you will have to select resistors R5 and R6 to obtain the required redistribution. The positive side of such a replacement is a decrease in the voltage drop across the current sensor, the negative side is a deterioration in the stability of the limiting current.

Author: I. Nechaev

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