Every person who collects electronic circuits needs a universal power source that allows a wide variation of the output voltage, current control and, if necessary, disconnect the powered device. In stores, such laboratory power supplies are very expensive, but you can assemble one yourself from common radio components. The presented power supply includes:
- Voltage regulation up to 24 volts;
- The maximum current given to the load is up to 5 amperes;
- Current protection with the choice of several fixed values;
- Active cooling for operation at high currents;
- Dial indicators of current and voltage;
Voltage regulator circuit
The simplest and most affordable version of the voltage regulator is a circuit on a special chip called a voltage regulator. The most suitable option is LM338, it provides a maximum current of 5 A and a minimum of ripple at the output. LM350 and LM317 are also suitable here, but the maximum current in this case will be 3 A and 1.5 A, respectively. A variable resistor serves to adjust the voltage, its rating depends on what maximum voltage you need to get at the output. If the maximum output requires 24 volts, you need a variable resistor with a resistance of 4.3 kOhm. In this case, you need to take a standard potentiometer at 4.7 kOhm and connect a constant at 47 kOhm in parallel with it, the total resistance will be about 4.3 kOhm. To power the entire circuit, you need a DC source with a voltage of 24-35 volts, in my case it is a normal transformer with a built-in rectifier. You can also use laptop chargers or other various switching sources suitable for current.
This voltage regulator is linear, which means that the whole difference between the input and output voltage falls on one chip and is dissipated on it in the form of heat. At high currents, this is very critical, so the microcircuit must be installed on a large radiator, the radiator from the computer processor, working in tandem with the fan, is best for this. In order for the fan not to rotate all the time in vain, but to turn on only when the radiator is heating, it is necessary to assemble a small temperature sensor.
Fan control circuit
It is based on an NTC thermistor, the resistance of which varies with temperature - with increasing temperature, the resistance decreases significantly, and vice versa. The operational amplifier acts as a comparator, registering a change in the resistance of the thermistor. When the threshold is reached, the voltage appears at the output of the op-amp, the transistor unlocks and starts the fan, with which the LED lights up. A trimming resistor is used to adjust the threshold, its value should be selected based on the resistance of the thermistor at room temperature. Suppose a thermistor has a resistance of 100 kOhm, in this case the tuning resistor should have a nominal value of about 150-200 kOhm. The main advantage of this scheme is the presence of hysteresis, i.e. differences between the thresholds for turning on and off the fan. Due to the hysteresis, the fan does not turn on and off frequently at a temperature close to the threshold. The thermistor is displayed on the wiring directly to the radiator and installed in any convenient place.
Current protection circuit
Perhaps the most important part of the entire power supply is current protection. It works as follows: the voltage drop across the shunt (resistor with a resistance of 0.1 Ohm) is amplified to a level of 7-9 volts and is compared with a reference using a comparator. The reference voltage for comparison is set by four tuning resistors in the range from zero to 12 volts, the input of the operational amplifier is connected to the resistors through a 4-position wrench switch. Thus, changing the position of the biscuit switch, we can choose from 4 predefined options for protection currents. For example, you can set the following values: 100 mA, 500 mA, 1.5 A, 3 A. If the current set by the dial switch is exceeded, the protection will work, the voltage will cease to be output and the LED will light up. To reset the protection, just briefly press the button, the output voltage will appear again. The fifth tuning resistor is necessary to set the gain (sensitivity), it must be set so that when the current through the shunt 1 Ampere, the voltage at the output of the op-amp was about 1-2 volts. The resistor for setting the protection hysteresis is responsible for the “clarity” of the circuit snapping, it must be adjusted if the output voltage does not disappear completely. This circuit is good because it has a high response speed, instantly turning on protection when the current is exceeded.
Current and voltage display unit
Most laboratory power supplies are equipped with digital voltmeters and ammeters, showing values in the form of numbers on the scoreboard. This option is compact and provides good accuracy of readings, but it is completely inconvenient for perception. That is why for indication it was decided to use arrowheads, the readings of which are easily and pleasantly perceived. In the case of a voltmeter, everything is simple - it connects to the output terminals of the power supply through a trimming resistor with a resistance of about 1-2 MOhm. For the ammeter to work properly, a shunt amplifier is required, the circuit of which is shown below.
A tuning resistor is needed to adjust the gain, in most cases it is enough to leave it in the middle position (about 20-25 kOhm). The switch head is connected via a dial switch, with which you can select one of three tuning resistors, with which the current of the maximum deviation of the ammeter is set. Thus, the ammeter can operate in three ranges - up to 50 mA, up to 500 mA, up to 5A, this ensures maximum accuracy of readings at any load current.
Power Supply Board Assembly
Printed circuit board:
moschnyj-laboratornyj-blok-pitanija-s-zaschitoj-po-toku.zip [135.37 Kb] (downloads: 336)
Now that all the theoretical aspects have been taken into account, we can begin to assemble the electronic part of the structure. All elements of the power supply - voltage regulator, radiator temperature sensor, protection unit, shunt amplifier for the ammeter are assembled on one board, the dimensions of which are 100x70 mm. The board is made by the LUT method, below are some photos of the manufacturing process.
Power paths along which the load current flows, it is desirable to tin with a thick layer of solder to reduce resistance. First, small parts are installed on the board.
After that, all the other components. The 78L12 microcircuit supplying the temperature sensor and cooler must be installed on a small radiator, a place for which is provided on the printed circuit board. Lastly, the wires are soldered to the board, on which the fan, thermistor, protection reset button, wrench switches, LEDs, LM338 chip, voltage input and output are output. The voltage input is most conveniently connected via a DC connector, while it must be borne in mind that it must provide a large current. All power wires must be used corresponding to the current cross-section, preferably copper. Plus, the output from the printed circuit board does not go to the output terminals directly, but through a toggle switch with two groups of contacts. The second group turns the LED on and off, indicating whether voltage is applied to the terminals.
The housing can be either found ready-made or assembled independently. You can make it, for example, from plywood and fiberboard, as I did. First of all, a rectangular front panel is cut out, on which all controls will be installed.
Then the walls and bottom of the box are made, the structure is fastened together with self-tapping screws. When the frame is ready, you can install all the electronics inside.
Controls, arrowheads, LEDs are installed in their places in the front panel, the board is placed inside the case, the radiator with fan is mounted on the rear panel. To mount the LEDs, special holders are used. It is desirable to duplicate the output terminals, especially since the place allows. The dimensions of the case are 290x200x120 mm, there is still a lot of free space inside the case, and there can fit, for example, a transformer for powering the entire device.
Despite a lot of tuning resistors, setting up the power supply is pretty straightforward. First of all, calibrate the voltmeter by connecting an external to the output terminals. Rotating the tuning resistor, connected in series with the arrow head of the voltmeter, we achieve equal readings. Then we connect any load with an ammeter to the output and calibrate the shunt amplifier. Rotating each and three interlinear resistors we achieve coincidence of readings on each of the three measuring ranges of the ammeter - in my case it is 50 mA, 500 mA and 5A. Next, we set the necessary protection currents with the help of four tuning resistors. It is not difficult to do this, given that the standard ammeter is already calibrated and shows the exact current. We gradually increase the voltage (the current also rises) and look at the current at which the protection is triggered. Then we rotate each of the resistors, setting the four necessary protection currents, between which you can switch using the dial switch. Now it remains only to set the desired threshold for the radiator temperature sensor - the setting is completed.