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Powerful current protection power supply

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


Powerful current protection power supply

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


Powerful current protection power supply

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.
Powerful current protection power supply

Powerful current protection power supply

Powerful current protection power supply

Current protection circuit
Powerful current protection power supply

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.
Powerful current protection power supply

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.
Powerful current protection power supply

Power Supply Board Assembly


Printed circuit board:
[135.37 Kb] (downloads: 321)

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.
Powerful current protection power supply

Powerful current protection power supply

Powerful current protection power supply

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.
Powerful current protection power supply

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.
Powerful current protection power supply

Powerful current protection power supply

Powerful current protection power supply

Powerful current protection power supply

Body assembly


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.
Powerful current protection power supply

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.
Powerful current protection power supply

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.
Powerful current protection power supply

Powerful current protection power supply

Powerful current protection power supply

Powerful current protection power supply

Powerful current protection power supply

Powerful current protection power supply

Powerful current protection power supply

Customization


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.
Powerful current protection power supply

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Comments (14)
  1. Sergey Zuev
    #1 Sergey Zuev Guests February 4, 2019 09:10
    12
    The work of course has been done, the scheme has been invented and executed, well done!
    however, there is no reason to fence such a garden now. At Aliexspress for 200 rubles you can buy a switching power supply with adjustable voltage 1.2-35 V and current 0 - 5A. And immediately with a digital display.
    1. Something tells me that you are disingenuous.
      #2 Something tells me that you are disingenuous. Guests March 13, 2019 1:59 p.m.
      0
      Something tells me that you are disingenuous.
  2. Guest Sergey
    #3 Guest Sergey Guests February 4, 2019 09:13
    5
    case-box is just class
  3. Guest Sergey
    #4 Guest Sergey Guests February 4, 2019 2:02 p.m.
    0
    I didn’t provide for ventilation of the case? But in vain.
  4. Valery Valerevich
    #5 Valery Valerevich Visitors February 4, 2019 7:58 p.m.
    0
    what nonsense !!! can still be made more compact so that less space is taken up on the desktop. due to excessive body weight. Well, if you get away from retro, well, at least a digital display with a current-voltage characteristic, otherwise what retro if modern LEDs stick out? so then insert ts270 trans and as indicators of a bulb. and if a real unit for work, then really make it simpler and more compact and easier. Well, I'm shocked guys. the modern age. Well, figs with it is an analogue ammeter for the fact that the digits are on and the analogue is cumbersome. but not the same !!! I am ashamed of those who do not take experience from the past and do not develop the future. let the coffins go do little there
    1. Malik Nizamutdinov
      #6 Malik Nizamutdinov Guests March 1, 2019 07:41 AM
      1
      And you yourself do good and lay out.
      This is an excellent power supply for beginners with their own shortcomings. But clearly and clearly. The author’s goal is not to boast of steepness, but to show that it is possible to assemble something working from the available components. It is necessary to perceive such works as "Laboratory work" for beginners.
  5. Sergey Ripenko
    #7 Sergey Ripenko Guests February 5, 2019 06:26
    1
    What are biscuits? Why are they? Why not a variable resistor?
    Why are the devices of different sizes? Case - tear off your hands! Shame on you! I would be shy
    show this "masterpiece" to someone
  6. Guest Alexander
    #8 Guest Alexander Guests February 7, 2019 01:36
    3
    Sorry, but how do you turn it on in 220? It turns out you still need a step-down transformer with a rectifier, or a switching power supply?
  7. Zxl
    #9 Zxl Guests February 14, 2019 00:32
    0
    Handmade ... heyyyyy ...
  8. Alexander Grigorievich
    #10 Alexander Grigorievich Guests March 1, 2019 10:15
    1
    Poor corps kills ......
  9. Guest Andrey
    #11 Guest Andrey Guests March 1, 2019 2:15 PM
    0
    Hello.
    Tell me how the display circuit is connected to the main circuit. I apologize for the stupid question, I just take the first steps and do not quite understand everything.
  10. Alexander Viktorovich
    #12 Alexander Viktorovich Guests March 13, 2019 17:50
    1
    Powerful heat-insulated case made of chipboard, and a cooler with a radiator. Like a heater in the refrigerator - who whom. Cool.
  11. Guest Victor
    #13 Guest Victor Guests March 13, 2019 18:49
    0
    1.current protection? But in LM 338, isn’t it? 2. Well-forgotten old, about 30 years ago, such circuits came into use, but without a passing power transistor 2N 2055, there will be no business in the plus bus circuit. A good primer on "The Art of Circuit Engineering" Horowitz and Hill.- a very detailed analysis of the circuit.3. We hope that the fate of consumers powered
  12. Guest Victor
    #14 Guest Victor Guests March 13, 2019 18:57
    0
    1. This kind of power supply began to come into use 30 years ago. and beautifully illuminated in the world bezler “The Art of Circuit Engineering Vol. 1, authors Horowitz and Hill. Without a 2N 3055 transistor in the plus bus circuit, it causes reliability of the circuit — sorry for the burnt consumers.

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