Lab Power Supply

When creating various electronic devices, sooner or later, the question arises of what to use as a power source for home-made electronics. Suppose you have assembled some LED flasher, now it needs to be powered from something neatly. Very often, for these purposes, use different chargers for phones, power supply units of computers, various network adapters, which do not limit the current delivered to the load.
Laboratory power supply
And if, say, on a board of this LED flasher, two closed tracks accidentally went unnoticed? By connecting it to a powerful computer power supply unit, the assembled device can easily burn if there is any installation error on the board. In order to avoid such unpleasant situations, there are laboratory power supply units with current protection. Knowing in advance what current will consume the connected device, we can prevent a short circuit, and, as a result,burnout of transistors and delicate microcircuits. In this article we will look at the process of creating just such a power supply unit to which you can connect the load, without fear that something will burn.

Power supply diagram

Laboratory Power Supply
The circuit contains the LM324 chip, which combines 4 operational amplifiers, you can put TL074 instead. OP1 operational amplifier is responsible for adjusting the output voltage, while OP2-OP4 monitors the current consumed by the load. Chip TL431 forms a reference voltage of approximately 10.7 volts, it does not depend on the magnitude of the supply voltage. Variable resistor R4 sets the output voltage, resistor R5, you can adjust the scope of voltage changes to fit your needs. The current protection works as follows: the load consumes the current that flows through the low-resistance resistor R20, which is called a shunt, the magnitude of the voltage drop across it depends on the current consumed. The OP4 operational amplifier is used as an amplifier, increasing the low voltage drop on the shunt to the level of 5-6 volts, the output voltage of the OP4 varies from zero to 5-6 volts depending on the load current.The OP3 cascade works as a comparator, comparing the voltage at its inputs. The voltage at one input is set by the variable resistor R13, which sets the protection threshold, and the voltage at the second input depends on the load current. Thus, as soon as the current exceeds a certain level, a voltage will appear at the output of OP3, opening the transistor VT3, which in turn pulls the base of the transistor VT2 to the ground, closing it. The closed transistor VT2 closes the power VT1, opening the load supply circuit. All these processes take place in a matter of a fraction of a second. The resistor R20 should be taken with a watt power of 5 to prevent its possible heating with long work. Trimmer resistor R19 sets the current sensitivity, the higher its value, the more sensitivity can be achieved. Resistor R16 adjusts the hysteresis of protection, I recommend not to get carried away with increasing its denomination. Resistance of 5-10 kOhm will provide a precise snap-on of the circuit when the protection is triggered, a greater resistance will give the effect of current limiting when the voltage on the output does not completely disappear. As a power transistor you can use domestic KT818, KT837, KT825 or imported TIP42. Particular attention should be paid to its cooling, because all the difference in input and output voltage will be dissipated in the form of heat on this transistor. That's why you should not use a power supply unit with a small output voltage and high current, the heating of the transistor will be maximum at the same time. So, let's move from words to deeds.

PCB Manufacturing and Assembly

Printed circuit board is performed using the LUT method, which has been repeatedly described on the Internet.
Laboratory Power Supply
Laboratory Power Supply
Laboratory Power Supply
On the printed circuit board, a LED with a resistor is added, which are not indicated in the diagram. A resistor for the LED is suitable for 1-2 kOhm. This LED turns on when the protection trips. Also added two contacts, marked with the word "Jamper", when they close the power supply comes out of protection, "flips". In addition, a 100 pF capacitor is added between the 1st and 2nd chip outputs,it serves to protect against interference and ensures stable operation of the circuit.
Laboratory Power Supply
Laboratory Power Supply
Laboratory Power Supply
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Setting up the power supply

So, after assembling the circuit, you can proceed to setting it up. First of all, we feed 15-30 volts and measure the voltage at the cathode of the TL431 chip, it should be approximately equal to 10.7 volts. If the voltage supplied to the input of the power supply is small (15-20 volts), then the resistor R3 should be reduced to 1 kΩ. If the reference voltage is OK, we check the operation of the voltage regulator; when the variable resistor R4 rotates, it should change from zero to maximum. Further, we rotate the resistor R13 in its most extreme position, the protection may be triggered when this resistor pulls the input OP2 to ground. You can install a resistor of 50-100 ohms between the ground and the output of the extreme output of R13, which is connected to ground. Connect any load to the power supply, set the R13 to the extreme position. We increase the voltage at the output, the current will increase and at some point the protection will work.We achieve the desired sensitivity trimmer R19, then you can solder the constant instead. This completes the process of assembling the laboratory power supply, you can install it in the case and use it.


Laboratory Power Supply
Laboratory Power Supply
To display the output voltage is very convenient to use a pointer head. Digital voltmeters, although they can show voltage up to hundredths of a volt, constantly running numbers are poorly perceived by the human eye. That is why it is more rational to use the switchheads. It is very simple to make a voltmeter from such a head - it is enough to put a trimmer with a nominal value of 0.5-1 MΩ in series with it. Now you need to apply a voltage, the value of which is known in advance and the trimmer to adjust the position of the arrow corresponding to the applied voltage. Successful build!

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