Radio-electronic structures based on op-amps can be powered from both unipolar and bipolar power sources. The best results of the design are obtained when they are powered from a bipolar source.
Therefore, let's consider a practical circuit of a bipolar power supply. The power supply is assembled on discrete elements and consists of two single-transistor voltage regulators.
Schematic diagram
The diagram of a bipolar power supply is shown in Fig. 1. The circuit is based on three conventional voltage stabilizers on one transistor. A special feature of the circuit is the presence of a voltage stabilizer on the VTZ transistor.
The presence of this cascade is dictated by the following considerations. It is clear from the diagram that the stabilizers on transistors VTI and VT2 do not have short circuit protection.
In the event of a short circuit in the output of the upper arm according to the circuit, the VTI emitter closes with the VT2 emitter and a voltage of minus 24 V appears on the lower arm relative to the “common” wire.
This situation can lead to failure of the power supply equipment. To prevent such a danger, a cascade on the VTZ transistor was introduced at the minus power supply.
Rice. 1. Schematic diagram of a bipolar unregulated power supply using discrete elements.
Parts and PCB
To operate the source, you need a power transformer that provides a voltage of 27...30 V on the secondary winding. For this purpose, a unified transformer TP8-18-220-50 with a magnetic core ШЛ 16x25 is suitable.
The parts of the power supply device, except for the transformer, are assembled on a printed circuit board made of single-sided foil PCB measuring 95x35 mm (Fig. 2).
Rice. 2. Printed circuit board for a bipolar voltage regulator-converter circuit.
Rice. 3. Installation of parts on a printed circuit board for a bipolar power supply.
To ensure normal temperature regime To ensure the operation of transistors, it is necessary to make heat sinks for them from duralumin.
With serviceable parts and correct assembly, the device begins to work immediately. The device does not require any special adjustment; it is advisable to check the output voltage with a voltmeter and if it differs from the required one, then you need to select zener diodes VD5, VD7 and resistors R1 and R2.
Literature: V.M. Pestrikov. - Encyclopedia of amateur radio.
When creating various electronic devices, sooner or later the question arises of what to use as a power source for homemade electronics. Let's say you've assembled some kind of LED flasher, now you need to carefully power it from something. Very often, for these purposes, various phone chargers, computer power supplies, and all kinds of network adapters are used, which do not in any way limit the current supplied to the load.
What if, say, on the board of this same LED flasher, two closed tracks accidentally went unnoticed? By connecting it to a powerful computer power supply, the assembled device can easily burn out if there is any installation error on the board. It is precisely to prevent such unpleasant situations from happening that there are laboratory power supplies with current protection. Knowing in advance approximately how much current the connected device will consume, we can prevent short circuits 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 to which you can connect a load without fear that something will burn out.
Power supply diagram
The circuit contains an LM324 chip, which combines 4 operational amplifiers; a TL074 can be installed instead. Operational amplifier OP1 is responsible for regulating the output voltage, and OP2-OP4 monitor the current consumed by the load. The TL431 microcircuit generates a reference voltage approximately equal to 10.7 volts; it does not depend on the value of the supply voltage. Variable resistor R4 sets the output voltage; resistor R5 can be used to adjust the voltage change frame to suit your needs. Current protection works as follows: the load consumes current, which flows through a low-resistance resistor R20, which is called a shunt, the magnitude of the voltage drop across it depends on the current consumed. Operational amplifier OP4 is used as an amplifier, increasing the low voltage drop across the shunt to a level of 5-6 volts, the voltage at the output of 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 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 transistor VT3, which, in turn, pulls the base of transistor VT2 to ground, closing it. The closed transistor VT2 closes the power VT1, opening the load power circuit. All these processes take place in a matter of seconds.
Resistor R20 should be taken with a power of 5 watts to prevent its possible heating during long-term operation. Trimmer resistor R19 sets current sensitivity; the higher its value, the greater sensitivity can be achieved. Resistor R16 adjusts the protection hysteresis; I recommend not to get carried away with increasing its value. A resistance of 5-10 kOhm will ensure a clear latching of the circuit when the protection is triggered; a higher resistance will give a current limiting effect when the voltage at 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 the entire difference between the input and output voltage will be dissipated in the form of heat on this transistor. That is why you should not use a power supply with a low output voltage and high current, as the heating of the transistor will be maximum. So, let's move from words to action.
PCB fabrication and assembly
The printed circuit board is made using the LUT method, which has been described many times on the Internet.
An LED with a resistor is added to the printed circuit board, which are not indicated in the diagram. A resistor for the LED is suitable with a nominal value of 1-2 kOhm. This LED turns on when the protection is triggered. Two contacts have also been added, marked with the word “Jamper”; when they are closed, the power supply comes out of the protection and “snaps off”. In addition, a 100 pF capacitor has been added between pins 1 and 2 of the microcircuit; it serves to protect against interference and ensures stable operation of the circuit.
Download the board:
(downloads: 951)
Setting up the power supply
So, after assembling the circuit, you can begin to configure it. First of all, we supply power of 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 resistor R3 should be reduced to 1 kOhm. If the reference voltage is OK, we check the operation of the voltage regulator; when rotating the variable resistor R4, it should change from zero to maximum. Next, we rotate the resistor R13 in its most extreme position; the protection may be triggered when this resistor pulls the OP2 input to ground. You can install a 50-100 Ohm resistor between ground and the outermost pin of R13, which is connected to ground. We connect any load to the power supply, set R13 to its extreme position. We increase the output voltage, the current will increase and at some point the protection will work. We achieve the required sensitivity with trimming resistor R19, then you can solder a constant one instead. This completes the process of assembling the laboratory power supply; you can install it in the case and use it.Indication
It is very convenient to use a pointer head to indicate the output voltage. 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 pointer heads. It is very simple to make a voltmeter from such a head - just place a trimming resistor in series with it with a nominal value of 0.5 - 1 MOhm. Now you need to apply a voltage, the value of which is known in advance, and use a trimming resistor to adjust the position of the arrow corresponding to the applied voltage. Happy build!
Hello everyone. This article is a companion piece to the video. Let's consider the powerful laboratory block power supply, which is not yet fully completed, but is functioning very well.
The laboratory source is single-channel, completely linear, with digital display, current protection, although there is also an output current limitation.
The power supply can provide an output voltage from zero to 20 volts and a current from zero to 7.5-8 Amps, but more is possible, at least 15, at least 20 A, and the voltage can be up to 30 Volts, but my option has a limitation due to with transformer.
Regarding stability and ripples, it is very stable, the video shows that the voltage at a current of 7 Amperes does not drop even by 0.1 V, and the ripples at currents of 6-7 Amperes are about 3-5 mV! in class it can compete with industrial professional power supplies for a couple of hundred dollars.
At a current of 5-6 Amps, the ripple is only 50-60 millivolts; budget Chinese industrial-style power supplies have the same ripples, but at currents of only 1-1.5 amperes, that is, our unit is much more stable and can compete in class with samples for a couple of hundred dollars
Despite the fact that the side is linear, it has high efficiency, it has an automatic winding switching system, which will reduce power losses on transistors at low output voltages and high current.
This system is built on the basis of two relays and a simple control circuit, but later I removed the board, since the relays, despite the declared current of more than 10 Amps, could not cope, I had to buy powerful 30 Ampere relays, but I have not yet made a board for them, but without a system The switching unit works great.
By the way, with the switching system, the unit will not need active cooling; a huge radiator at the rear will be enough.
The case is from an industrial network stabilizer, the stabilizer was bought new, from the store, just for the sake of the case.
I left only a voltmeter, a power switch, a fuse and a built-in socket.
There are two LEDs under the voltmeter, one shows that the stabilizer board is receiving power, the second, red, shows that the unit is operating in current stabilization mode.
The display is digital, designed by a good friend of mine. This is a personalized indicator, as evidenced by the greeting, you will find the firmware with the board at the end of the article, and below is the indicator diagram
But essentially this is a volt/ampere wattmeter, there are three buttons under the display that will allow you to set the protection current and save the value, the maximum current is 10 Amps. The protection is relay, the relay is again weak, and at high currents there is quite a strong heating of the contacts.
There are power terminals at the bottom and a fuse at the output. By the way, foolproof protection is implemented here if you use a power supply as a charger and accidentally reverse the polarity of the connection, the diode will open, burning the fuse.
Now about the scheme. This is a very popular variation based on three op-amps, the Chinese are also churning out en masse, in this source it is the Chinese board that is used, but with major changes.
Here is the diagram that I got, with what was changed highlighted in red.
Let's start with the diode bridge. The bridge is full-wave, made on 4 powerful dual Schottky diodes type SBL4030, 40 volts 30 amperes, diodes in a TO-247 package.
There are two diodes in one case, I paralleled them, and as a result I got a bridge on which there is a very low voltage drop, and therefore losses, at maximum currents “that bridge is barely warm, but despite this the diodes are installed on an aluminum heat sink, represented by a massive plate The diodes are isolated from the radiator with a mica gasket.
A separate board was created for this node.
Next is the power part. The original circuit is only 3 Amperes, but a modified one can easily give out 8 Amps in this situation. There are already two keys. These are powerful composite transistors 2SD2083 with a collector current of 25 Amps. It would be appropriate to replace it with KT827, they are cooler.
The keys are essentially parallelized; in the emitter circuit there are equalizing resistors of 0.05 Ohm 10 watts, or rather, for each transistor, 2 resistors of 5 watts 0.1 Ohm are used in parallel.
Both keys are installed on a massive radiator, their substrates are isolated from the radiator; this can not be done, since the collectors are common, but the radiator is screwed to the body, and any short circuit can have disastrous consequences.
The smoothing capacitors after the rectifier have a total capacitance of about 13,000 µF and are connected in parallel.
The current shunt and the specified capacitors are located on the same printed circuit board.
A fixed resistor was added on top (in the diagram) of the variable resistor responsible for regulating the voltage. The fact is that when power is supplied (say 20 Volts) from the transformer, we get some drop on the diode rectifier, but then the capacitors are charged to the amplitude value (about 28 Volts), that is, at the output of the power supply the maximum voltage will be greater than the voltage supplied transformer. Therefore, when connecting a load to the output of the block, there will be a large drawdown, this is unpleasant. The task of the previously indicated resistor is to limit the voltage to 20 Volts, that is, even if you turn the variable to maximum, it is impossible to set more than 20 Volts at the output.
The transformer is a converted TS-180, provides an alternating voltage of about 22 volts and a current of at least 8 A, there are 9 and 15 volt taps for the switching circuit. Unfortunately, there was no normal winding wire at hand, so the new windings were wound with mounting, stranded copper wire 2.5 sq. mm. This wire has thick insulation, so it was impossible to wind the winding at a voltage of more than 20-22V (this takes into account the fact that that I left the original filament windings at 6.8V, and connected the new one in parallel with them).
Hello dear readers. There are many circuits where the wonderful high-power composite transistors KT827 are used with great success and naturally sometimes there is a need to replace them. When the code for these transistors is not found at hand, we begin to think about their possible analogues.
I have not found complete analogues among foreign-made products, although there are many proposals and statements on the Internet about replacing these transistors with TIP142. But for these transistors the maximum collector current is 10A, for 827 it is 20A, although their powers are the same and equal to 125W. For 827, the maximum collector-emitter saturation voltage is two volts, for TIP142 it is 3V, which means that in pulse mode, when the transistor is in saturation, with a collector current of 10A, a power of 20 W will be released on our transistor, and in the bourgeois mode - 30 W , so you will have to increase the size of the radiator.
A good replacement could be the KT8105A transistor, see the data on the plate. With a collector current of 10A, the saturation voltage of this transistor is no more than 2V. This is good.
In the absence of all these replacements, I always assemble an approximate analogue using discrete elements. Transistor circuits and their appearance are shown in photo 1.
I usually assemble by hanging installation, one of possible options shown in photo 2.
Depending on the required parameters of the composite transistor, you can select replacement transistors. The diagram shows diodes D223A, I usually use KD521 or KD522.
In photo 3, the assembled composite transistor operates on a load at a temperature of 90 degrees. The current through the transistor in this case is 4A, and the voltage drop across it is 5 volts, which corresponds to the released thermal power of 20W. I usually perform this procedure on semiconductors within two or three hours. For silicon this is not at all scary. Of course, for such a transistor to work on this radiator inside the device case, additional airflow will be required.
To select transistors, I provide a table with parameters.
