Power supply with changeable polarity. IP with a smooth change in polarity. Power supply circuit with polarity adjustment
Entire treatises have been written on what bipolar nutrition is, from 2 paragraphs to an article 40 pages long, so we will not describe these details here, we will note only the most important points. This type nutrition is most often used measuring technology and various analog equipment, especially in audio and video - the reason for this is quite simple: many signals that need to be measured and processed have not only positive value, but also negative, in accordance with the non-electric generating them physical phenomenon. A striking example of such a phenomenon are sound waves that rock the membrane of a dynamic microphone, generating a current in the coil, the direction of which shows the position of this very membrane relative to the resting point. Therefore, the processing circuit for such a signal should work normally for any sign of the input voltage. There are a huge number of such circuits, but many of them require bipolar power supply.
Again, there are a huge number of various circuits for obtaining bipolar power - from primitive to very non-standard ones, using completely non-obvious circuit solutions. You can consider the advantages of abstract schemes and solutions used in them for an infinitely long time, and the best option simply does not exist, because in each specific case there are certain requirements (including the availability of the necessary components at the current time), which determine the final version of the device assembly.
Selecting a bipolar power supply circuit
Taking into account the above, we will assemble a small adjustable stabilized bipolar one for use in laboratory conditions when setting up low-power low-frequency amplifiers, measuring circuits containing operational amplifiers, and other devices that, for one reason or another, require bipolar power supply. Let's add that this source must have low level self-noise and the lowest possible output voltage ripple. Additionally, it is required that it be sufficiently reliable and can survive the connection of an incorrectly assembled device to it. I would also like to make it in the form of a universal module that could be used for quick prototyping of new designs or temporarily installed in a device for which the final version of the power supply has not yet been manufactured. Having determined the technical specifications, you can proceed to selecting the circuit diagram for the future device.
All circuits of single-to-bipolar power supply converters, similar to those shown in Fig. 1, we do not consider, because their use is possible only with a strictly defined load. So, for example, if a short circuit occurs in a circuit connected to one of the arms, an unpredictable imbalance of voltages or currents will occur, which in turn can lead to failure of both the source and the circuit under study.
Rice. 1 - Inappropriate schemes converters
An excellent circuit for converting unipolar power supply to bipolar power supply, but, alas, without adjusting the output voltage, is given in the magazine “Radioamator” No. 6 for 1999:
Let's immediately discard the idea of simple pulse source, because when using the simplest schemes that contain minimum set components - the source turns out to be very noisy, i.e. at its output there is quite a lot of noise and various types of interference, which are not so easy to get rid of.
Rice. 3 - Scheme from the book “500 schemes for radio amateurs. Power supplies", author A.P. Family man
At the same time, for powering ULF on a TDA chip, this is an excellent option, but for a microphone amplifier with a high gain, it’s not so much. In addition, you will still have to make separate stabilization and short circuit protection units. Although, if we needed a source with a power of 150 W or more - construction pulse block A power supply with regulation, good filtration and built-in protection would be an excellent, and also cost-effective solution.
The simplest and most reliable solution for our task would be to use a transformer with a power of about 30 W with two windings or a winding with a tap from midpoint. These transformers are widely distributed on the market, they are easy to find in outdated equipment, and in extreme cases you can always add additional winding to the existing one. this moment in stock.
Rice. 4 - Transformers
Since we need a stabilized source, then, accordingly, after the transformer and diode bridge we need some adjustable block voltage stabilization with short circuit protection (although short circuit protection can be added after).
The next step is to reject all variants of stabilizers, assembled on discrete elements and consisting of a huge number of parts, as too complex for the task. In addition, in the vast majority of cases they require careful configuration with the selection of certain elements.
Most simple solution in our case we will use adjustable linear stabilizers such as LM317. I would immediately like to warn against the fundamentally wrong idea of using two positive stabilizers, included as shown below. This scheme, although it can work, it functions incorrectly and is unstable!
Rice. 6 - Scheme using two positive stabilizers
Accordingly, you will have to use “complementary” adjustable stabilizer LM337. The advantage of both stabilizers is the built-in protection against overheating and short circuit at the output, as well as a simple switching circuit and no need for configuration. You can see a typical connection diagram for these stabilizers in the datasheet from the manufacturer:
Rice. 7 - Typical connection diagram for LM337 stabilizers
Having modified it a little, we get the final version of the module of an adjustable bipolar power supply, which we will assemble according to the following scheme:
Rice. 8 - Scheme adjustable bipolar power supply module
The circuit seems complicated due to the fact that we have marked on it all the recommended wiring parts, namely, shunt capacitors and diodes that serve to discharge capacitors. To make sure that most of them need to be installed, you can refer to the datasheet again:
Rice. 9 - Wiring diagram from datasheet
To simplify manufacturing, namely, reduce the number of operations required for assembly, we use surface mounting technology, i.e. All parts in our design will be SMD. One more important point There will be the fact that our module will not have a network transformer, we will make it pluggable. The reason lies in the fact that when big difference between the supply and output voltages, and when working with maximum current, the difference between the power supplied and supplied to the load must be dissipated on the regulatory elements of our circuit, and specifically on integrated regulators. The maximum power dissipation for such stabilizers is already small, and when using SMD packages it becomes even less, and as a result, the maximum current of such a stabilizer operating with a difference between input and output voltages of 20 V can easily drop to 100 mA, and this for our tasks are no longer enough. This problem can be solved by reducing the difference between these voltages, for example, by connecting a transformer with secondary winding voltages closest to what is currently required.
Selection of components
One of difficult moments The implementation of our idea suddenly turned out to be a selection of integrated stabilizers in the required housing. Despite the fact that I was reliably aware of their existence in all possible SMD packages, viewing the datasheets of various manufacturers did not allow me to find the exact markings, and a search for parameters from several global suppliers showed only individual options, and most often from different manufacturers. As a result, the desired combination in SOT-223 cases, also from the same series, was found on the website Texas Instruments: LM337IMP and LM317EM:
Rice. 10 - I integral stabilizers LM337IMP and LM317EM
It is worth noting that a great variety of different pairs consisting of different polarity voltage stabilizers can be selected, but the manufacturer recommends a pair of stabilizers of the same series. Both stabilizers provide a maximum current of up to 1 A with a difference between input and output voltages of up to 15 V inclusive, however, the rated current at which the stabilizer is guaranteed not to go into overheating protection can be considered 0.5-0.8 A. A current of 500 mA at those applications for which we build this stabilizer more than enough, so we will consider the task of selecting stabilizers completed.
Let's move on to the remaining components.
Diode bridge - any, with a rated current of 1-2 A. for a voltage of at least 50 V, we used DB155S.
Almost any electrolytic capacitors can be used in this circuit, with a small voltage reserve. The selection is made based on the following considerations: since the range of the supply voltage that we require does not exceed 15 V, and the recommended maximum for stabilizers is 20 V, 25 V capacitors have a reserve of at least 25%. All electrolytic capacitors must be shunted with film or ceramic ones with ratings according to the diagram, for a voltage of at least 25 V. We used standard size 0805 and dielectric type X7R (NP0 can be used, and Z5U or Y5V are not recommended due to poor TKS and TKE, although in the absence of an alternative - These will also work).
Resistors of a constant value - any, in the voltage divider responsible for the stabilization voltage it is better to use more accurate ones, with a tolerance of 1%. The standard size of all resistors is -1206, solely for ease of installation, but you can safely use 0805. A 100 Ohm trimmer is multi-turn, for precise adjustment (use 3224W-1-101E). The resistor used to adjust the output voltage is rated at 5 KOhm, any available, we took 3314G-1-502E for a screwdriver, but you can also use a variable resistor for mounting on the case, connecting it to the stabilizer board with wires. It is advisable to use high-speed diodes, with a current of at least 1 A and a voltage of 50 V or more, for example HS1D.
The LED power indicator is designed according to the following principle: the current through the zener diode at the highest input voltage should not exceed 40 mA, when a voltage of up to 30 V is applied to the input, the value of the current-limiting resistor will be equal to 750 Ohms, for reliability it is better to use 820 Ohms. It is pointless to supply the stabilizers with a voltage less than 8 V per arm (since the internal structure of the microcircuit contains 6.3 V zener diodes), so at a voltage of 16 V the current through the zener diode will be 20 mA, and through the LED connected in parallel to it - about 8 mA, which will be enough to light up an SMD LED. Any zener diode with a stabilization voltage of 3.3 V (DL4728A is used), and accordingly a current-limiting resistor for the LED of 150 Ohms to ensure its long-term operation at the maximum current through the zener diode.
Manufacturing of the device
We draw the printed circuit board of our device, Special attention paying attention to the contact pads for large SMD capacitors. The following difficulty may arise with them - they are basically intended for soldering in an oven, i.e. It is quite difficult to solder them from below, especially with a low-power soldering iron, but the capacitor leads are accessible from the side and you can firmly solder it, provided that the thickness of the tracks suitable to it is sufficient to ensure the mechanical strength of the connection. Also, it is important that the positive and negative stabilizers have different pinouts, i.e. just mirror one half printed circuit board It won't work during wiring.
We transfer the printed circuit board design onto a previously prepared piece of foil fiberglass laminate, and send it to be etched in a solution of ammonium persulfate (or another similar reagent of your choice).
Rice. 12 - Board with transferred pattern + etching
After the board has been etched, we remove the protective coating and apply flux to the tracks, tin them to protect the copper from oxidation, and then begin soldering the components, starting with the smallest height. There shouldn’t be any special problems, and we prepared in advance for possible difficulties with SMD electrolytes.
Rice. 13 - Board after etching + apply flux + tinning
After all the components are soldered and the board is washed of flux, you need to use a 100 Ohm trimmer to adjust the voltage on the negative side so that it matches the voltage on the positive side.
Rice. 14 - Finished board
Rice. 15 - Adjustment voltage on the negative side
Testing the assembled device
Let's connect a transformer to our stabilizer and try to load both of its arms, and each of the arms independently of each other, simultaneously controlling the currents and voltage at the outputs.
Rice. 16 - First dimension
After several attempts to take measurements at the maximum current, it became clear that the tiny transformer was not able to provide a current of 1.5 A, and the voltage on it sags by more than 0.5 V, so the circuit was switched to a laboratory power supply that provides current up to 5 A.
Everything is working as normal. This regulated bipolar power supply, assembled from high-quality components, due to its simplicity and versatility, will take its rightful place in a home laboratory or small repair shop.
Measurements and commissioning work were carried out on the basis of the testing laboratory of JSC "KPPS", for which special thanks to them!
A controlled constant stabilized current source with good dynamic characteristics allows you to change the magnitude and polarity of the output current under the influence of the input control voltage. The source can be part of various devices and systems. The accuracy of the output current matching the input control voltage allows the source to be used for critical applications. The operation of the current source can be explained using the example of controlling an LED indicator.
Using a current source to control LEDs
It is more convenient to change the brightness of LEDs by adjusting the current flowing through the LED, rather than the voltage applied to the LED. Using a controlled source of stabilized current, you can change and adjust the brightness of conventional or laser LEDs. By changing the polarity, you can select a group of working LEDs. With one polarity of the current, LEDs H1-H6 will light up, with the opposite polarity, LEDs H7-H12. If the LEDs have different color, for example, H1-H6 are red, and H7-H12 are green, it is possible to indicate the normal and critical value of the controlled value.
A source of constant stabilized current is necessary to regulate the magnitude of the constant magnetic field. The control voltage can come from a digital-to-analog converter of a specialized controller or other device.
Application of a current source to control electric motors
Using source direct current, which has the ability to change the direction of the current, it is quite simple to regulate the rotation speed and change the direction of rotation of the electric motor rotor. To transmit a command that sets the rotation parameters, one two-wire line is enough. Forward rotation occurs when the current polarity is positive on pin 1 and the current polarity is negative on pin 2 of the current source output connector U1.
Motor reversal occurs when the polarity of the control voltage and the resulting change in the polarity of the output current are changed. With the help of one source of current changing the direction, two electric motors can be controlled. With a positive polarity of the output current at pin 1, current flows through the diode VD2 and the electric motor M2 operates; with a negative polarity of the current at pin 1, current flows through the diode VD1 and the electric motor M1 operates. There is no motor reversal with this connection scheme.
A voltage-controlled current source is used in the transmission of analog signals. With this method of organizing communication, the current value is proportional to the analog value. The distortion of the signal transmitted by current by electromagnetic interference is significantly less compared to in the usual way voltage signal transmission.
The use of a current signal requires installation in transmitting and receiving equipment special modules transmitting and receiving current. In this case, digital coding of the transmitted data can be eliminated. A voltage-controlled current source is used for smooth control of solenoid-based electromagnetic regulators in hydraulic systems. Based on a controlled current source, it is easy to build a universal device for charging batteries of different types.
Current source operation
Current generated ideal source, is stable when the resistance of the connected load changes. To maintain the current value constant, the value of the source emf changes. A change in load resistance causes a change in the emf of the current source in such a way that the current value remains unchanged.
Real current sources maintain current at the required level over a limited range of voltage generated across varying load resistance. This range is limited by the power supply power of the current source. If it is necessary to maintain a current of 1 amp into a 20 ohm load, this means that the load will have a voltage of 20 volts. When the load resistance decreases or a short circuit occurs, the output voltage will decrease, and when the load resistance increases, the power supply must be able to operate at voltages above 20 volts.
Operation of the current source requires a power supply. A current stabilizer is connected in series with the power source. The output of such a device is considered as a current source. The power supply parameters of the current source are finite, this limits the maximum load resistance that can be connected to the current source. To ensure reliable operation, the power supply must have an overload reserve. The limited power supply limits the maximum current that the current source can deliver to the load.
The current source can operate with a load resistance close to zero. Shorting the output of the current source does not lead to a device failure or protection. If a short circuit occurs in the output of the current source caused by high humidity or careless handling of the equipment by maintenance personnel, after eliminating the causes of the short circuit, the device instantly returns to normal operation.
Controlled current source circuit
- Supply voltage………….100…260 V, 47…440 Hz
- Input voltage………….±10 V
- Output current………………….± 100 mA
- Load resistance……..0.1…120 Ohm
- Temperature range……-50…+75 ±С
- Conversion accuracy……0.5%
Simplified current source circuit
The operation of the circuit is based on the ability of the operational amplifier to change the output voltage of the operational amplifier so as to equalize the voltage at the inputs thanks to the circuits feedback. The control voltage through resistor R1 is supplied to the inverting input of the operational amplifier and causes a change in the voltage at its output.
A change in voltage at the amplifier output causes current to flow through resistor R5 and the load. The output voltage is fed through feedback circuits to the inputs of the operational amplifier. The resistor resistances have values that provide the desired proportionality between the influence on the control voltage and the current through the load.
When a positive control voltage is supplied to the inverting input of the operational amplifier, a negative voltage is generated at its output. A current flows through the resistor and the load, creating a voltage across resistor R5. The potential at the junction of resistors R3 and R5 is lower than at the junction of resistors R4, R5 and the load.
Due to the fact that the total resistance of resistors R4 and R5 is equal to the resistance of R3, there is a potential at the output of the amplifier that compensates the control voltage at the inputs of the operational amplifier through feedback resistors. The amplifier output potential will drop as much as necessary to compensate for the effect of the positive control voltage on the inverting input of the operational amplifier.
Compensation for the effect of the control voltage on the inputs of the operational amplifier occurs depending on the voltage across resistor R5 caused by the flowing current. If the control voltage is fixed, then the feedback effect on the operational amplifier inputs depends on the voltage across resistor R5.
A change in load resistance causes a change in the potential at the non-inverting input of the operational amplifier through resistor R4. As the load resistance decreases, the potential at the non-inverting input of the operational amplifier decreases and the voltage between the inputs of the operational amplifier increases, which causes a decrease in the potential at the output of the amplifier. At the same time, the applied voltage decreases at a decreased load resistance, preventing the current from increasing.
The proportionality between the control voltage and the output current is established by the resistances of the resistors. The resistance of resistor R5 should be small; the output current flows through it, causing heating. Reducing resistance R5 expands the range of resistance of connected loads. The resistances of resistors R1 and R2 are equal, their values are chosen such that they eliminate overload of the control voltage source. Resistor resistances are calculated using the following formulas:
I = (U*R3)/(R1*R5)
- U - control voltage
- I - output current
One of the important parameters of any current source, and in our case a voltage-to-current converter, is the resistance range of the connected loads. The idealized model of the device provides the required current in the range of load resistance from 0 to infinity.
In real devices this is impossible and unnecessary, since the resistance of the wires, connector contacts, and elements of other circuits is added to the load resistance. The property of a current source to ensure the operation of the system regardless of the load resistance is very useful. Thanks to this property, it increases the reliability of the system in which the current source is involved.
The disadvantage of the current source is the power released at the output amplifier. In each case, you will need to choose a compromise between the load resistance margin and the heat generated at the output amplifier. To provide a wide range of load resistances, it is necessary to use a device power supply with a sufficient voltage margin.
with change in current direction
The practical implementation of the source is shown in the electrical circuit diagram. To accurately match the circuit calculations, the resistances are assembled from resistors connected in series or parallel. The output amplifier consists of transistors VT1 and VT2. With an output current of one hundred milliamps at a twenty-ohm load, the voltage will be two volts, across the regulating transistor the voltage drop is approximately 0.6 volts, and across resistor R5 the voltage drop is 0.1 volt. With a power supply of 15 volts, the voltage on one of the two transistors of the amplifier will be 15V-2.7V=12.3V, and a power of about 12.3V*100mA=1.23 W will be released in the form of heat.
Capacitor C4 is necessary to suppress interference induced on the line connected to the control input of the device, capacitor C5 prevents excitation of the circuit. Capacitor C1 reduces device interference into the power supply. Power is supplied from a network of 220 volts, 50 Hz.
Thanks to the DA1 pulse voltage converter, there are no voltage stability requirements for the power supply. Circuit breaker Q1 acts as a power switch and protects the 220-volt network from overload in the event of a device failure. H1 – power supply indicator. Transyl diode VD1 protects the power source from exceeding the mains voltage above a critical value. The voltage converter provides the device circuit with bipolar power, necessary for the operation of the operational amplifier and the formation of an output current of two polarities.
Circuit components
| Positional designation |
Name |
| Capacitors | |
| C1 | K73-16 0.01 µF ± 20%, 630 V |
| C2, C3 | C4 | 100 pF-J-1H-H5 50 Volt, f. Hitano | C5 | 0.47 µF-K-1N-N5 50 Volts, f. Hitano |
| Resistors | |
| R1, R2 | C2-29B-0.125-101 Ohm ± 0.05% | R3 | C2-23-0.25-33 Ohm ± 5% | R4 | C2-29B-0.125-101 Ohm ± 0.05% | R5 | 1 Ohm ± 0.01% Astro 2000 axial f. Megatron Electronic | R6, R7 | C2-29B-0.125-200 Ohm ± 0.05% | R8, R9 | C2-29B-0.125-10 kOhm ± 0.05% |
| Transistors and diodes | VT1 | TIP3055 f. Motorola |
| VT2 | TIP2955 f. Motorola |
| VD1 | Bidirectional transyl diode 1.5KE350CA f. STMicroelectronics |
| Circuits and modules | H1 | LED switch lamp SKL-14BL-220P “Proton” | DA1 | Voltage converter TML40215 f. TRACO POWER | DA2 | OP2177AR operational amplifier chip | Q1 | Automatic switch Ukrem VA-2010-S 2p 4A “Asko” |
Capacitor C1 can be of any type. An important requirement for this component is an operating voltage level of at least 630 volts. Capacitors C2...C5 can be used ceramic or multilayer. All resistors except R3 must have the highest possible accuracy. It is better to make resistor R5 a composite of four resistors with a resistance of 1 ohm.
Two circuits consisting of two 1 ohm resistors connected in series are connected in parallel. As a result, the total resistance is 1 ohm, and the power dissipation is quadrupled. Wire-type resistor R5 cannot be used. Pulse converter voltage DA1 can be replaced with a bipolar power supply, providing an output current in each arm of 500 milliamps and a ripple level of no more than 50 millivolts.
For achievement high precision To convert the control voltage into output current, the operational amplifier must have a low voltage zero offset. This is especially important for reducing the output current to zero under the influence of control voltage. With a slight decrease in accuracy, OP213 or OP177 microcircuits are suitable as a replacement for DA1. Circuit output application powerful transistors increases the reliability of the device. Transistors must be installed on radiators.
The circuit can be used for other output currents and control voltages. To do this, you will need to make calculations using the formulas given earlier in the article. When performing calculations, you should take into account the possibility of using resistors from the standard range of resistances.
When checking the operation of the circuit, it is necessary to check with an oscilloscope over the entire range of voltages, currents and load resistance that there are no oscillations at the circuit output. If there are fluctuations, increase capacitance C4 or C5.
Platon Konstantinovich Denisov, Simferopol
[email protected]
The peculiarity of this circuit is that by turning the control knob you can change not only the output voltage, but also its polarity. Adjustment is made in the range from +12V to -12V.
Power supply circuit with polarity adjustment
Essentially, these are two separate voltage stabilizers - positive and negative with a common regulating resistor R5.
The transformer for the source is also required with double winding.
When the resistor R5 slider is in the middle position, both stabilizers are closed and the output voltage will be zero. When the engine is moved in one direction or another, one of the adjustable stabilizers will open - either “positive” or “negative” and, accordingly, the output voltage will change.
The capacitances of capacitors C1 and C2 should not be less than 1000 µF. Instead of transistors KT816 and KT817, you can use more powerful ones - for example, KT818 and KT819. The power of the power source itself directly depends on the power of the transformer used.
The transformer must have two output windings of at least 12 Volts each.
Instead of the KTs405 diode assembly, you can use four simple diodes connected in a bridge.
The peculiarity of this power source is that by rotating the control knob you can not only change the output voltage, but also its polarity. Practically adjustable from +12V to - 12V. This is achieved thanks to the slightly unusual inclusion of stabilizers of a bipolar power supply, so that both stabilizers are regulated using one variable resistor.
The schematic diagram is shown in the figure. The rectifier is bipolar, made according to a standard circuit on a T1 transformer with a secondary winding tapped from the middle, a diode bridge VD 1 and capacitors C1 and C2. As a result, its output produces a bipolar voltage of +-16.., 20V. This voltage is supplied to two transistor stabilizers VT 1 and VT 3 (positive voltage regulation) and on transistors VT 2 and VT 4 (negative voltage adjustment). The difference from the standard bipolar circuit is that the outputs of the stabilizers are connected together, and that one common variable resistor is used to regulate the voltage R5. Thus, if the slider of this resistor is installed exactly in the middle, and the voltage across it relative to the common wire is zero, then both stabilizers are closed, and the voltage at the output of the circuit is also zero. Now, if the engine begins to move towards positive voltages (up in the circuit), the positive voltage stabilizer on the transistors begins to open VT 1 and VT 3, and the stabilizer negative voltages (VT 4 and VT 2) still remains closed. INThe result is a positive voltage at the output. Now, if the slider is moved in the direction of negative voltages (down the circuit), the positive voltage at the circuit terminal will decrease in the middle position R 5 the voltage will become zero. The positive voltage regulator will close. If the engine is moved further in the same direction, the negative voltage stabilizer on VT 2 and VT 4 (in this case, the positive voltage stabilizer will be closed) and the negative voltage at the output will increase.
The design uses a ready-made transformer"TAIWAN" with a power of 10 W, producing two alternating voltages of 12 V each on the secondary winding.
The capacitances of capacitors C1 and C2 should not be less than 1000 μF; it must be taken into account that the level of ripple at the output depends on them. Zener diodes can be any low-power voltage 12V. The KT817 transistor can be replaced with KT815, KT807, KT819. Transistor KT816 - on KT814, KT818. Transistors KT502 and KT503 can be replaced, respectively, with KT361 and KT315. You can use another rectifier bridge, for example KTs402, or assemble it from diodes like D226 or KD105.
Transistors VT 1 and VT 2 need to be placed on small heat sinks.