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Power supply unit 0-30V 10A
This rather powerful power supply provides a stabilized voltage from 1 to 30 volts at a current of up to 10 amperes.
Unlike other power supplies described on this site, in addition to a voltmeter, it has a current measurement function, which can be used, for example, in electroplating.
The front panel contains (from top to bottom):
- green LED for switching on the power supply unit;
- red LED of current protection operation;
- head for measuring voltage (upper scale) and current (lower scale);
- to the left of the icon - voltage indication switch - current;
- to the right of the icon - current protection reset button;
- output voltage regulator;
- load connection terminals.
The transformer must have a power of 300 W with a secondary voltage of 23 volts alternating with an output from the middle of the secondary. The output is needed to implement the current protection circuit (below). The protection key is assembled on the transistor T1. The voltage drop across the resistor R2 leads to the opening of this transistor, the thyristor optocoupler AOU103 is triggered, the relay is triggered, the contacts of which break the load at the PSU output and light up the red LED. After the protection is triggered, it is better to reset the voltage with the alternator and return the unit to work with the START button. The stabilizer itself is assembled on a DA2 stabilizer and two powerful transistors VT3 and VT4, operating in parallel.
Here I brought a spreading :) of some active elements, so that you do not have to rummage in reference books.
Do not forget that there is a collector on the case of 2N3055 transistors, so they must be isolated from the radiator with a mica or ceramic gasket lubricated with silicone grease for thermal conductivity.
The front panel on the back is unsoldered without any surprises. A circuit with trimming resistors for calibrating the measured current and voltage is mounted directly on the terminals of the measuring head.
View of the right wall from the inside.
A relay is attached closer to the corner. I don't know the type of relay, the operating voltage on the winding is 12 volts DC, the winding resistance is 123 ohms, the current is 84 mA. Normally closed contacts switch the load, normally open for protection actuation signaling (red LED).
In the foreground are power transistors on a copper heat sink through ceramic spacers. Copper is used as an excellent heat-conducting material, second only to silver in this respect. The copper radiator transfers heat further to the duralumin radiator. Under the transistors are current equalizing resistors R9 and R10.
There is a ballast resistor under the relay, the voltage drop across which the measuring head operates in the current measurement mode. I will not give specific numbers, it all depends on which head you find. I will only tell you how this resistor can be made. Firstly, according to your calculations, its resistance will be quite small, and Secondly, its resistance should be pretty accurate. Therefore, we find nichrome. It doesn't matter what diameter, because you can play with the number of wires. The main thing is to measure its diameter and use the tables I gave to determine its linear resistance. This is already enough to calculate the length and number of wires according to Ohm's law. Next, we collect the wires in a bundle, put them in copper tubes of a suitable diameter and flatten them while observing the required length of the wires. That's it, the ballast is ready. It can be soldered to the contacts.
Left and back wall.
A printed circuit board is attached at the top of the left wall, on which all the small things are located. Scheme printed circuit board and its view further.
The power diode assembly BB36931 is attached to the radiator of the left wall itself. It operates up to 80 volts at up to 10 amps. For high-quality thermal contact, we sit on an organosilicon ointment. I use vixint for this. The good thing about this assembly is that no gaskets are required.
The back panel contains the fuses and the main capacitor. The capacitor is shunted with a resistor just in case.
On the left is a diagram of the printed circuit board from the side of the hinged elements. On the right side of the back. Further - already live views.
Arrangement of elements internal device power supply is not arbitrary. All of them are located in such a way that when assembling all the walls together, they do not interfere with each other, and each protrusion goes into the corresponding recess. As you can see in the next photo.
And finally, the back wall is outside. Do not torture yourself in vain, because often when carrying, the lace dangles and interferes. Make brackets for winding the wire and select its length for the most convenient winding. Do not follow the example of factory products. After all, they are not made for people, but for sale. And you still do for yourself, beloved :)
In addition, on these brackets, the unit can work lying on its back.
The main unit is an adjustable compensation-type voltage regulator. It contains an input differential stage on transistors VT5, VT7, two amplification stages on transistors VT3 and VT2, and a regulating transistor VT 1. Elements VT4, VT6, VD4, VD5, R5 - R8, R10 form current stabilizers. Capacitor C8 prevents self-excitation of the unit. The output voltage is regulated by resistor R13. The upper voltage limit is a trimming resistor R14. Construction and details. The power of the transformer T1 must be at least 100 - 160 watts, the current of winding II - at least 4 - 6 amperes. Winding current III - within 1 ... 2 amperes. Transistor VT1 should be installed on finned aluminum radiators with an area of more than 1450 sq. Cm. Resistor R4 is selected experimentally, according to the protection operation current.
Resistors R 7 and R 14 are multiturn SP5-2. Resistor - R13 any variable. Microcircuits DA1 and DA2 can be replaced with similar domestic KR142EN5A and KR1162EN5A. Their power allows a stabilized voltage of ± 5 volts to power external loads with a current consumption of up to 1 ampere. This load is a digital panel, which is used for digital indication of voltage and current in power supplies. If you do not use a digital panel, then DA1 and DA2 microcircuits can be replaced with 78L05 and 79L05 microcircuits. Diodes VD3 - VD5 can be replaced with diodes KD522B. The digital panel consists of an input voltage and current divider, a KR572PV2A microcircuit and an indication of four seven-segment LED indicators. Resistor R4 of the digital panel consists of two pieces of constantan wire = 1 mm and a length of 50 mm. The difference in the resistor value must exceed 15 - 20%. Resistors R2 and R6 of the SP5-2 and SP5-16VA brands. Switch of modes of indication of voltage and current type P2K. The KR572PV2A microcircuit is a 3.5 decimal place converter operating on the principle of sequential counting with double integration, with automatic correction zero and determining the polarity of the input signal. Imported LEDs were used for indication. seven-segment indicators KINGBRIGT DA56 - 11 SRWA with common anode. It is advisable to use film capacitors C2 - C4 of the K73-17 type. Instead of imported seven-segment LEDs, domestic ones with a common anode of the ALS324B type can be used.
All radio components of the device:
VD1 - VD4 - RS600
VD5 - VD8 - KS407A
VD9 - AL307B
VD10 - KD102A
VD11 - 1N4148
VD12 - 1N4148
C1 - 10000 μF x 50 volts
C2 - 100 μF
C3 - 100 uF
C4 - 10 μF
C5 - 10 μF
C6 - 10 n
C7 - 10 n
C8 - 33 n
R1 - 330 Ohm
R2 - 3 kOhm
R3 - 33 Ohm
R4 - 2.4 kOhm
R5 - 150 Ohm
R6 - 2.2 kOhm
R7 - 10 kOhm
R8 - 330 kOhm
R9 - 6.8 kOhm
R10 - 1 kOhm
R11 - 5.1 kOhm
R12 - 5.1 kOhm
R13 - 10 kOhm
R14 - 2.2 kOhm
VT1 - KT827A
VT2 - KT815G
VT3 - KT3107A
VT4 - KT3102A
VT5 - KT315D
VT6 - KT315D
VT7 - KT315D
After turning on the power and error-free installation, if the parts are intact, the indication segments HG1-HG3 should light up. According to the voltmeter, the resistor R2 at pin 36 of the KR572PV2 microcircuit sets a voltage of 1 volt. Connect the power supply to legs (a) and (b). At the output of the power supply, a voltage of 5 ... 15 volts is set and a resistor R 10 (roughly) is selected, replacing it, for a while, with a variable.
With the resistor R8, a more accurate voltage reading is set. After that, a variable resistor with a power of 10 ... 30 watts is connected to the output of the power supply, the current is set to 1 ampere by the ammeter, and the value on the indicator is set by the resistor R 6. The reading should be 1.00. At a current of 500 mA - 0.50, at a current of 50 mA - 0.05. Thus, the indicator can indicate a current of 10 mA, that is, 0.01.
The maximum current indication is 9.99 amperes. For a greater digit capacity of the indication, you can use the circuit on the KR572PV6. Contact pads U and I on the printed circuit board of the digital panel, using flexible conductors, are connected to the points of the corresponding indicators HG 2 and HG 1. The KR572PV2A microcircuit can be replaced with an imported ICL7107CPL microcircuit.
The simplest power supply unit 0-30 Volts for a radio amateur.
Scheme.
In this article, we continue the topic of power supply circuitry for radio amateur laboratories. This time we will talk about the simplest device, assembled from Russian-made radio components, and with a minimum number of them.
And so, a schematic diagram of the power supply:
As you can see, everything is simple and accessible, the element base is widespread and does not contain any deficiencies.
Let's start with the transformer. Its power should be at least 150 watts, the voltage of the secondary winding - 21 ... 22 volts, then after the diode bridge on the capacitance C1 you will get about 30 volts. Calculate so that the secondary winding can supply 5 Amps.
After the step-down transformer, there is a diode bridge assembled on four 10-ampere D231 diodes. The current reserve is certainly good, but the design is rather cumbersome. The best option would be to use an imported diode assembly of the RS602 type, with small dimensions it is designed for a current of 6 Amperes.
Electrolytic capacitors are rated for an operating voltage of 50 volts. C1 and C3 can be set from 2000 to 6800 uF.
Zener diode D1 - it sets the upper limit for adjusting the output voltage. In the diagram, we see the inscription D814D x 2, which means that D1 consists of two series-connected zener diodes D814D. The stabilization voltage of one such zener diode is 13 Volts, which means that two connected in series will give us an upper voltage adjustment limit of 26 volts minus the voltage drop at the junction of transistor T1. As a result, you will get a smooth adjustment from zero to 25 volts.
KT819 is used as a regulating transistor in the circuit, they are produced in plastic and metal cases... See the next two pictures for the pinout, package sizes and parameters of this transistor.
It makes sense not only for a keen radio amateur to make a power supply unit with your own hands. A homemade power supply unit (PSU) will create convenience and save a considerable amount also in the following cases:
- For powering low-voltage power tools, in order to save the resource of an expensive rechargeable battery (accumulator battery);
- For the electrification of premises that are especially dangerous in terms of the degree of electric shock: basements, garages, sheds, etc. When powered by alternating current, a large amount of it in low-voltage wiring can interfere household appliances and electronics;
- In design and creativity for accurate, safe and waste-free cutting of foam plastic, foam rubber, low-melting plastics with heated nichrome;
- In lighting design - the use of special power supplies will extend life led strip and get stable lighting effects. Power supply of underwater illuminators, etc., from a household electrical network is generally unacceptable;
- For charging phones, smartphones, tablets, laptops away from stable power sources;
- For electroacupuncture;
- And many other goals not directly related to electronics.
Acceptable simplifications
Professional PSUs are designed to power loads of any kind, incl. reactive. Precision equipment is among the potential consumers. The preset voltage of the pro-PSU must be maintained with the highest accuracy indefinitely long time, and its design, protection and automation must allow operation by unqualified personnel in difficult conditions, for example. biologists to power their devices in a greenhouse or on an expedition.
An amateur laboratory power supply unit is free from these restrictions and therefore can be significantly simplified while maintaining quality indicators sufficient for their own use. Further, through also simple improvements, it is possible to obtain a special-purpose power supply unit from it. What we are going to do now.
Abbreviations
- Short circuit - short circuit.
- XX - idle, i.e. sudden disconnection of the load (consumer) or an open circuit in its circuit.
- KSN - voltage stabilization coefficient. It is equal to the ratio of the change in the input voltage (in% or times) to the same output voltage at a constant consumption current. Ex. the mains voltage dropped "to full", from 245 to 185V. Relative to the norm of 220V, this will be 27%. If the VSD of the PSU is equal to 100, the output voltage will change by 0.27%, which at its value of 12V will give a drift of 0.033V. For amateur practice, more than acceptable.
- PPI is a source of unstabilized primary voltage. It can be a transformer on iron with a rectifier or a pulse mains voltage inverter (IIN).
- IIN - operate at an increased (8-100 kHz) frequency, which allows the use of lightweight compact transformers on ferrite with windings of several or several tens of turns, but they are not without drawbacks, see below.
- RE is a regulating element of a voltage stabilizer (CH). Maintains the specified value at the output.
- ION - a source of reference voltage. Sets its reference value, according to which, together with signals feedback OS control unit CU acts on the RE.
- SNN - continuous voltage stabilizer; simply "analog".
- ISN - pulse voltage regulator.
- UPS - pulse unit nutrition.
Note: both SNN and IIN can operate both from the IIN of industrial frequency with a transformer on iron, and from the IIN.
About computer power supplies
UPSs are compact and economical. And in the closet, many have a power supply unit from an old computer, morally outdated, but quite serviceable. So, is it possible to adapt a switching power supply from a computer for amateur / work purposes? Unfortunately, a computer UPS is a highly specialized device and the possibilities of its use in everyday life / at work are very limited:
To use a UPS converted from a computer, it is advisable for an ordinary amateur, perhaps, only to power a power tool; see below about this. The second case is if an amateur is engaged in PC repair and / or creation of logic circuits. But then he already knows how to adapt the power supply from the computer for this:
- Load the main channels + 5V and + 12V (red and yellow wires) with nichrome coils at 10-15% of the rated load;
- Green soft start wire (with a low-current button on the front panel of the system unit) pc on short-circuit to common, i.e. on any of the black wires;
- Switch on / off mechanically, with a toggle switch on the rear panel of the power supply unit;
- With a mechanical (iron) I / O "duty room", i.e. independent power supply of + 5V USB ports will also be turned off.
Get down to business!
Due to the shortcomings of the UPS, plus their fundamental and schematic complexity, we will only at the end consider a couple of such, but simple and useful ones, and talk about the IIN repair method. The main part of the material is devoted to SNV and IIT with power frequency transformers. They allow a person who has just picked up a soldering iron to build a power supply unit very High Quality... And having it on the farm, it will be easier to master the technique "thinner".
IIT
Let us first consider the IIT. We will leave the impulse ones in more detail until the section on repair, but they have something in common with the "iron" ones: a power transformer, a rectifier and a ripple suppression filter. Together, they can be implemented in various ways in accordance with the purpose of the power supply unit.
Pos. 1 in Fig. 1 - half-wave (1P) rectifier. The voltage drop across the diode is the smallest, approx. 2B. But the ripple of the rectified voltage - with a frequency of 50 Hz and "ragged", ie. with intervals between pulses, therefore, the capacitor of the ripple filter Cf should be 4-6 times larger than in other circuits. Usage power transformer Power consumption - 50%, because only 1 half-wave is rectified. For the same reason, an imbalance of the magnetic flux occurs in the magnetic circuit Tr and the network "sees" it not as an active load, but as an inductance. Therefore, 1P rectifiers are used only at low power and where there is no other way, for example. in IIN on blocking generators and with a damper diode, see below.
Note: why 2V, and not 0.7V, at which the p-n junction in silicon opens? The reason is the through current, about which see below.
Pos. 2 - 2-half-cycle with a midpoint (2PS). Losses on diodes are the same as before. case. The ripple is 100 Hz solid, so Sph needs the smallest possible. Use of Tr - 100% Disadvantage - double copper consumption for the secondary winding. In the days when rectifiers were made on kenotron lamps, this did not matter, but now it is decisive. Therefore, 2PS is used in low-voltage rectifiers, mainly of increased frequency with Schottky diodes in UPS, however, 2PS does not have fundamental power limitations.
Pos. 3 - 2-half-period bridge, 2RM. Losses on diodes - doubled in comparison with pos. 1 and 2. The rest is the same as in 2PS, but copper for the secondary needs almost half as much. Almost - because several turns have to be completed in order to compensate for the losses on a pair of "extra" diodes. The most common circuit for voltage from 12V.
Pos. 3 - bipolar. The "bridge" is depicted conventionally, as is customary in schematic diagrams (get used to it!), And rotated 90 degrees counterclockwise, but in fact it is a pair of 2PS connected in different polarities, as can be clearly seen further in Fig. 6. Copper consumption as in 2PS, diode losses as in 2PM, the rest as in both. It is built mainly to power analog devices that require voltage symmetry: Hi-Fi UMZCH, DAC / ADC, etc.
Pos. 4 - bipolar according to the parallel doubling scheme. Gives, without additional measures, increased voltage symmetry, because secondary asymmetry is excluded. The use of Tr is 100%, the ripple is 100 Hz, but torn, therefore, Sph needs double the capacity. Losses on diodes are approximately 2.7V due to mutual exchange of through currents, see below, and at a power of more than 15-20 W they increase sharply. They are built mainly as low-power auxiliary for independent power supply of operational amplifiers (OA) and other low-power analog units, but demanding on the quality of power supply.
How to choose a transformer?
In a UPS, the entire circuit is most often clearly tied to the size (more precisely, to the volume and cross-sectional area Sс) of the transformer / transformers, since the use of subtle processes in ferrite makes it possible to simplify the circuit with its greater reliability. Here "somehow in its own way" comes down to the exact observance of the developer's recommendations.
A transformer on iron is chosen taking into account the characteristics of CHN, or is consistent with them when calculating it. The voltage drop across the RE Ure should not be taken less than 3V, otherwise the SVR will drop sharply. With an increase in Ure, the KCH slightly increases, but the power dissipated by the RE grows much faster. Therefore, Ure take 4-6 V. To it we add 2 (4) V losses on the diodes and the voltage drop across the secondary winding Tr U2; for a power range of 30-100 W and voltages of 12-60 V, we take it 2.5 V. U2 arises mainly not on the ohmic resistance of the winding (it is generally negligible for powerful transformers), but as a result of losses due to magnetization reversal of the core and the creation of a stray field. Simply, part of the network energy, "pumped" by the primary winding into the magnetic circuit, evaporates into world space, which is taken into account by the value of U2.
So, we counted, for example, for a bridge rectifier, 4 + 4 + 2.5 = 10.5V excess. We add it to the required output voltage of the PSU; let it be 12V, and divide by 1.414, we get 22.5 / 1.414 = 15.9 or 16V, this will be the lowest allowable voltage of the secondary winding. If Tr is factory-made, we take 18V from the standard range.
Now the secondary current is used, which, of course, is equal to the maximum load current. Let us need 3A; multiply by 18V, it will be 54W. We have obtained the overall power Tr, Pg, and we will find the passport P by dividing Pg by the efficiency Tr η, which depends on Pg:
- up to 10W, η = 0.6.
- 10-20 W, η = 0.7.
- 20-40 W, η = 0.75.
- 40-60 W, η = 0.8.
- 60-80 W, η = 0.85.
- 80-120 W, η = 0.9.
- from 120 W, η = 0.95.
In our case, it will be P = 54 / 0.8 = 67.5W, but there is no such typical value, so you will have to take 80W. In order to get at the output 12Vx3A = 36W. A locomotive, and nothing more. It's time to learn how to calculate and wind "trances" yourself. Moreover, in the USSR, methods for calculating transformers on iron were developed, allowing without loss of reliability to squeeze 600W out of the core, which, when calculated according to radio amateur reference books, is capable of producing only 250W. The Iron Trance is not as dumb as it seems.
SNN
The rectified voltage needs to be stabilized and, more often than not, regulated. If the load is more powerful than 30-40 W, short-circuit protection is also necessary, otherwise a power supply failure may cause a network failure. All this together is done by SNN.
Simple reference
It is better for a beginner not to immediately go into high powers, but to make a simple highly stable CHN for 12v for the sample according to the diagram in Fig. 2. It can then be used as a source of reference voltage (its exact value is set by R5), for checking instruments or as a high-quality SNV reference voltage. The maximum load current of this circuit is only 40mA, but the KCH on the antediluvian GT403 and the same ancient K140UD1 is more than 1000, and if VT1 is replaced with a silicon medium power and DA1 for any of the modern op amps, it will exceed 2000 or even 2500. The load current will also increase to 150 -200 mA, which is already good for business.
0-30
The next step is a voltage regulated power supply. The previous one is made according to the so-called. compensation circuit of comparison, but it is difficult to remake this for a high current. We will make a new SNN based on an emitter follower (EP), in which the RE and UU are combined in only 1 transistor. KSN will be released somewhere 80-150, but this will be enough for an amateur. On the other hand, SNN on the electric drive allows, without any special tricks, to obtain an output current of up to 10A or more, how much Tr will give and withstand the RE.
A diagram of a simple power supply unit for 0-30V is shown in pos. Fig. 1 3. IIT for it is a ready-made transformer of the TPP or TS type for 40-60 W with a secondary winding for 2x24V. Rectifier type 2PS on diodes 3-5A and more (KD202, KD213, D242, etc.). VT1 is installed on a radiator with an area of 50 sq. cm; an old PC from a processor will work very well. Under such conditions, this SNN is not afraid of a short circuit, only VT1 and Tr will warm up, so a 0.5A fuse in the primary winding circuit Tr will be enough for protection.
Pos. 2 shows how convenient it is for a fan of SNN on an electric drive: there is a 5A power supply circuit with adjustment from 12 to 36 V. This power supply can also give 10A to the load if there is a 400W 36V Tr. Its first feature is the integral SNN K142EN8 (preferably with the index B) acts in an unusual role of the control unit: to its own 12V output, all 24V is added, partially or completely, all 24V, the voltage from the ION to R1, R2, VD5, VD6. Capacities C2 and C3 prevent excitation on the HF DA1 operating in an unusual mode.
The next point is a short circuit protection device (UZ) on R3, VT2, R4. If the voltage drop across R4 exceeds approximately 0.7V, VT2 will open, close the base circuit VT1 to the common wire, it will close and disconnect the load from voltage. R3 is needed so that the extra current when the ultrasound is triggered does not disable DA1. There is no need to increase its denomination, because when the ultrasound is triggered, VT1 must be securely locked.
And the last is the apparent excess capacitance of the capacitor of the output filter C4. In this case, it is safe, because the maximum collector current VT1 of 25A ensures its charge when turned on. But on the other hand, this SNN can deliver a current of up to 30A to the load within 50-70 ms, so this simple power supply is suitable for powering a low-voltage power tool: it starting current does not exceed this value. It is only necessary to make (at least from plexiglass) a contact block-shoe with a cable, put on the heel of the handle, and let the "Akumych" rest and save the resource before leaving.
About cooling
Let's say, in this circuit, the output is 12V with a maximum of 5A. This is just the average power of the jigsaw, but, unlike a drill or screwdriver, he takes it constantly. C1 keeps about 45V, i.e. on the RE VT1, it remains somewhere around 33V at a current of 5A. The power dissipation is more than 150W, even more than 160, considering that the VD1-VD4 also needs to be cooled. Hence, it is clear that any powerful regulated power supply unit must be equipped with a very efficient cooling system.
A ribbed / needle radiator on natural convection does not solve the problem: the calculation shows that a scattering surface of 2000 sq. see and the thickness of the radiator body (the plate from which the ribs or needles extend) from 16 mm. It was and remains a dream in a crystal castle to get so much aluminum in a shaped article as a property for an amateur. A fan-cooled processor cooler is also not suitable, it is designed for less power.
One of the options for the home craftsman is an aluminum plate with a thickness of 6 mm and more and dimensions of 150x250 mm with holes of increasing diameter drilled along the radii from the installation site of the cooled element in a checkerboard pattern. It will also serve as the back wall of the PSU case, as in Fig. 4.
An indispensable condition for the effectiveness of such a cooler is a weak, but continuous flow of air through the perforations from the outside to the inside. For this, a low-power exhaust fan is installed in the housing (preferably at the top). Suitable for a computer with a diameter of 76 mm, for example. add. cooler HDD or video card. It is connected to pins 2 and 8 of DA1, there is always 12V.
Note: in fact, a radical way to overcome this problem is the secondary winding of Tr with taps at 18, 27 and 36V. The primary voltage is switched depending on which tool is in operation.
Still, UPS
The described PSU for the workshop is good and very reliable, but it's hard to carry it with you on the road. This is where a computer power supply comes in handy: the power tool is insensitive to most of its shortcomings. Some refinement is most often reduced to installing an output (closest to the load) electrolytic capacitor of large capacity for the purpose described above. There are a lot of recipes for altering computer power supplies for power tools (mainly screwdrivers, as they are not very powerful, but very useful) in runet there are a lot, one of the methods is shown in the video below, for a 12V tool.
Video: 12V power supply from computer
With 18V tools it is even easier: with the same power, they consume less current. Much more might come in handy here. available device ignition (ballast) from a housekeeping lamp for 40 W or more; it can be completely placed in the case from the unusable battery, and only the cable with the mains plug will remain outside. How to make a power supply for an 18V screwdriver out of ballast from a burned-out housekeeper, see the following video.
Video: BP 18V for a screwdriver
High class
But back to SNN on EP, their capabilities are far from being exhausted. In Fig. 5 is a bipolar powerful power supply unit with 0-30 V adjustment, suitable for Hi-Fi sound equipment and other fastidious consumers. Setting the output voltage is done with one knob (R8), and the symmetry of the channels is maintained automatically at any value and any load current. A formalist pedant at the sight of this scheme may turn gray before our eyes, but for the author, such a power supply unit has been working properly for about 30 years.
The main stumbling block in its creation was δr = δu / δi, where δu and δi are small instantaneous voltage and current increments, respectively. For the development and adjustment of high-quality equipment, it is necessary that δr does not exceed 0.05-0.07 Ohm. Simply, δr determines the power supply's ability to instantly respond to inrush current consumption.
For SNN on ED, δr is equal to that of the ION, i.e. zener diode divided by the current transfer coefficient β RE. But in powerful transistors β on a large collector current drops sharply, and δr of a zener diode ranges from units to tens of ohms. Here, in order to compensate for the voltage drop across the OM and reduce the temperature drift of the output voltage, we had to dial their whole chain in half with diodes: VD8-VD10. Therefore, the reference voltage from the ION is removed through an additional electric drive at VT1, its β is multiplied by β RE.
The next feature of this design is short circuit protection. The simplest one, described above, does not fit into the bipolar circuit, so the protection task is solved according to the principle of "no reception against scrap": there is no protective module as such, but there is redundancy in the parameters of powerful elements - KT825 and KT827 at 25A and KD2997A at 30A. T2 is not capable of giving such a current, but while it warms up, FU1 and / or FU2 will have time to burn.
Note: it is not necessary to indicate blown fuses on miniature incandescent bulbs. It was just that then LEDs were still quite scarce, and there were several handfuls of SMok in the store.
It remains to save the RE from the extracurrents of the discharge of the filter of pulsations C3, C4 at short circuit. For this, they are connected through low-resistance limiting resistors. In this case, pulsations with a period equal to the time constant R (3,4) C (3,4) may appear in the circuit. They are prevented by C5, C6 of smaller capacity. Their extracurrents are no longer dangerous for electronic devices: the charge will drain faster than the crystals of powerful KT825 / 827 will heat up.
The output symmetry is provided by op-amp DA1. The RE of the negative channel VT2 opens with a current through R6. As soon as the minus of the output in modulus exceeds the plus, it will slightly open VT3, and it will close VT2 and the absolute values of the output voltages will be equal. Operational control over the symmetry of the output is carried out using a dial gauge with a zero in the middle of the P1 scale (in the inset - its appearance), and adjustment if necessary - R11.
The last highlight is the output filter C9-C12, L1, L2. Such a construction is necessary to absorb possible HF interference from the load, so as not to rack your brains: the prototype is buggy or the power supply unit is "bogged down". With some electrolytic capacitors shunted with ceramics, there is no complete certainty here, the large self-inductance of "electrolytes" interferes. And the chokes L1, L2 share the "return" of the load across the spectrum, and - to each his own.
This power supply unit, unlike the previous ones, requires some adjustment:
- Connect a load of 1-2 A at 30V;
- R8 is set to the maximum, to the extreme upper position according to the scheme;
- Using a reference voltmeter (any digital multimeter) and R11 set the channel voltages equal in absolute value. Maybe, if the op-amp is without the possibility of balancing, you will have to choose R10 or R12;
- With the trimmer R14, set P1 exactly to zero.
About BP repair
PSUs fail more often than other electronic devices: they take the first blow of the network surges, they also get a lot from the load. Even if you do not intend to make your own power supply, there is a UPS, except for a computer, in a microwave oven, washing machine and other household appliances. The ability to diagnose a power supply unit and knowledge of the basics of electrical safety will make it possible, if not to eliminate the malfunction yourself, then knowingly bargain about the price with the repairmen. Therefore, let's see how the diagnostics and repair of the power supply unit is carried out, especially with IIN, tk. over 80% of refusals are accounted for by them.
Saturation and draft
First of all - about some of the effects, without understanding which it is impossible to work with the UPS. The first of these is the saturation of ferromagnets. They are not able to accept energies of more than a certain value, depending on the properties of the material. On iron, amateurs rarely encounter saturation; it can be magnetized up to several T (Tesla, a unit for measuring magnetic induction). When calculating iron transformers, the induction is taken 0.7-1.7 T. Ferrites withstand only 0.15-0.35 T, their hysteresis loop is "rectangular", and operate at higher frequencies, so that the probability of "jumping into saturation" is several orders of magnitude higher.
If the magnetic circuit is saturated, the induction in it no longer grows and the EMF of the secondary windings disappears, even if the primary has already melted (remember school physics?). Now turn off the primary current. A magnetic field in soft magnetic materials (hard magnetic materials are permanent magnets) cannot exist stationary, as electric charge or water in the tank. It will begin to dissipate, the induction will drop, and an EMF will be induced in all windings of the opposite polarity relative to the original polarity. This effect is widely used in IIN.
Unlike saturation, through-current in semiconductor devices (simply a draft) is certainly harmful. It arises due to the formation / resorption of space charges in the p and n regions; for bipolar transistors - mainly in the base. Field-effect transistors and Schottky diodes are practically draft-free.
For example, when the voltage is applied / removed to the diode, it conducts current in both directions until the charges are collected / dissipated. That is why the voltage loss across the diodes in rectifiers is more than 0.7V: at the moment of switching, part of the charge of the filter capacitor has time to drain through the winding. In a parallel doubling rectifier, the draft flows through both diodes at once.
Draft of transistors causes a voltage surge on the collector, which can damage the device or, if a load is connected, damage it with a through extra current. But even without that, a transistor draft increases dynamic energy losses, like a diode draft, and reduces the efficiency of the device. Powerful field-effect transistors are almost not subject to it, because do not accumulate charge in the base due to its absence, and therefore switch very quickly and smoothly. "Almost", because their source-gate circuits are protected from reverse voltage by Schottky diodes, which are a little, but show through.
TIN types
UPSs trace their ancestry to the blocking generator, pos. 1 in Fig. 6. When Uin VT1 is turned on, it is slightly opened by current through Rb, current flows through the winding Wk. It cannot instantly grow to the limit (we recall school physics again), an EMF is induced in the base Wb and the load winding Wн. With Wb, it forces the unlocking of VT1 through Sat. The current does not flow through Wn yet, does not start up VD1.
When the magnetic circuit is saturated, the currents in Wb and Wn stop. Then, due to the dissipation (resorption) of energy, the induction drops, an EMF of the opposite polarity is induced in the windings, and the reverse voltage Wb instantly locks (blocks) VT1, saving it from overheating and thermal breakdown. Therefore, such a scheme is called a blocking generator, or simply blocking. Rk and Ck cut off HF interference, which blocking gives more than enough. Now, some useful power can be removed from Wn, but only through the 1P rectifier. This phase continues until Sat is fully recharged or until the stored magnetic energy runs out.
This power, however, is small, up to 10W. If you try to take more, VT1 will burn out from the strongest draft before being blocked. Since Tr is saturated, the blocking efficiency is useless: more than half of the energy stored in the magnetic circuit flies away to warm other worlds. True, due to the same saturation, blocking to some extent stabilizes the duration and amplitude of its pulses, and its scheme is very simple. Therefore, blocking-based tax identification numbers are often used in cheap telephone chargers.
Note: The value of Sat in many ways, but not completely, as they say in amateur reference books, determines the pulse repetition period. The value of its capacity should be linked to the properties and dimensions of the magnetic circuit and the speed of the transistor.
Blocking at one time gave rise to a line scan of televisions with cathode ray tubes (CRT), and she - TIN with a damper diode, pos. 2. Here, the control unit, according to the signals from Wb and the DSP feedback circuit, forcibly opens / locks VT1 before Tr is saturated. When VT1 is locked, the reverse current Wc is closed through the same damper diode VD1. This is the working phase: already larger than in blocking, part of the energy is removed into the load. Large because with full saturation, all excess energy flies away, but here this is too little. In this way, it is possible to remove power up to several tens of watts. However, since the CU cannot operate until Tr has approached saturation, the transistor still shows through strongly, the dynamic losses are high and the efficiency of the circuit leaves much to be desired.
IIN with a damper are still alive in televisions and displays with CRTs, since in them the IIN and the horizontal scan output are combined: a powerful transistor and Tr are common. This greatly reduces production costs. But, frankly, an IIN with a damper is fundamentally stunted: the transistor and the transformer are forced to work all the time on the verge of an accident. Engineers who have managed to bring this scheme to acceptable reliability deserve the deepest respect, but it is strongly discouraged to stick a soldering iron in there, except for professionals who have undergone professional training and have relevant experience.
Push-pull INN with a separate feedback transformer is most widely used, because has the best quality indicators and reliability. However, in terms of high-frequency interference, and he is terribly sinning in comparison with the "analog" power supply unit (with transformers on iron and SNN). Currently, this scheme exists in many modifications; powerful bipolar transistors in it are almost completely replaced by field-effect, controlled by specials. IC, but the principle of operation remains unchanged. It is illustrated by the original diagram, pos. 3.
The limiting device (UO) limits the charging current of the capacitors of the input filter Sfvh1 (2). Their large size is an indispensable condition for the operation of the device, because in one working cycle, a small fraction of the stored energy is taken from them. Roughly speaking, they play the role of a water tank or air receiver... When charging "shortly", the extra current of the charge can exceed 100A for a period of up to 100 ms. Rc1 and Rc2 with a resistance of the order of MΩ are needed to balance the filter voltage, since the slightest imbalance in his shoulders is unacceptable.
When Sfvh1 (2) is charged, the ultrasonic triggering device generates a trigger pulse that opens one of the arms (which is all the same) of the VT1 VT2 inverter. A current flows through the winding Wk of a large power transformer Tr2, and the magnetic energy from its core through the winding Wn almost completely goes to rectification and to the load.
A small part of the energy Tr2, determined by the value of Rlim, is removed from the Woc1 winding and fed to the Woc2 winding of a small basic feedback transformer Tr1. It quickly saturates, the open arm closes and, due to dissipation in Tr2, the previously closed arm opens, as described for blocking, and the cycle repeats.
In essence, a push-pull IIN is 2 blockings, "shoving" each other. Since the powerful Tr2 is not saturated, the draft VT1 VT2 is small, completely "sinks" in the magnetic circuit Tr2 and ultimately goes into the load. Therefore, a push-pull IIN can be built for power up to several kW.
Worse if it ends up in XX mode. Then, in a half-cycle, Tr2 will have time to get enough and the strongest draft will burn both VT1 and VT2 at once. However, power ferrites for induction up to 0.6 T are now on sale, but they are expensive and degrade from accidental magnetization reversal. Ferrites with a capacity of more than 1 T are being developed, but in order for IIN to achieve "iron" reliability, at least 2.5 T is needed.
Diagnostic technique
When searching for faults in the "analog" power supply unit, if it is "stupidly silent", first check the fuses, then the protection, RE and ION, if it has transistors. They call normally - we go further element by element, as described below.
In the IIN, if it “starts up” and immediately “stalls”, the UO is checked first. The current in it is limited by a powerful low-resistance resistor, then shunted by an opto-thyristor. If the "rezik" is apparently burnt, change it and the optocoupler. Other elements of the UO fail extremely rarely.
If the IIN is “silent, like a fish on ice,” the diagnosis is also started with the UO (maybe the “rezik” is completely burnt out). Then - US. In cheap models, they use transistors in the avalanche breakdown mode, which is far from very reliable.
The next stage, in any PSU, is electrolytes. The destruction of the case and the leakage of electrolyte are far from being as common as they write in the Russian Internet, but the loss of capacity happens much more often than the failure of active elements. Electrolytic capacitors are checked with a multimeter with the ability to measure capacitance. Below the face value by 20% or more - we put the "dead" in the sludge and put a new, good one.
Then - active elements. You probably know how to ring diodes and transistors. But there are 2 tricks here. First, if a Schottky diode or a zener diode is called by a tester with a 12V battery, then the device may show a breakdown, although the diode is completely serviceable. It is better to call these components with a dial gauge with a 1.5-3 V battery.
The second is powerful field workers. Above (noticed?) It is said that their E-Z are protected by diodes. Therefore, powerful field-effect transistors seem to ring like serviceable bipolar transistors, even unusable, if the channel is "burned out" (degraded) not completely.
Here the only way available at home is to replace it with a known serviceable one, and both at once. If a burned one remains in the circuit, it will immediately pull a new serviceable one. Electronics engineers joke that powerful field workers cannot live without each other. Another prof. joke - "replacement of a gay couple." This means that the transistors of the IIN arms must be strictly of the same type.
Finally, there are film and ceramic capacitors. They are characterized by internal breaks (they are found with the same tester with checking the "air conditioners") and leakage or breakdown under voltage. To "catch" them, you need to assemble a simple diagram according to Fig. 7. A step-by-step check of electrical capacitors for breakdown and leakage is carried out as follows:
- We put on the tester, without connecting it anywhere, the smallest DC voltage measurement limit (most often 0.2V or 200mV), detect and record the device's own error;
- We turn on the measurement limit of 20V;
- We connect a suspicious capacitor to points 3-4, the tester to 5-6, and to 1-2 we supply a constant voltage of 24-48 V;
- We switch the voltage limits of the multimeter down to the lowest;
- If on any tester it showed at least something other than 0000.00 (at the smallest - something other than its own error), the tested capacitor is not suitable.
This is where the methodological part of the diagnosis ends and the creative part begins, where all the instructions are your own knowledge, experience and considerations.
A pair of impulses
UPS is a special article due to their complexity and circuit variety. Here we will start by looking at a couple of pulse width modulated (PWM) samples to get the best quality UPS. There are many PWM circuits in Runet, but PWM is not as terrible as it is painted ...
For lighting design
You can simply light the LED strip from any power supply described above, except for the one in Fig. 1 by setting the required voltage. CHN with pos. Fig. 1 3, it is easy to make 3 of these, for channels R, G and B. But the durability and stability of the LED glow depend not on the voltage applied to them, but on the current flowing through them. Therefore, a good power supply for an LED strip should include a load current regulator; technically - a source of stable current (IST).
One of the schemes for stabilizing the current of the light strip, available for repetition by amateurs, is shown in Fig. 8. It was assembled on an integral 555 timer (domestic analogue - K1006VI1). Provides a stable tape current from a power supply unit with a voltage of 9-15 V. The value of a stable current is determined by the formula I = 1 / (2R6); in this case - 0.7A. A powerful transistor VT3 is necessarily field-effect, from a draft due to the charge of the base of the bipolar PWM, it simply will not form. The choke L1 is wound on a ferrite ring 2000NM K20x4x6 with a bundle 5xPE 0.2 mm. Number of turns - 50. Diodes VD1, VD2 - any silicon HF (KD104, KD106); VT1 and VT2 - KT3107 or analogs. With KT361, etc. the input voltage and dimming ranges will decrease.
The circuit works like this: first, the timing capacitance C1 is charged through the R1VD1 circuit and discharged through VD2R3VT2, open, i.e. in saturation mode through R1R5. The timer generates a sequence of pulses with maximum frequency; more precisely - with a minimum duty cycle. The inertia-free key VT3 generates powerful pulses, and its VD3C4C3L1 strapping smooths them down to direct current.
Note: the duty cycle of a series of pulses is the ratio of their repetition period to the pulse duration. If, for example, the pulse duration is 10 μs, and the interval between them is 100 μs, then the duty cycle will be 11.
The current in the load increases, and the voltage drop across R6 opens VT1, i.e. transfers it from cut-off (locking) mode to active (amplifying) mode. This creates a base current leakage circuit VT2 R2VT1 + Usup and VT2 also goes into active mode. The discharge current C1 decreases, the discharge time increases, the duty cycle of the series increases, and the average value of the current falls to the norm set by R6. This is the essence of PWM. At the minimum current, i.e. at maximum duty cycle, C1 is discharged along the VD2-R4-internal timer circuit.
In the original design, the ability to quickly adjust the current and, accordingly, the brightness of the glow, is not provided; there are no 0.68 ohm potentiometers. The easiest way to adjust the brightness is by turning on the 3.3-10 kOhm potentiometer R * after adjustment in the gap between R3 and the emitter VT2, highlighted in brown. Moving its slider down the diagram, we will increase the C4 discharge time, duty cycle and decrease the current. Another way is to bypass the base transition VT2 by turning on the potentiometer by about 1 MΩ at points a and b (highlighted in red), it is less preferable, because the adjustment will be deeper, but coarse and sharp.
Unfortunately, to establish this useful not only for IST light strips, an oscilloscope is needed:
- The minimum + Usup is supplied to the circuit.
- By selecting R1 (impulse) and R3 (pause), a duty cycle of 2 is achieved, i.e. the pulse duration must be equal to the pause duration. You cannot give a duty cycle less than 2!
- Serve maximum + Usup.
- By selecting R4, the rated value of the stable current is achieved.
For charging
In Fig. 9 is a diagram of the simplest ISN with PWM, suitable for charging a phone, smartphone, tablet (a laptop, unfortunately, will not pull) from a homemade solar battery, wind generator, motorcycle or car battery, magneto flashlight-"bug" and other low-power unstable random power supplies. See the diagram for the input voltage range, there is no error. This ISN is indeed capable of outputting a voltage greater than the input voltage. As in the previous one, here there is the effect of reversing the polarity of the output relative to the input, this is generally a proprietary chip of PWM circuits. Let's hope that after reading carefully the previous one, you will figure out the work of this little one yourself.
Along the way about charging and charging
The charging of batteries is a very complex and delicate physicochemical process, violation of which reduces their resource by several times and tens of times, i.e. number of charge-discharge cycles. The charger must, based on very small changes in the battery voltage, calculate how much energy is received and adjust the charge current accordingly according to a certain law. That's why Charger By no means and by no means a power supply unit, and you can only charge batteries from conventional power supply units in devices with a built-in charge controller: phones, smartphones, tablets, individual models of digital cameras. And charging, which is a charger, is a subject of a separate conversation.
Voprosy-remont.ru said:
There will be sparks from the rectifier, but perhaps it's okay. The point is the so-called. differential output impedance of the power supply. In alkaline batteries, it is of the order of mΩ (milliohm), in acidic ones it is even less. For a trance with a bridge without smoothing - tenths and hundredths of ohms, i.e. approx. 100 - 10 times more. And the starting current of a collector DC motor can be more than the working current by 6-7 or even 20 times. Yours is most likely closer to the latter - fast-accelerating motors are more compact and more economical, and the huge overload capacity of the batteries allows you to give the engine current how much it will eat overclocking. A trans with a rectifier will not give so much instantaneous current, and the motor accelerates more slowly than it is designed for, and with a large armature slip. From this, from a large slip, a spark arises, and then it is kept in operation due to self-induction in the windings.
What can you advise here? First: take a closer look - how does it spark? You need to look at work, under load, i.e. during sawing.
If the sparkles dance in certain places under the brushes, it's okay. I have a powerful Konakovskaya drill from birth so sparkles, and even henna. For 24 years I changed the brushes once, washed it with alcohol and polished the collector - that's all. If you have connected an 18 V instrument to the 24 V output, slight arcing is normal. Unwind the winding or extinguish the excess voltage with something like a welding rheostat (a resistor of approx. 0.2 Ohm for a dissipated power of 200 W) so that the motor has a nominal voltage in operation and, most likely, the spark will go away. If you connected to 12 V, hoping that after rectification it would be 18, then in vain - the rectified voltage under load sits down a lot. And the collector electric motor, by the way, does not care whether it is powered by direct current or by alternating current.
Specifically: take 3-5m steel wire with a diameter of 2.5-3mm. Roll into a spiral with a diameter of 100-200 mm so that the turns do not touch each other. Place on a non-combustible dielectric pad. Strip the ends of the wire to a shine and roll up the "ears". It is best to immediately grease with graphite grease so as not to oxidize. This rheostat is included in the break of one of the wires leading to the instrument. It goes without saying that the contacts must be screw, tightly tightened, with washers. Connect the entire circuit to the 24V output without rectification. The spark is gone, but the power on the shaft has also dropped - the rheostat needs to be reduced, switch one of the contacts 1-2 turns closer to the other. It still sparks, but less - the rheostat is too small, you need to add turns. It is better to immediately make the rheostat known to be large, so as not to screw on the additional sections. It is worse if the fire is along the entire contact line of the brushes with the collector or spark tails are trailing behind them. Then the rectifier needs a smoothing filter somewhere, according to your data, from 100,000 uF. An expensive pleasure. The "filter" in this case will be the energy storage for the motor acceleration. But it may not help - if the overall capacity of the transformer is not enough. Efficiency of DC collector motors approx. 0.55-0.65, i.e. trance is needed from 800-900 watts. That is, if the filter is installed, but still sparks with fire under the entire brush (under both, of course), then the transformer does not hold out. Yes, if you put a filter, then the diodes of the bridge must also have a triple operating current, otherwise they can fly out from the charge current surge when connected to the network. And then the tool can be launched 5-10 seconds later after being connected to the network, so that the "banks" have time to "pump up".
And worst of all, if the tails of sparks from the brushes reach or almost reach the opposite brush. This is called an all-round fire. It very quickly burns out the collector until it is completely unusable. There can be several reasons for the all-round fire. In your case, the most likely one is that the motor was switched on at 12V with rectification. Then, at a current of 30 A, the electric power in the circuit is 360 W. The slide of the armature goes more than 30 degrees per revolution, and this is necessarily a continuous all-round fire. It is also possible that the motor armature is wound with a simple (not double) wave. Such electric motors are better able to overcome instantaneous overloads, but they have a starting current - mom, do not worry. More precisely, I can’t say in absentia, and I don’t need anything - there’s hardly anything fixable here with my own hands. Then, probably, it will be cheaper and easier to find and purchase new batteries. But first try to turn on the engine for a bit. increased voltage through a rheostat (see above). Almost always in this way it is possible to shoot down a solid all-round fire at the cost of a small (up to 10-15%) decrease in shaft power.
Evgeniy said:
More cuts needed. So that all the text is made up of abbreviations. Fuck that no one understands, but you can not write the same word, which is repeated THREE times in the text.
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Today we will assemble a laboratory power supply unit with our own hands. We will understand the block device, select the correct components, learn how to solder correctly, assemble elements on printed circuit boards.
This is a high quality laboratory (and not only) power supply with variable adjustable voltage from 0 to 30 volts. The circuit also includes an electronic output current limiter that effectively regulates the output current of 2 mA from the maximum possible in this circuit (3 A). This characteristic makes this power supply indispensable in the laboratory, as it makes it possible to regulate the power, limit the maximum current that the connected device can consume, without fear of damaging it if something goes wrong.
There are also visual sign that this limiter is active (LED) so you can see that your circuit is out of range.
The schematic diagram of the laboratory power supply is presented below:
Laboratory Power Supply Specifications
Input voltage: ……………. 24 V AC;
Input current: ……………. 3 A (max.);
Output voltage: …………. 0-30 V - adjustable;
Output current: …………. 2 mA -3 A- adjustable;
Output voltage ripple:…. 0.01% maximum.
Peculiarities
- Small size, easy to make, simple design.
- The output voltage is easily adjustable.
- Limiting the output current with visual indication.
- Protection against overload and wrong connection.
Principle of operation
To begin with, a transformer with a secondary winding of 24V / 3A is used for the laboratory power supply, which is connected through input terminals 1 and 2 (the quality of the output signal is proportional to the quality of the transformer). The alternating current voltage from the secondary winding of the transformer is rectified by a diode bridge formed by diodes D1-D4. The ripple of the rectified DC voltage at the output of the diode bridge smooths out the filter formed by the resistor R1 and the capacitor C1. The circuit has some features that make this PSU stand out from other PSUs in its class.
Instead of using feedback to control the output voltage, our circuit uses an operational amplifier to provide the required voltage for stable operation. This voltage drops at the output of U1. The circuit is powered by a 5.6V D8 Zener diode, which operates here at zero temperature coefficient of current. The voltage at the output of U1 drops on diode D8 turning it on. When this happens, the circuit also stabilizes the voltage of the diode (5.6) drops across the resistor R5.
The current that flows through the opera. the amplifier changes insignificantly, which means the same current will flow through the resistors R5, R6, and since both resistors have the same voltage value, the total voltage will be added as if they were connected in series. Thus, the voltage obtained at the output of the opera. amplifier will be equal to 11.2 volts. Chain with operas. amplifier U2 has a constant gain of approximately 3, according to the formula A = (R11 + R12) / R11 increases the voltage of 11.2 volts to approximately 33 volts. Trimmer RV1 and resistor R10 are used to set the output voltage parameters so that it does not decrease to 0 volts, regardless of the magnitude of other components in the circuit.
Another very important characteristic circuits are the ability to obtain the maximum output current that can be obtained from p.s.u. To make this possible, the voltage drops across a resistor (R7), which is connected in series with the load. The IC responsible for this circuit function is U3. An inverted signal to the input U3 equal to 0 volts is fed through R21. At the same time, without changing the signal of the same IC, you can set any voltage value by means of P2. Assuming a few volts for a given output, P2 is set so that there is a 1 volt signal at the IC input. If the load is boosted, the output voltage is constant and the presence of R7 in series with the output will have little effect because of its low value and because of its position outside the control loop. As long as the load and the output voltage are constant, the circuit works stably. If the load is increased so that the voltage across R7 is greater than 1 volt, U3 is turned on and stabilizes to its original parameters. U3 works without changing the signal to U2 through D9. Thus, the voltage across R7 is constant and does not increase above a given value (1 volt in our example), reducing the output voltage of the circuit. It is within the power of the device to keep the output constant and accurate, which makes it possible to obtain 2 mA at the output.
Capacitor C8 makes the circuit more stable. Q3 is needed to drive the LED whenever you use the limiter indicator. To make this possible for U2 (changing the output voltage down to 0 volts), it is necessary to provide a negative connection, which is done through the circuit C2 and C3. The same negative relationship is used for U3. Negative voltage is supplied stabilized by R3 and D7.
To avoid uncontrollable situations, there is a kind of protection circuit built around Q1. The IC is internally protected and cannot be damaged.U1 is a reference voltage source, U2 is a voltage regulator, U3 is a current stabilizer.
Power supply design.
First of all, let's go over the basics in building electronic circuits on printed circuit boards - the basics of any laboratory power supply. The board is made of a thin insulating material covered with a thin conductive layer of copper, which is formed in such a way that the circuit elements can be connected with conductors as shown in schematic diagram... It is necessary to properly design the printed circuit board to avoid malfunctioning of the device. To protect the board from oxidation in the future and keep it in excellent condition, it must be covered with a special varnish that protects against oxidation and facilitates soldering.
Soldering the elements into a board is the only way to assemble a laboratory power supply unit with high quality, and the success of your work will depend on how you do it. This is not very difficult if you follow a few rules and then you will not have any problems. The power of the soldering iron you are using should not exceed 25 watts. The tip must be thin and clean throughout the entire work. There is a kind of damp sponge for this, and from time to time you can clean the hot sting to remove all the residues that accumulate on it.
- DO NOT attempt to clean a dirty or worn out tip with a file or sandpaper. If it cannot be cleaned, replace it. There are many different soldering irons on the market and you can also buy a good flux to get a good connection during soldering.
- DO NOT use flux if you are using solder that already contains it. Large amounts of flux are one of the main causes of circuit failure. If, however, you must use additional flux, such as when tinning copper wires, you must clean the work surface after finishing work.
In order to solder the element correctly, you must do the following:
- Clean the terminals of the elements with sandpaper (preferably with a small grain).
- Bend the component leads at the correct distance from the exit from the case for a comfortable position on the board.
- You may come across elements whose pins are thicker than the holes in the board. In this case, it is necessary to widen the holes a little, but do not make them too large - this will complicate the soldering.
- Insert the element so that its leads protrude slightly from the board surface.
- When the solder is melted, it will spread evenly over the entire area around the hole (this can be achieved with the correct soldering iron temperature).
- Soldering of one element should be no more than 5 seconds. Remove excess solder and wait for the solder on the board to cool naturally (without blowing on it). If done correctly, the surface should have a bright metallic hue and the edges should be smooth. If the solder looks dull, cracked, or has a droplet appearance, this is called dry soldering. You have to remove it and do it all over again. But be careful not to overheat the tracks, otherwise they will lag behind the board and break easily.
- When you solder the sensing element, you must hold it with metal tweezers or tongs, which will absorb excess heat so as not to burn the element.
- When you complete your work, trim off excess from the cell leads and you can clean the board with rubbing alcohol to remove all flux residues.
Before starting the assembly of the power supply, you need to find all the elements and divide them into groups. To get started, install the ICs sockets and external connection pins and solder them in place. Then resistors. Remember to place R7 at a certain distance from the PCB as it gets very hot, especially when a lot of current is flowing, and this can damage it. This is also recommended for R1. then place the capacitors without forgetting the polarity of the electrolytic and finally solder the diodes and transistors, but be careful not to overheat them and solder them as shown in the diagram.
Set the power transistor to heatsink. To do this, follow the diagram and remember to use an insulator (mica) between the body of the transistor and the heatsink and a special cleaning fiber to insulate the screws from the heatsink.
Connect insulated wire to each terminal, be careful to do good quality connection, since a large current flows here, especially between the emitter and collector of the transistor.
Also, when assembling the power supply, it would be nice to figure out where which element will be located, in order to calculate the length of the wires that will be between the PCB and the potentiometers, the power transistor and for the input and output connections.
Connect potentiometers, LED and power transistor and connect two pairs of ends for input and output connections. Make sure from the diagram that you are doing everything correctly, try not to confuse anything, since there are 15 external connections in the chain and after making a mistake it will be difficult to find it later. It would also be nice to use wires of different colors.
The printed circuit board of the laboratory power supply, below will be a link to download the seal in .lay format:
The layout of the elements on the power supply board:
Connection diagram of variable resistors (potentiometers) to regulate the output current and voltage, as well as the connection of the contacts of the power transistor of the power supply:
Designation of terminals of transistors and operational amplifier:
Terminal designation in the diagram:
- 1 and 2 to the transformer.
- 3 (+) and 4 (-) DC OUTPUT.
- 5, 10 and 12 at P1.
- 6, 11 and 13 at P2.
- 7 (E), 8 (B), 9 (E) to Q4.
- The LED must be installed on the outside of the board.
When all external connections are made, it is necessary to check the board and clean it to remove the remaining solder. Make sure that there is no connection between adjacent tracks that could lead to a short circuit and if all goes well, connect the transformer. And connect the voltmeter.
DO NOT TOUCH ANY PART OF THE CIRCUIT WHILE IT IS LIVE.
The voltmeter should show a voltage between 0 and 30 volts, depending on which position P1 is in. Turning P2 counterclockwise should turn on the LED, indicating that our limiter is working.
List of elements.
R1 = 2.2 kΩ 1W
R2 = 82 Ohm 1 / 4W
R3 = 220 Ohm 1 / 4W
R4 = 4.7 kΩ 1 / 4W
R5, R6, R13, R20, R21 = 10 kΩ 1 / 4W
R7 = 0.47 Ohm 5W
R8, R11 = 27 kΩ 1 / 4W
R9, R19 = 2.2 kΩ 1 / 4W
R10 = 270 kΩ 1 / 4W
R12, R18 = 56kΩ 1 / 4W
R14 = 1.5 kΩ 1 / 4W
R15, R16 = 1 kΩ 1 / 4W
R17 = 33 Ohm 1 / 4W
R22 = 3.9 kΩ 1 / 4W
RV1 = 100K trimmer
P1, P2 = 10KOhm linear potentiometer
C1 = 3300 uF / 50V electrolytic
C2, C3 = 47uF / 50V electrolytic
C4 = 100nF polyester
C5 = 200nF polyester
C6 = 100pF ceramic
C7 = 10uF / 50V electrolytic
C8 = 330pF ceramic
C9 = 100pF ceramic
D1, D2, D3, D4 = 1N5402,3,4 diode 2A - RAX GI837U
D5, D6 = 1N4148
D7, D8 = 5.6V zenerevsky
D9, D10 = 1N4148
D11 = 1N4001 diode 1A
Q1 = BC548, NPN transistor or BC547
Q2 = 2N2219 NPN transistor - (Replace with KT961A- everything works)
Q3 = BC557 PNP transistor or BC327
Q4 = 2N3055 NPN power transistor ( replace with KT 827A)
U1, U2, U3 = TL081, op. amplifier
D12 = LED diode
As a result, I independently assembled a laboratory power supply unit, but in practice I came across the fact that I consider it necessary to correct it. Well, first of all, it's a power transistor. Q4 = 2N3055 it urgently needs to be deleted and forgotten. I do not know about other devices, but in this regulated power supply it does not fit. The fact is that given type transistors fail instantly with a short circuit and a current of 3 amperes does not pull at all !!! I did not know what was the matter until I changed it to our dear soviet KT 827 A... After installing it on the radiator, I didn't even know grief and never returned to this question.
As for the rest of the circuitry and details, there are no difficulties. Except for the transformer - I had to wind it. Well, this is purely because of greed, half a bucket of them is in the corner - do not buy it =))
Well, in order not to break the good old tradition, I post the result of my work to the general court 🙂 I had to shaman with a column, but on the whole it turned out not bad:
Actually the front panel - put the potentiometers on the left side, on the right side there are an ammeter and a voltmeter + a red LED to indicate the current limit.
On next photo back view. Here I wanted to show how to mount a cooler with a radiator from motherboard... A power transistor is perched on this radiator from the back.
Here it is, the power transistor KT 827 A. Mounted on the back wall. I had to drill holes for the legs, lubricate all contact parts with heat-conducting paste and fasten them to the nuts.
Here they are… .insides! Actually, everything is in a heap!
Slightly larger inside the case
Bezel on the other side
Closer, you can see how the power transistor and transformer are mounted.
Power supply board on top; here I cheated and packed low-power transistors from the bottom of the board. You can't see them here, so don't be surprised if you don't find them.
Here is the transformer. I rewound the TVS-250 output voltage by 25 volts. Rough, sour, not aesthetically pleasing, but everything works like a clock =) I did not use the second part. Left room for creativity.
Somehow like this. A little creativity and patience. The unit has been working wonderfully for 2 years now. To write this article, I had to disassemble and reassemble it. It's just awful! But all for you, dear readers!
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