Heavy duty garage power supply
This AC-powered power supply is designed to supply power to devices and mechanisms of electrical equipment of the car, auxiliary tools and appliances, installed in it electronic equipment during the repair or maintenance work in the garage. A valuable quality of the source is protection against short circuits in the load circuit. It can also power portable sound amplifying or receiving and transmitting equipment, and around the clock.
The power supply outputs a stabilized direct voltage of 13.8 V with a load current of up to 50 A. Increasing the load current to the limit value causes the output voltage to decrease by no more than 0.2 V.
Schematic diagram of the device is shown in the figure. The device consists of a line transformer T1, a rectifier on Schottky diodes VD1-VD3, a battery of filter capacitors CЗ-C14, transistors VT1, VT2, the control unit, the switching element on the field-effect transistors VT3, VT4, storage choke L1 with switching diode VD6, output voltage regulator on the chip DA2 and optocoupler U2, output circuit short circuit protection device on the stabilizing diode VD5 and optocoupler U1, signal LEDs HL1, HL2, indicating an operating mode of power supply. On the diode bridge VD4, connected to the mains transformer winding III, and on the stabilizer DA1 there is a power supply of the control unit.
When the contacts of the switch SA1 are closed, the mains voltage is supplied to the primary winding of the transformer T1. Reduced to 24 V voltage from winding II rectifies a powerful diode bridge VD1-VD3. Using Schottky diodes in it allowed to almost halve the power dissipated by the rectifier. The filter capacitorsЗ-С14 are charged to the amplitude value, i.e. almost to 35 V. The 15 V voltage from transformer winding III is rectified by diode bridge VD4, and stabilizer DA1 limits it at 12 V.
The stabilized voltage is fed to the control transistors VT1, VT2. Since no current flows through the emitting diodes of optocouples U1 and U2, the opto-transistors are closed and the base current of transistor VT1 flows through resistors R5 and R6. Therefore it opens and transistor VT2 remains closed.
Through VT1 to the gates of transistors VT3 and VT4 with respect to their sources comes the opening voltage of 12 V. Resistors R7 and R8 eliminate the high-frequency self-excitation of the stage at switching moments. When the transistors of the switching element open, a current begins to flow through the storage choke L1, the voltage at the filter capacitor C15 increases. At the same time the voltage at the control input of the stabilizer DA2, set by the divider R9R10, increases.
As soon as the voltage at the capacitor C15 becomes equal to 13.8 V, the voltage at the control input of chip DA2 reaches 2.5 V and it opens. Through the emitting diode of the optocoupler U2 and LED HL2 begins to flow current, limited by resistor R11. The green LED HL2 turns on and signals that the power supply output has reached nominal voltage. At this point the opto-transistor of the optocoupler U2 opens and the base resistor R6 through a small collector-emitter resistor connects to the negative output of the power supply of the control unit.
As a result, transistor VT1 closes and VT2 opens and the gate-source capacity of the switching element transistors is rapidly discharged through resistors R7, R8 and open transistor VT2. Transistors VT3, VT4 are thus closed.
The energy stored in the magnetic field of chokes L1 is converted into electric current, closed through the load by the opening of the switching diode VD6. The need for two field-effect transistors working in parallel is explained by the fact that the current through the choke L1 has a triangular shape and at the output current of 50 A the current amplitude through the choke and transistors reaches 100 A. In addition to reducing the current flowing through each of these transistors, connecting them in parallel has halved the power dissipated by the switching element and has eased the thermal conditions under the casing of the device.
At the rated load of the power supply its output voltage is 13,8 V, and at the stabilizing diode VD5 – 12,5 V. The emitting diode of optocoupler U1 and LED HL1 are closed. If the unit output is shorted, the output voltage becomes close to zero and a current limited by resistor R4 flows through the LED HL1 and the emitting diode of optocoupler U1 from voltage regulator R3VD5. LED HL2 green glow goes out and HL1 red glow turns on. The optocoupler U1 opens, which causes the closing of the switching element. After the cause of the short circuit has been eliminated, the output of the unit will automatically return to the operating mode.
The power supply has a considerable power reserve, therefore in case of current overloads the output fuses FU2 and FU3 with the rated current of 30 A switched in parallel have time to close. In this case the indicators HL1 and HL2 will light up simultaneously. Thus the two LEDs indicate the three states of the power supply. The glow of HL2 indicates the rated output voltage, HL1 – overload, HL1 and HL2 simultaneously – blown fuses FU2, FU3. Adjustment of the power supply consists in selecting the resistor R10 to set the required output voltage.
Control transistors VT1 and VT2 in the device can be of the specified types with any letter index. If necessary, each of them can be replaced by a pair of high-frequency Darlington transistors: KT972A will replace KT315 and KT6114, and KT973A – KT361 and KT6115 with any letter indices. The pairs of KT315, KT815 and KT361, KT814 perform somewhat worse. Switching transistors VT3, VT4 are chosen on conditions: drain-source voltage – not less than 50 V, and constant drain current – not less than 50 A. These requirements are satisfied, for example, by field-effect transistors IRFZ48N, IRF1010N, IRL3705N, IRL2505.
The mains transformer T1 is an industrial one, OS-1,0-220/24, for a secondary voltage of 24 V, 1 kW capacity. There are also similar transformers, marked OS-1,0-220/50-24. Winding III is wound with mounting wire MGSHV-0,5 in the free space of transformer window without disassembling its magnetic core. Initially ten turns are placed, the resulting voltage is measured, and then the required number of turns to achieve 15 V is calculated and the necessary turns are placed. Instead of winding III, a separate network transformer can be used with an output AC voltage of 12. 17 V with a capacity of 3. 5 W.
If you can not buy a ready-made powerful mains transformer, you can make it from an old nine-amp LATR. As the primary, you should use the existing winding as a whole (250 V), isolating the leads to 127 and 220 V. The enameled track on the turns of the winding should be cleaned of dust and covered with two layers of nitrolac NC221 or NC222 (in a pinch, glue BF-2).
After the varnish dries, the winding is covered with lacotta tape or cotton cloth insulation tape (two layers). Then, lay the secondary winding, consisting of 27 turns of insulated copper wire of rectangular cross section of 5×2 mm (or a bundle of wires of a smaller thickness of the total cross section of not less than 10 mm2). Winding III – 19 turns of any mounting wire.
Diodes VD1-VD3, VD6 – assemblies of two Schottky diodes with common cathode for reverse voltage not less than 50 V and rectified current 60 A. Instead of 60CPQ150 you can use 60CMQ050, C60P05Q, FST6050 . Two of these – VD1, VD2 with common cathode – are conveniently replaced by one with a common anode FST16050A, S60D50A or SR5050A.
Rectifier bridge VD4 can be assembled from four diodes with a reverse voltage of at least 30V and rectified current of 0.1A. Instead of 7812 (DA1; output voltage 12 V) you can use stabilizers KA7812, 78M12 or domestic KR142EN8B, KR142EN8D, KR1180EN12A, KR1180EN12B. Stabilizer TL431LPA (DA2) replaced with domestic KR142EN19A, but in this case, as measurements have shown, the ripple output voltage source increased from 63 to 80 mV.
Transistor optocouplers, except as shown in the schematic, can be 4N25-4N28, TLP331 or domestic AOT128A. LEDs HL1 and HL2 – any red and green colors. Resistors – MJT and C2-23; oxide capacitors – imported, and C14, C16 – K73-17. The fuses FU2, FU3 for 30A are automotive ones.
The choke L1 is homemade, made as follows. Several layers of polyethylene film are wound on a smooth mandrel with an outer diameter of 35 mm. On top of it lay coil to coil the first layer of winding – 9 turns of copper insulated wire of rectangular cross section of 5×2 mm and wind a layer of glass fiber impregnated with epoxy glue. It should be burned in the flame of a blowtorch or a gas burner before impregnation.
The glass fabric is fastened with threads, the second layer of the winding is wound – 8 turns, then the second layer of glass fabric soaked with epoxy glue, which is again fastened with threads, and the last layer of the winding is wound – 7 turns. The total number of winding turns is 24. For protection against sprawling turns, the winding is tied with thread, covered with a thin layer of epoxy glue from the outside and left in a warm place until it is fully polymerized.
All work must be done wearing rubber gloves.
After 24 hours, the finished coil is removed from the mandrel. The polyethylene film is removed. Deburrs and glue deposits are removed with a file. For the manufacture of the magnetic core it is necessary to grind the fragments of various ferrite products, up to the magnetic cores of deflection systems of kinescopes in a mortar and sieve them. Homemade mortar is made from a scrap of steel tube with a diameter of about 160 mm, welded to a scrap of steel sheet 3 mm thick. The mortar is a steel rod with a diameter of 30 mm.
The ferrite fragments should be preheated several times in an oven to a temperature of 160 °C and quickly cooled in cold water, in which case micro-cracks are formed in the ferrite, greatly facilitating the grinding process. The ferrite powder sifted through a fine sieve (can be made from a kapron stocking) is kneaded on epoxy glue to the thickness of a sour cream. The coil is placed vertically on a sheet of organic glass (epoxy does not stick to it), the gaps between the sheet and the coil are sealed with auto sealant, and the cavity inside the coil is filled with the mass obtained. After filling in the mass, the heads of one or two M4 brass screws should be put in, which will later serve to mount the choke to the power chassis.
The choke, made according to the described technology, turned out to be very “quiet” and practically does not heat up during operation. The design of a power supply can be in many ways arbitrary. Diode assemblies VD1-VD3 are mounted on a common heat sink with a usable area of 600 cm2, while transistors VT3, VT4 and diode VD6 are on a second one, 800 cm2. Operation has shown that the heat sinks practically do not heat up and therefore their size can be significantly reduced.
The device was assembled by hinged mounting on pieces of technological assembly boards. On one of them with dimensions of 50×30 mm there are parts of the power supply of the control unit, optocouplers and transistors of the unit. The board on racks is fixed on the heat sink of the switching element transistors.
Stabilizer of output voltage and elements of overload protection node are located on the second board with dimensions 30×20 mm. It is mounted on the chassis near the output fuses.
High-current part of the source should be mounted with wire pieces of cross-section 8. 10 mm2. Since it is difficult to find an installation wire of this cross-section, you can make it yourself from the shielding braid of the coaxial cable RK-75. A bunch of braid pieces of necessary length taken from the cable is stretched in heat-shrinkable polymeric tube with 8 mm diameter. After heating the tube with hot air from a hair dryer, the wire is used for installation.
In experiments to increase output power of the described power supply a high-power bridge rectifier was assembled on diodes B320-2, a diode FC171-320 was used as a commutator (VD6), the number of transistors of commutating element was increased to five, and resistance of resistors in a circuit of their gates was increased to 22 Ohm. The mains transformer was a welding transformer, and one more diode FF171-320 was connected in parallel to the capacitor C16 with the cathode to the plus side and the anode to the minus side. Fuses FU2, FU3 were replaced with a self-made one for the current about 150A.
In such a design the source confidently turned the crankshaft of the car “Volga” with a starter.
Garage equipment. Findings, mistakes, ideas. Electricity
What can I say about electricity? It’s good to live with electricity. It’s even better to live with it if they never turn it off. Alas, unrealistic dreams. Some goat will turn on a goat with unprecedented power at night, or something will be flooded with rains somewhere. Consequently, it is necessary to have in a garage backup power at least in order to find the door in complete darkness and for security system not to shut off. I will tell you about backup power, because I already know better than any academic about switches and sockets. Another good reason to have 12 volts in the garage is to power twelve-volt tools like a tire compressor or miniature drills. On the forum forumhouse.ru in the section “Autonomous power sources” there are a lot of cool guys with gasoline generators and solar panels. I read it and thought, “Do I need it? All I need is five amps for the compressor and for the alarm to stay on. So I need a battery with periodic recharging. I ruled out step-down transformer on thick iron. This is the twenty-first century, what transformer? There are plenty of discarded computer power supplies lying around. I lied a little bit about the mountain, but almost all of us have upgraded our computer and it is not a problem to find a power supply unit. And the voltage is stable and short-circuit proof, and most importantly, ATX standard units have PS_ON control input. If you put ground to PS_ON, the unit turns off, if you put + 5 volt, it turns on. Everything is good, but there is one little disadvantage – all computer power supplies deliver exactly 12 volts. If we are going to use it to power the drill, then it’s okay, but if you need to charge the lead battery, then 12 volts is not enough. To recharge it, you need to raise the voltage to at least 14.5 volts. This is where we can get in trouble. If you made your block with the old TL494 chip and its clones, it’s not hard to raise the voltage. If you are unlucky and your block is made with new generation chips, then you’ll have to do a lot of juggling. There are too many different protections there, including overvoltage protection. However, with some patience this problem can be solved. Here are a few links on remaking power supplies and decide for yourself whether you should get into it. http://www.qrz.ru/schemes/contribute/power/ua4nx/ and more http://forum.cxem.net/index.php?showtopic=36540 Well, let’s say you converted the power supply to 14.8 volts and successfully recharge the lead starter battery from a car. Well, you charged it. And then what? And leave it on a permanent charge? Not desirable. Practice shows that lead batteries, standing on a permanent charge in uninterruptible power supplies, do not live there long. I won’t go into details, I’d rather refer you for cations and anions to Khrustalev’s book “Batteries”. It is on the Internet, those who want to find it will. There is also a “Basic information on acid batteries” at forum.fonarevka.ru If you don’t have time to get into the theory, you can follow this link to the Radiokot site. http://radiokot.ru/circuit/power/charger/28/ I took the scheme called “Swing” from there. Here it is a little modified. The idea is simple – a two level comparator watches the battery voltage. If it is less than 11.9 volts, it flips the RS trigger on DA3, DA4 and on the PS_ON bus it starts charging. If the voltage is more than 13.8 volts, it turns it off. Although two-level comparator with hysteresis can be implemented easier, but then I had these parts just lying in the table, and I did not invent anything.
