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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Welding machine assembled from parts of old TVs. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / welding equipment Many in the household would need an apparatus for electric welding of parts made of ferrous metals. Since mass-produced welding machines are quite expensive, many radio amateurs take on their own production. This article is about one of these devices. From the very beginning of my work, I set myself the task of creating the most simple and cheap welding machine using widely used parts and assemblies in it. Of the two main options for the design of the apparatus - with a welding transformer or based on a converter - the second one was chosen. Indeed, a welding transformer is a large and heavy magnetic circuit and a lot of copper wire for windings, which is inaccessible to many. Electronic components for the converter, with their correct choice, are not scarce and relatively cheap.
As a result of rather lengthy experiments with various types of converters based on transistors and trinistors, the circuit shown in Fig. 1. Simple transistor converters turned out to be extremely capricious and unreliable, while trinistor converters withstand the output shorting without damage until the fuse blows. In addition, trinistors heat up much less than transistors. As you can easily see, the circuit design is not original - it is an ordinary single-cycle converter, its advantage is in the simplicity of design and the absence of scarce components, the device uses a lot of radio components from old TVs. And, finally, it practically does not require adjustment. The welding machine has the following main characteristics: Limits of regulation of welding current, A ........ 40 ... 130 Maximum voltage on the electrode at idle, V .......................................... 90 Maximum current consumed from the network, A.......................20 Voltage in the supply network of alternating current with a frequency of 50 Hz, V .............. 220 The maximum diameter of the welding electrode, mm .......... 3 Load duration (PN), %, at an air temperature of 25 °C and output current 100 A ...................... 60
Apparatus dimensions, mm......................350x 180x 105 Mass of the device without supply cables and electrode holder, kg.......................5,5 Kind of welding current - constant, regulation - smooth. When butt-welding steel sheets 3 mm thick with an electrode 3 mm in diameter, the steady current consumed by the machine from the mains does not exceed 10 A. The welding voltage is turned on by a button located on the electrode holder, which allows, on the one hand, to use an increased arc ignition voltage and increase electrical safety, on the other hand, since when the electrode holder is released, the voltage on the electrode is automatically turned off. The increased voltage facilitates the ignition of the arc and ensures the stability of its burning. The use of direct welding current with the reverse polarity of the welding voltage allows you to connect thin-sheet parts. Mains voltage rectifies the diode bridge VD1-VD4. The rectified current, flowing through the lamp HL1, begins to charge the capacitor C5. The lamp serves as a charging current limiter and an indicator of this process. Welding should be started only after the HL1 lamp goes out. At the same time, battery capacitors C1-C6 are charged through the inductor L17. The glow of the HL2 LED indicates that the device is connected to the network. Trinistor VS1 is still closed. When you press the SB1 button, a pulse generator is started at a frequency of 25 kHz, assembled on a unijunction transistor VT1. The generator pulses open the VS2 trinistor, which, in turn, opens the VS3-VS7 trinistors connected in parallel. Capacitors C6-C17 are discharged through the inductor L2 and the primary winding of the transformer T1. Circuit choke L2 - primary winding of transformer T1 - capacitors C6-C17 is an oscillatory circuit. When the direction of the current in the circuit changes to the opposite, the current begins to flow through the diodes VD8, VD9, and the trinistors VS3-VS7 close until the next pulse of the generator on the transistor VT1. Then the process is repeated. The pulses that appear on the winding III of the transformer T1 open the trinistor VS1. which directly connects the mains rectifier on diodes VD1 -VD4 with a trinistor converter. The HL3 LED serves to indicate the process of generating a pulsed voltage. Diodes VD11-VD34 rectify the welding voltage, and capacitors C19-C24 smooth it out, thereby facilitating the ignition of the welding arc. Switch SA1 is a packet or other switch for a current of at least 16 A. Section SA1.3 closes capacitor C5 to resistor R6 when turned off and quickly discharges this capacitor, which allows, without fear of electric shock, to inspect and repair the device. The VN-2 fan (with an M1 electric motor according to the scheme) provides forced cooling of the device components. Less powerful fans are not recommended, or you will have to install several of them. Capacitor C1 - any designed to operate at an alternating voltage of 220 V. Rectifier diodes VD1-VD4 must be rated for a current of at least 16 A and a reverse voltage of at least 400 V. They must be installed on plate-shaped corner heat sinks 60x15 mm in size, 2 mm thick, made of aluminum alloy. Instead of a single capacitor C5, you can use a battery of several connected in parallel for a voltage of at least 400 V each, while the battery capacity may be greater than that indicated in the diagram. Choke L1 is made on a steel magnetic core PL 12,5x25-50. Any other magnetic circuit of the same or larger cross section is also suitable, provided that the winding is placed in its window. The winding consists of 175 turns of wire PEV-2 1,32 (a wire of smaller diameter cannot be used!). The magnetic circuit must have a non-magnetic gap of 0,3 ... 0,5 mm. Choke inductance - 40±10 μH. Capacitors C6-C24 should have a small dielectric loss tangent, and C6-C17 should also have an operating voltage of at least 1000 V. The best capacitors I have tested are K78-2, used in TVs. You can use more widespread capacitors of this type of a different capacity, bringing the total capacitance to that indicated in the diagram, as well as imported film ones. Attempts to use paper or other capacitors designed for operation in low-frequency circuits, as a rule, lead to their failure after a while. Trinistors KU221 (VS2-VS7) are preferably used with the letter index A or, in extreme cases, B or G. As practice has shown, during the operation of the device, the cathode terminals of the trinistors are noticeably heated, which may lead to the destruction of solder joints on the board and even failure trinistors. Reliability will be higher if either piston tubes made of tinned copper foil with a thickness of 0,1 ... along the entire length. The piston (bandage) should cover the entire length of the lead almost to the base. It is necessary to solder quickly so as not to overheat the trinistor. You will have a question: is it possible to install one powerful one instead of several relatively low-power trinistors? Yes, this is possible when using a device that is superior (or at least comparable) in its frequency characteristics to the KU221A trinistors. But among those available, for example, from the PM or TL series, there are none. The transition to low-frequency devices will force the operating frequency to be lowered from 25 to 4 ... 6 kHz, and this will lead to a deterioration in many of the most important characteristics of the device and a loud shrill squeak during welding. In addition, it has been found that one powerful trinistor is less reliable than several connected in parallel, since it is easier for them to provide better conditions for heat removal. It is enough to install a group of trinistors on one heat-removing plate with a thickness of at least 3 mm. Since the current equalizing resistors R14-R18 (C5-16 V) can get very hot during welding, they must be freed from the plastic shell before installation by firing or heating with a current, the value of which must be selected experimentally. Diodes VD8 and VD9 are installed on a common heat sink with trinistors, and the VD9 diode is isolated from the heat sink with a mica gasket. Instead of KD213A, KD213B and KD213V, as well as KD2999B, KD2997A, KD2997B, are suitable. When mounting diodes and trinistors, the use of heat-conducting paste is mandatory. Inductor L2 is a frameless spiral of 11 turns of wire with a cross section of at least 4 mm2 in heat-resistant insulation, wound on a mandrel with a diameter of 12...14 mm. The throttle during welding is very hot, therefore, when winding the spiral, a gap of 1 ... 1.5 mm should be provided between the turns, and the throttle must be positioned so that it is in the air flow from the fan.
The magnetic circuit of the T1 transformer is made up of three PK30x16 magnetic circuits folded together from ferrite 3000NMS-1 (they were used for horizontal transformers of old TVs). The primary and secondary windings are divided into two sections each (see Fig. 2), wound with wire PSD1,68x10,4 in fiberglass insulation and connected in series according to. The primary winding contains 2x4 turns, the secondary - 2x2 turns. Sections are wound on a specially made wooden mandrel. The sections are protected from unwinding by two bandages made of tinned copper wire with a diameter of 0,8 ... 1 mm. Bandage width - 10...11 mm. A strip of electric cardboard is placed under each bandage or several turns of fiberglass tape are wound. After winding, the bandages are soldered. One of the bandages of each section serves as the output of its beginning. To do this, the insulation under the shroud is made so that from the inside it is in direct contact with the beginning of the section winding. After winding, the bandage is soldered to the beginning of the section, for which the insulation is removed from this section of the coil in advance and it is tinned. It should be borne in mind that winding I operates in the most severe thermal conditions. For this reason, when winding its sections and during assembly, it is necessary to provide air gaps between the outer parts of the turns by inserting between the turns short, lubricated with heat-resistant glue, fiberglass inserts. In general, the more air gaps in the windings, the more efficient the heat removal from the transformer will be. It is also appropriate to note here that winding sections made with the mentioned inserts and gaskets with wire of the same section 1,68x10,4 mm2 without insulation will be cooled better under the same conditions. Next, both sections of the primary winding are stacked together one on top of the other so that the directions of their winding (counted from their ends) are opposite, and the ends are on the same side (see Fig. 2). The bandages in contact are connected by soldering, and it is advisable to solder a copper pad in the form of a short piece of wire from which the section is made to the front ones, which serve as the leads of the sections. The result is a rigid one-piece primary winding of the transformer. The secondary is made in the same way. The difference is only in the number of turns in the sections and in the fact that it is necessary to provide an output from the midpoint. The windings are installed on the magnetic circuit in a strictly defined way - this is necessary for the correct operation of the VD11 - VD32 rectifier. The winding direction of the upper winding section I (when looking at the transformer from above) must be counterclockwise, starting from the upper terminal, which must be connected to the L2 choke. The winding direction of the upper winding section II, on the contrary, is clockwise, starting from the upper output, it is connected to the VD21-VD32 diode block. Winding III is a coil of any wire with a diameter of 0,35 ... 0,5 mm in heat-resistant insulation that can withstand a voltage of at least 500 V. It can be placed last in any place of the magnetic circuit from the side of the primary winding.
To ensure the electrical safety of the welding machine and effective cooling of all elements of the transformer with air flow, it is very important to maintain the necessary gaps between the windings and the magnetic wire. This task is performed by four fixing plates laid in the windings during the final assembly of the assembly. The plates are made of fiberglass with a thickness of 1,5 mm in accordance with the drawing in fig. 3. After the final adjustment of the plate, it is advisable to fix it with heat-resistant glue. The transformer is attached to the base of the apparatus with three brackets bent from brass or copper wire with a diameter of 3 mm. The same brackets fix the mutual position of all elements of the magnetic circuit. Before mounting the transformer on the base, between the halves of each of the three sets of the magnetic circuit, it is necessary to insert non-magnetic gaskets made of electric cardboard, getinaks or textolite with a thickness of 0,2 ... 0,3 mm. For the manufacture of a transformer, magnetic cores of other sizes with a cross section of at least 5,6 cm2 can be used. Suitable, for example, W20x28 or two sets of W 16x20 from ferrite 2000NM1. Winding I for the armored magnetic circuit is made in the form of a single section of eight turns, winding II - similarly to that described above, from two sections of two turns.
The welding rectifier on diodes VD11-VD34 is structurally a separate unit, made in the form of a bookcase (see Fig. 4). It is assembled in such a way that each pair of diodes is placed between two heat-removing plates 44x42 mm in size and 1 mm thick, made of sheet aluminum alloy. The whole package is pulled together by four steel threaded studs with a diameter of 3 mm between two flanges 2 mm thick (of the same material as the plates), to which two boards are screwed on both sides, forming the rectifier leads. All diodes in the block are oriented in the same way - with the cathode leads to the right according to the figure - and the leads are soldered into the holes of the board, which serves as a common positive lead of the rectifier and the device as a whole. The anode terminals of the diodes are soldered into the holes of the second board. Two groups of conclusions are formed on it, connected to the extreme conclusions of the winding II of the transformer according to the scheme. Considering the large total current flowing through the rectifier, each of its three terminals is made of several pieces of wire 50 mm long, each soldered into its own hole and connected by soldering at the opposite end. A group of ten diodes is connected in five segments, of fourteen - in six, the second board with a common point of all diodes - in six. It is better to use a flexible wire, with a cross section of at least 4 mm. In the same way, high-current group outputs from the main printed circuit board of the device are made. The rectifier boards are made of foil fiberglass 0,5 mm thick and tinned. Four narrow slots in each board help to reduce the stress on the diode leads during thermal deformations. For the same purpose, the diode leads must be molded, as shown in Fig. four. In the welding rectifier, you can also use more powerful diodes KD2999B, 2D2999B, KD2997A, KD2997B, 2D2997A, 2D2997B. Their number may be less. So, in one of the variants of the apparatus, a rectifier of nine 2D2997A diodes successfully worked (five in one arm, four in the other). The area of the heat sink plates remained the same, it was possible to increase their thickness up to 2 mm. The diodes were placed not in pairs, but one in each compartment. Publication: radioradar.net
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