Homemade resistance spot welding. Do-it-yourself resistance welding from a microwave

Do-it-yourself welding in this case does not mean welding technology, but homemade equipment for electric welding. Working skills are acquired through industrial practice. Of course, before going to the workshop, you need to master the theoretical course. But you can put it into practice only if you have something to work with. This is the first argument in favor of, when mastering welding on your own, first taking care of the availability of appropriate equipment.

Second, a purchased welding machine is expensive. Rent is also not cheap, because... the probability of its failure due to unskilled use is high. Finally, in the outback, getting to the nearest point where you can rent a welder can be simply long and difficult. All in all, It is better to start your first steps in metal welding by making a welding installation with your own hands. And then - let it sit in a barn or garage until the opportunity arises. It’s never too late to spend money on branded welding if things work out.

What are we going to talk about?

This article discusses how to make equipment at home for:

• Electric arc welding with alternating current of industrial frequency 50/60 Hz and direct current up to 200 A. This is enough to weld metal structures up to approximately a corrugated fence on a frame made of corrugated pipe or a welded garage.
• Micro-arc welding of twisted wires is very simple and useful when laying or repairing electrical wiring.
• Spot pulse resistance welding - can be very useful when assembling products from thin steel sheets.

What we won't talk about

First, let's skip gas welding. The equipment for it costs pennies compared to consumables, you can’t make gas cylinders at home, and a homemade gas generator is a serious risk to life, plus carbide is expensive now, where it is still on sale.

The second is inverter electric arc welding. Indeed, a semi-automatic inverter welding allows a novice amateur to weld quite important structures. It is light and compact and can be carried by hand. But purchasing at retail the components of an inverter that allows for consistent high-quality welding will cost more than a finished machine. And an experienced welder will try to work with simplified homemade products, and refuse - “Give me a normal machine!” Plus, or rather minus - in order to make a more or less decent welding inverter, you need to have fairly solid experience and knowledge in electrical engineering and electronics.

The third is argon-arc welding. With whose light hand the assertion that it is a hybrid of gas and arc began to circulate in RuNet is unknown. In fact, this is a type of arc welding: the inert gas argon does not participate in the welding process, but creates a cocoon around the working area, isolating it from air. As a result, the welding seam is chemically pure, free from impurities of metal compounds with oxygen and nitrogen. Therefore, non-ferrous metals can be cooked under argon, incl. heterogeneous. In addition, it is possible to reduce the welding current and arc temperature without compromising its stability and weld with a non-consumable electrode.

It is quite possible to make equipment for argon-arc welding at home, but gas is very expensive. It is unlikely that you will need to cook aluminum, stainless steel or bronze as part of routine economic activity. And if you really need it, it’s easier to rent argon welding - compared to how much (in money) gas will go back into the atmosphere, it’s pennies.

Transformer

The basis of all “our” types of welding is a welding transformer. The procedure for its calculation and design features differ significantly from those of power supply (power) and signal (sound) transformers. The welding transformer operates in intermittent mode. If you design it for maximum current like continuous transformers, it will turn out to be prohibitively large, heavy and expensive. Ignorance of the features of electrical transformers for arc welding is the main reason for the failures of amateur designers. Therefore, let’s take a walk through welding transformers in the following order:

• a little theory - on the fingers, without formulas and brilliance;
• features of magnetic cores of welding transformers with recommendations for choosing from random ones;
• testing of available used equipment;
• calculation of a transformer for a welding machine;
• preparation of components and winding of windings;
• trial assembly and fine-tuning;
• commissioning.

An electrical transformer can be likened to a water supply storage tank. This is a rather deep analogy: a transformer operates due to the reserve of magnetic field energy in its magnetic circuit (core), which can be many times greater than that instantly transmitted from the power supply network to the consumer. And the formal description of losses due to eddy currents in steel is similar to that for water losses due to infiltration. Electricity losses in copper windings are formally similar to pressure losses in pipes due to viscous friction in the liquid.

Note: the difference is in losses due to evaporation and, accordingly, magnetic field scattering. The latter in the transformer are partially reversible, but smooth out the peaks of energy consumption in the secondary circuit.

External characteristics of electrical transformers

An important factor in our case is the external current-voltage characteristic (VVC) of the transformer, or simply its external characteristic (VC) - the dependence of the voltage on the secondary winding (secondary) on the load current, with a constant voltage on the primary winding (primary). For power transformers, the VX is rigid (curve 1 in the figure); they are like a shallow, vast pool. If it is properly insulated and covered with a roof, then water losses are minimal and the pressure is quite stable, no matter how consumers turn the taps. But if there is gurgling in the drain - sushi oars, the water is drained. In relation to transformers, the power source must keep the output voltage as stable as possible to a certain threshold less than the maximum instantaneous power consumption, be economical, small and light. For this:

• The steel grade for the core is selected with a more rectangular hysteresis loop.
• Design measures (core configuration, calculation method, configuration and arrangement of windings) reduce dissipation losses, losses in steel and copper in every possible way.
• The magnetic field induction in the core is taken to be less than the maximum permissible current form for transmission, because its distortion reduces efficiency.

Note: transformer steel with “angular” hysteresis is often called magnetically hard. This is not true. Magnetically hard materials retain strong residual magnetization; they are made by permanent magnets. And any transformer iron is soft magnetic.

You cannot cook from a transformer with a hard VX: the seam is torn, burned, and the metal splatters. The arc is inelastic: I moved the electrode slightly wrong and it goes out. Therefore, the welding transformer is made to look like a regular water tank. Its CV is soft (normal dissipation, curve 2): as the load current increases, the secondary voltage gradually drops. The normal scattering curve is approximated by a straight line incident at an angle of 45 degrees. This allows, due to a decrease in efficiency, to briefly extract several times more power from the same hardware, or resp. reduce the weight, size and cost of the transformer. In this case, the induction in the core can reach a saturation value, and for a short time even exceed it: the transformer will not go into a short circuit with zero power transfer, like a “silovik”, but will begin to heat up. Quite long: the thermal time constant of welding transformers is 20-40 minutes. If you then let it cool down and there is no unacceptable overheating, you can continue working. The relative drop in the secondary voltage ΔU2 (corresponding to the range of the arrows in the figure) of normal dissipation gradually increases with increasing range of fluctuations of the welding current Iw, which makes it easy to hold the arc during any type of work. The following properties are provided:

• The steel of the magnetic circuit is taken with hysteresis, more “oval”.
• Reversible scattering losses are normalized. By analogy: the pressure has dropped - consumers will not pour out much and quickly. And the water utility operator will have time to turn on the pumping.
• The induction is chosen close to the overheating limit; this allows, by reducing cosφ (a parameter equivalent to efficiency) at a current significantly different from the sinusoidal one, to take more power from the same steel.

Note: reversible scattering loss means that part of the power lines penetrates the secondary through the air, bypassing the magnetic circuit. The name is not entirely apt, just like “useful scattering”, because “reversible” losses for the efficiency of a transformer are no more useful than irreversible ones, but they soften the I/O.

As you can see, the conditions are completely different. So, should you definitely look for iron from a welder? Not necessary, for currents up to 200 A and peak power up to 7 kVA, but this is enough for the farm. Using design and design measures, as well as with the help of simple additional devices (see below), we will obtain on any hardware a VX curve 2a that is somewhat more rigid than normal. The efficiency of welding energy consumption is unlikely to exceed 60%, but for occasional work this is not a problem. But on delicate work and low currents, holding the arc and welding current will not be difficult, without much experience (ΔU2.2 and Iw1), at high currents Iw2 we will get acceptable weld quality, and it will be possible to cut metal up to 3-4 mm.

There are also welding transformers with a steeply falling VX, curve 3. This is more like a booster pump: either the output flow is at nominal level, regardless of the feed height, or there is none at all. They are even more compact and lightweight, but in order to withstand the welding mode at a steeply falling VX, it is necessary to respond to fluctuations ΔU2.1 of the order of a volt within a time of about 1 ms. Electronics can do this, which is why transformers with a “steep” VX are often used in semi-automatic welding machines. If you cook from such a transformer manually, then the seam will be sluggish, undercooked, the arc will again be inelastic, and when you try to light it again, the electrode will stick every now and then.

Magnetic cores

The types of magnetic cores suitable for the manufacture of welding transformers are shown in Fig. Their names begin with the letter combination respectively. standard size. L means tape. For a welding transformer L or without L, there is no significant difference. If the prefix contains M (SHLM, PLM, ShM, PM) - ignore without discussion. This is iron of reduced height, unsuitable for a welder despite all its other outstanding advantages.

Magnetic cores of transformers

After the letters of the nominal value there are numbers indicating a, b and h in Fig. For example, for W20x40x90, the cross-sectional dimensions of the core (central rod) are 20x40 mm (a*b), and the window height h is 90 mm. Core cross-sectional area Sc = a*b; window area Sok = c*h is needed for accurate calculation of transformers. We will not use it: for an accurate calculation, we need to know the dependence of losses in steel and copper on the value of induction in a core of a given standard size, and for them, the grade of steel. Where will we get it if we run it on random hardware? We will calculate using a simplified method (see below), and then finalize it during testing. It will take more work, but we will get welding that you can actually work on.

Note: if the iron is rusty on the surface, then nothing, the properties of the transformer will not suffer from this. But if there are spots of tarnish on it, this is a defect. Once upon a time, this transformer overheated very much and the magnetic properties of its iron were irreversibly deteriorated.

Another important parameter of the magnetic circuit is its mass, weight. Since the specific density of steel is constant, it determines the volume of the core, and, accordingly, the power that can be taken from it. Magnetic cores with the following weight are suitable for the manufacture of welding transformers:

• O, OL – from 10 kg.
• P, PL – from 12 kg.
• W, SHL – from 16 kg.

Why Sh and ShL are needed heavier is clear: they have an “extra” side rod with “shoulders”. OL may be lighter because it does not have corners that require excess iron, and the bends of the magnetic force lines are smoother and for some other reasons, which will be discussed later. section.

The cost of toroid transformers is high due to the complexity of their winding. Therefore, the use of toroidal cores is limited. A torus suitable for welding can, firstly, be removed from the LATR - a laboratory autotransformer. Laboratory, which means it should not be afraid of overloads, and the hardware of LATRs provides a VH close to normal. But…

LATR is a very useful thing, first of all. If the core is still alive, it is better to restore the LATR. Suddenly you don’t need it, you can sell it, and the proceeds will be enough for welding suitable for your needs. Therefore, “bare” LATR cores are difficult to find.

Secondly, LATRs with a power of up to 500 VA are weak for welding. From the LATR-500 iron you can achieve welding with a 2.5 electrode in the mode: cook for 5 minutes - it cools down for 20 minutes, and we heat up. As in Arkady Raikin’s satire: mortar bar, brick yok. Brick bar, mortar yok. LATRs 750 and 1000 are very rare and useful.

Another torus suitable for all properties is the stator of an electric motor; Welding from it will turn out to be good enough for an exhibition. But it is no easier to find than LATR iron, and it is much more difficult to wind on it. In general, a welding transformer from an electric motor stator is a separate topic, there are so many complexities and nuances. First of all, with a thick wire wound around the donut. Having no experience in winding toroidal transformers, the probability of damaging an expensive wire and not getting welded is close to 100%. Therefore, alas, you will have to wait a little longer with the cooking apparatus on a triode transformer.

Armor cores are structurally designed for minimal dissipation, and it is almost impossible to standardize it. Welding on a regular Sh or ShL will turn out to be too tough. In addition, the cooling conditions for the windings on Ш and ШЛ are the worst. The only armored cores suitable for a welding transformer are those of increased height with spaced biscuits windings (see below), on the left in Fig. The windings are separated by dielectric non-magnetic heat-resistant and mechanically strong gaskets (see below) with a thickness of 1/6-1/8 of the core height.

Plates of armored magnetic circuits and biscuits windings

For welding, the core Ш is welded (assembled from plates) necessarily across the roof, i.e. yoke-plate pairs are alternately oriented back and forth relative to each other. The method of normalizing dissipation by a non-magnetic gap is unsuitable for a welding transformer, because the losses are irreversible.

If you come across a laminated Sh without a yoke, but with a cut in the plates between the core and the lintel (in the center), you are in luck. The plates of the signal transformers are laminated, and the steel on them, to reduce signal distortion, is used to initially give normal VX. But the likelihood of such luck is very low: signal transformers with kilowatt power are a rare curiosity.

Note: do not try to assemble a high Ш or ШЛ from a pair of ordinary ones, as on the right in Fig. A continuous straight gap, albeit a very thin one, means irreversible scattering and a steeply falling CV. Here, dissipation losses are almost similar to water losses due to evaporation.

Winding transformer windings on a rod core

Rod cores are most suitable for welding. Of these, those laminated in pairs of identical L-shaped plates, see Fig., their irreversible scattering is the smallest. Secondly, the P and PL windings are wound in exactly the same halves, with half turns for each. The slightest magnetic or current asymmetry - the transformer hums, heats up, but there is no current. The third thing that may not seem obvious to those who have not forgotten the school gimlet rule is that the windings are wound onto the rods in one direction. Does something seem wrong? Does the magnetic flux in the core have to be closed? And you twist the gimlets according to the current, and not according to the turns. The directions of the currents in the half-windings are opposite, and magnetic fluxes are shown there. You can also check if the wiring protection is reliable: apply the network to 1 and 2’, and close 2 and 1’. If the machine does not immediately knock out, the transformer will howl and shake. However, who knows what's going on with your wiring. Better not.

Note: You can also find recommendations - to wind the windings of the welding P or PL on different rods. Like, VH is softening up. That’s how it is, but for this you need a special core, with rods of different sections (the secondary is smaller) and recesses that release power lines into the air in the desired direction, see fig. on right. Without this, we will get a noisy, shaking and gluttonous, but not cooking transformer.

If there is a transformer

A 6.3 A circuit breaker and an AC ammeter will also help determine the suitability of an old welder lying around God knows where and God knows how. You need either a non-contact induction ammeter (current clamp) or a 3 A pointer electromagnetic ammeter. A multimeter with alternating current limits will not lie, because the shape of the current in the circuit will be far from sinusoidal. Also, a long-neck liquid household thermometer, or, better yet, a digital multimeter with the ability to measure temperature and a probe for this. The step-by-step procedure for testing and preparing for further operation of an old welding transformer is as follows:

Calculation of a welding transformer

In RuNet you can find different methods for calculating welding transformers. Despite the apparent inconsistency, most of them are correct, but with full knowledge of the properties of steel and/or for a specific range of standard values ​​of magnetic cores. The proposed methodology developed in Soviet times, when instead of choice there was a shortage of everything. For a transformer calculated using it, the VX drops a little steeply, somewhere between curves 2 and 3 in Fig. at first. This is suitable for cutting, but for thinner work the transformer is supplemented with external devices (see below) that stretch the VX along the current axis to curve 2a.

The basis of the calculation is common: the arc burns stably under a voltage Ud of 18-24 V, and to ignite it an instantaneous current is required that is 4-5 times greater than the rated welding current. Accordingly, the minimum open-circuit voltage Uхх of the secondary will be 55 V, but for cutting, since everything possible is squeezed out of the core, we take not the standard 60 V, but 75 V. Nothing more: it is unacceptable according to technical regulations, and the iron will not pull out. Another feature, for the same reasons, is the dynamic properties of the transformer, i.e. its ability to quickly transition from short-circuit mode (say, when shorted by drops of metal) to working mode is maintained without additional measures. True, such a transformer is prone to overheating, but since it is our own and in front of our eyes, and not in the far corner of a workshop or site, we will consider this acceptable. So:

• According to the formula from paragraph 2 previous. list we find the overall power;
• We find the maximum possible welding current Iw = Pg/Ud. 200 A is guaranteed if 3.6-4.8 kW can be removed from the iron. True, in the first case the arc will be sluggish, and it will be possible to cook only with a deuce or 2.5;
• We calculate the operating current of the primary at the maximum permissible network voltage for welding I1рmax = 1.1Pg(VA)/235 V. In fact, the norm for the network is 185-245 V, but for a homemade welder at the limit this is too much. We take 195-235 V;
• Based on the found value, we determine the tripping current of the circuit breaker as 1.2I1рmax;
• We assume the current density of the primary J1 = 5 A/sq. mm and, using I1рmax, we find the diameter of its copper wire d = (4S/3.1415)^0.5. Its full diameter with self-insulation is D = 0.25+d, and if the wire is ready - tabular. To operate in the “brick bar, mortar yoke” mode, you can take J1 = 6-7 A/sq. mm, but only if the required wire is not available and is not expected;
• We find the number of turns per volt of the primary: w = k2/Sс, where k2 = 50 for Sh and P, k2 = 40 for PL, ShL and k2 = 35 for O, OL;
• We find the total number of its turns W = 195k3w, where k3 = 1.03. k3 takes into account the energy loss of the winding due to leakage and in copper, which is formally expressed by the somewhat abstract parameter of the winding’s own voltage drop;
• We set the laying coefficient Kу = 0.8, add 3-5 mm to a and b of the magnetic circuit, calculate the number of layers of the winding, the average length of the turn and the footage of the wire
• We calculate the secondary similarly at J1 = 6 A/sq. mm, k3 = 1.05 and Ku = 0.85 for voltages of 50, 55, 60, 65, 70 and 75 V, in these places there will be taps for rough adjustment of the welding mode and compensation for fluctuations in the supply voltage.

Winding and finishing

The diameters of the wires in the calculation of windings are usually greater than 3 mm, and varnished winding wires with d>2.4 mm are rarely widely sold. In addition, the welder windings experience strong mechanical loads from electromagnetic forces, so finished wires are needed with an additional textile winding: PELSH, PELSHO, PB, PBD. They are even more difficult to find, and they are very expensive. The meterage of the wire for the welder is such that it is possible to insulate cheaper bare wires yourself. An additional advantage is that by twisting several stranded wires to the required S, we get a flexible wire, which is much easier to wind. Anyone who has tried to manually lay a tire of at least 10 square meters on a frame will appreciate it.

Isolation

Let's say there is a 2.5 sq.m. wire available. mm in PVC insulation, and for the secondary you need 20 m by 25 squares. We prepare 10 coils or coils of 25 m each. We unwind about 1 m of wire from each and remove the standard insulation, it is thick and not heat-resistant. We twist the exposed wires with a pair of pliers into an even, tight braid, and wrap it in order of increasing insulation cost:

• Using masking tape with an overlap of 75-80% turns, i.e. in 4-5 layers.
• Calico braid with an overlap of 2/3-3/4 turns, i.e. 3-4 layers.
• Cotton electrical tape with an overlap of 50-67%, in 2-3 layers.

Note: the wire for the secondary winding is prepared and wound after winding and testing the primary, see below.

A thin-walled homemade frame will not withstand the pressure of turns of thick wire, vibrations and jerks during operation. Therefore, the windings of welding transformers are made of frameless biscuits, and they are secured to the core with wedges made of textolite, fiberglass or, in extreme cases, bakelite plywood impregnated with liquid varnish (see above). The instructions for winding the windings of a welding transformer are as follows:

• We prepare a wooden boss with a height equal to the height of the winding and with dimensions in diameter 3-4 mm larger than a and b of the magnetic circuit;
• We nail or screw temporary plywood cheeks to it;
• We wrap the temporary frame in 3-4 layers of thin polyethylene film, covering the cheeks and wrapping them on the outside so that the wire does not stick to the wood;
• We wind the pre-insulated winding;
• Along the winding, we impregnate it twice with liquid varnish until it drips through;
• Once the impregnation has dried, carefully remove the cheeks, squeeze out the boss and peel off the film;
• We tightly tie the winding in 8-10 places evenly around the circumference with thin cord or propylene twine - it is ready for testing.

Finishing and finishing

We mix the core into a biscuit and tighten it with bolts, as expected. Winding tests are carried out in exactly the same way as tests of a questionable finished transformer, see above. It is better to use LATR; Iхх at an input voltage of 235 V should not exceed 0.45 A per 1 kVA of the overall power of the transformer. If it’s more, the primary is wound up. Winding wire connections are made with bolts (!), insulated with heat-shrinkable tube (HERE) in 2 layers or with cotton electrical tape in 4-5 layers.

Based on the test results, the number of turns of the secondary is adjusted. For example, the calculation gave 210 turns, but in reality Ixx fit into the norm at 216. Then we multiply the calculated turns of the secondary sections by 216/210 = 1.03 approx. Do not neglect decimal places, the quality of the transformer largely depends on them!

After finishing, we disassemble the core; We wrap the biscuit tightly with the same masking tape, calico or “rag” tape in 5-6, 4-5 or 2-3 layers, respectively. Wind across the turns, not along them! Now saturate it with liquid varnish again; when it dries - twice undiluted. This galette is ready, you can make a secondary one. When both are on the core, we test the transformer again now at Ixx (suddenly it curled somewhere), fix the biscuits and impregnate the entire transformer with normal varnish. Phew, the most dreary part of the work is over.

But he’s still too cool for us, remember? Needs to be softened. The simplest method - a resistor in the secondary circuit - does not suit us. Everything is very simple: at a resistance of only 0.1 Ohm at a current of 200, 4 kW of heat will be dissipated. If we have a welder with a capacity of 10 kVA or more, and we need to weld thin metal, we need a resistor. Whatever current is set by the regulator, its emissions when the arc is ignited are inevitable. Without active ballast, they will burn the seam in places, and the resistor will extinguish them. But for us, weaklings, it will be of no use.

Adjusting the welding mode with a reactive coil

The reactive ballast (inductor, choke) will not take away excess power: it will absorb current surges, and then smoothly release them to the arc, this will stretch the VX as it should. But then you need a throttle with dispersion adjustment. And for it, the core is almost the same as that of a transformer, and the mechanics are quite complex, see fig.

Homemade welding transformer ballast

We will go the other way: we will use active-reactive ballast, colloquially called gut by old welders, see fig. on right. Material – steel wire rod 6 mm. The diameter of the turns is 15-20 cm. How many of them are shown in Fig. Apparently, for power up to 7 kVA this gut is correct. The air gaps between the turns are 4-6 cm. The active-reactive choke is connected to the transformer with an additional piece of welding cable (hose, simply), and the electrode holder is attached to it with a clothespin clamp. By selecting the connection point, it is possible, coupled with switching to secondary taps, to fine-tune the operating mode of the arc.

Note: An active-reactive choke can become red-hot during operation, so it requires a fireproof, heat-resistant, dielectric, non-magnetic lining. In theory, a special ceramic cradle. It is acceptable to replace it with a dry sand cushion, or formally with a violation, but not grossly, the welding gut is laid on bricks.

But other?

Primitive welding electrode holder

This means, first of all, an electrode holder and a connecting device for the return hose (clamp, clothespin). Since our transformer is at its limit, we need to buy them ready-made, but those like those in Fig. right, no need. For a 400-600 A welding machine, the quality of contact in the holder is hardly noticeable, and it will also withstand simply winding up the return hose. And our homemade one, working with effort, can go haywire, seemingly for some unknown reason.

Next, the body of the device. It must be made of plywood; preferably bakelite impregnated, as described above. The bottom is 16 mm thick, the panel with the terminal block is 12 mm thick, and the walls and cover are 6 mm thick, so that they do not come off during transportation. Why not sheet steel? It is ferromagnetic and in the stray field of a transformer can disrupt its operation, because we get everything we can out of him.

As for the terminal blocks, the terminals themselves are made from M10 bolts. The base is the same textolite or fiberglass. Getinax, bakelite and carbolite are not suitable; pretty soon they will crumble, crack and delaminate.

Let's try a permanent one

Welding with direct current has a number of advantages, but the input voltage of any welding transformer becomes more severe at constant current. And ours, designed for the minimum possible power reserve, will become unacceptably stiff. The choke-intestine will no longer help here, even if it worked on direct current. In addition, it is necessary to protect expensive 200 A rectifier diodes from current and voltage surges. We need a reciprocal-absorbing infra-low frequency filter, FINCH. Although it looks reflective, you need to take into account the strong magnetic coupling between the halves of the coil.

Direct current electric arc welding diagram

The circuit of such a filter, known for many years, is shown in Fig. But immediately after its implementation by amateurs, it became clear that the operating voltage of capacitor C is low: voltage surges during arc ignition can reach 6-7 values ​​of its Uхх, i.e. 450-500 V. Further, capacitors are needed that can withstand the circulation of high reactive power, only and only oil-paper ones (MBGCH, MBGO, KBG-MN). The following gives an idea of ​​the weight and dimensions of single “cans” of these types (by the way, not cheap ones). Fig., and a battery will need 100-200 of them.

Oil-paper capacitors

With a coil magnetic circuit it is simpler, although not entirely. Suitable for it are 2 PL power transformers TS-270 from old tube “coffin” TVs (the data is in reference books and in RuNet), or similar ones, or SLs with similar or larger a, b, c and h. From 2 submarines, an SL is assembled with a gap, see figure, of 15-20 mm. It is fixed with textolite or plywood spacers. Winding - insulated wire from 20 sq. mm, how much will fit in the window; 16-20 turns. Wind it into 2 wires. The end of one is connected to the beginning of the other, this will be the middle point.

Armored magnetic core with non-magnetic gap

The filter is adjusted in an arc at the minimum and maximum values ​​of Uхх. If the arc is sluggish at minimum, the electrode sticks, the gap is reduced. If the metal burns at maximum, increase it or, which will be more effective, cut off part of the side rods symmetrically. To prevent the core from crumbling, it is impregnated with liquid and then normal varnish. Finding the optimum inductance is quite difficult, but then welding works flawlessly on alternating current.

Microarc

The purpose of microarc welding is discussed at the beginning. The “equipment” for it is extremely simple: a step-down transformer 220/6.3 V 3-5 A. In tube times, radio amateurs connected to the filament winding of a standard power transformer. One electrode – the twisting of the wires itself (copper-aluminum, copper-steel is possible); the other is a graphite rod like a 2M pencil lead.

Nowadays, for micro-arc welding, they use more computer power supplies, or, for pulsed micro-arc welding, capacitor banks, see the video below. On direct current, the quality of work, of course, improves.

Video: homemade machine for welding twists

Contact! There is contact!

Resistance welding in industry is mainly used in spot, seam and butt welding. At home, primarily in terms of energy consumption, pulsed point is feasible. It is suitable for welding and welding thin, from 0.1 to 3-4 mm, steel sheet parts. Arc welding will burn through a thin wall, and if the part is the size of a coin or less, then the softest arc will burn it entirely.

Resistance spot welding diagram

The principle of operation of resistance spot welding is illustrated in the figure: copper electrodes forcefully compress the parts, a current pulse in the steel-to-steel ohmic resistance zone heats the metal until electrodiffusion occurs; metal does not melt. The current needed for this is approx. 1000 A per 1 mm of thickness of the parts being welded. Yes, a current of 800 A will grab sheets of 1 and even 1.5 mm. But if this is not a craft for fun, but, say, a galvanized corrugated fence, then the very first strong gust of wind will remind you: “Man, the current was rather weak!”

However, resistance spot welding is much more economical than arc welding: the no-load voltage of the welding transformer for it is 2 V. It consists of 2-contact steel-copper potential differences and the ohmic resistance of the penetration zone. The transformer for resistance welding is calculated in the same way as for arc welding, but the current density in the secondary winding is 30-50 or more A/sq. mm. The secondary of the contact-welding transformer contains 2-4 turns, is well cooled, and its utilization factor (the ratio of welding time to idling and cooling time) is many times lower.

There are many descriptions on the RuNet of homemade pulse-spot welders made from unusable microwave ovens. They are, in general, correct, but repetition, as written in “1001 Nights,” is of no use. And old microwaves don’t lie in heaps in trash heaps. Therefore, we will deal with designs that are less known, but, by the way, more practical.

Easy DIY Resistance Welding Installation

In Fig. – construction of a simple apparatus for pulsed spot welding. They can weld sheets up to 0.5 mm; It is perfect for small crafts, and magnetic cores of this and larger sizes are relatively affordable. Its advantage, in addition to its simplicity, is the clamping of the running rod of the welding pliers with a load. To work with a contact welding pulser, a third hand would not hurt, and if one has to forcefully squeeze the pliers, then it is generally inconvenient. Disadvantages – increased risk of accidents and injuries. If you accidentally give a pulse when the electrodes are brought together without the parts being welded, then the plasma will shoot out from the tongs, metal splashes will fly, the wiring protection will be knocked out, and the electrodes will fuse tightly.

The secondary winding is made of a 16x2 copper busbar. It can be assembled from strips of thin sheet copper (it will turn out flexible) or made from a piece of flattened refrigerant supply tube of a household air conditioner. The bus is isolated manually as described above.

Here in Fig. – drawings of a pulse spot welding machine are more powerful, for welding sheets up to 3 mm, and more reliable. Thanks to a fairly powerful return spring (from the armored mesh of the bed), accidental convergence of the pliers is excluded, and the eccentric clamp provides strong, stable compression of the pliers, on which the quality of the welded joint significantly depends. If something happens, the clamp can be instantly released with one blow on the eccentric lever. The disadvantage is the insulating pincer units, there are too many of them and they are complex. Another one is aluminum pincer rods. Firstly, they are not as strong as steel ones, and secondly, they are 2 unnecessary contact differences. Although the heat dissipation of aluminum is certainly excellent.

About electrodes

Resistance welding electrode in an insulating sleeve

In amateur conditions, it is more advisable to insulate the electrodes at the installation site, as shown in Fig. on right. There is no conveyor at home; you can always let the device cool down so that the insulating bushings do not overheat. This design will allow you to make rods from durable and cheap steel corrugated pipe, and also lengthen the wires (up to 2.5 m is permissible) and use a contact welding gun or external pliers, see fig. below.

In Fig. On the right, another feature of electrodes for resistance spot welding is visible: a spherical contact surface (heel). Flat heels are more durable, so electrodes with them are widely used in industry. But the diameter of the flat heel of the electrode must be equal to 3 times the thickness of the adjacent material being welded, otherwise the weld spot will be burned either in the center (wide heel) or along the edges (narrow heel), and corrosion will occur from the welded joint even on stainless steel.

Gun and external pliers for resistance welding

The last point about electrodes is their material and size. Red copper burns out quickly, so commercial electrodes for resistance welding are made of copper with a chromium additive. These should be used; at current copper prices it is more than justified. The diameter of the electrode is taken depending on the mode of its use, based on a current density of 100-200 A/sq. mm. According to heat transfer conditions, the length of the electrode is at least 3 of its diameters from the heel to the root (the beginning of the shank).

How to give impetus

In the simplest homemade pulse-contact welding machines, the current pulse is given manually: they simply turn on the welding transformer. This, of course, does not benefit him, and welding is either insufficient or burnt out. However, automating the supply and standardization of welding pulses is not so difficult.

Diagram of a simple pulse former for resistance welding

A diagram of a simple but reliable welding pulse generator, proven by long practice, is shown in Fig. Auxiliary transformer T1 is a regular 25-40 W power transformer. The voltage of winding II is indicated by the backlight. You can replace it with 2 LEDs connected back-to-back with a quenching resistor (usual, 0.5 W) 120-150 Ohm, then the voltage II will be 6 V.

Voltage III - 12-15 V. 24 is possible, then capacitor C1 (regular electrolytic) is needed for a voltage of 40 V. Diodes V1-V4 and V5-V8 - any rectifier bridges for 1 and from 12 A, respectively. Thyristor V9 - 12 or more A 400 V. Optothyristors from computer power supplies or TO-12.5, TO-25 are suitable. Resistor R1 is a wire-wound resistor; it is used to regulate the pulse duration. Transformer T2 – welding.

Many people do not want to depend on circumstances. If you suddenly need welding, you want to solve the problem in your workshop. A do-it-yourself resistance welding machine is a solution in the right direction.

To perform resistance welding with your own hands, you need to purchase or make a special machine yourself.

Of course, if you need to weld large metal structures, then it is difficult for resistance welding to compete with other types. At the same time, at home there is a great need for welding small parts. Such problems become easily solvable if you make your own resistance welding machine.

Resistance Welding Basics

In general, resistance welding is welding using electric current, when it passes through the contact zone of the metals being welded under the influence of compressive pressure. The principle of contact welding is based on the fact that when an electric current is applied, an arc appears at the point of contact of two metals, which melts them. The duration of exposure to welding current is very short (0.01-0.1 s). The main parameters of any resistance welding are: the strength of the welding current, the time of application of the current and the amount of compression of the metals in the contact zone. There are the following main ones: spot, relief, seam and butt welding.

Fundamentals of apparatus design

To carry out resistance welding, it is necessary to assemble a resistance welding machine. When manufacturing the apparatus and accessories, several basic rules must be taken into account. Typically, spot or butt welding machines are used for domestic purposes. Then you should think about what type of device it will be - stationary or portable, which determines its weight and dimensions. It is necessary to decide on the basic parameters of the device:

1. Type of welding current (alternating, direct) and its strength.
2. Voltage in the welding zone.
3. Welding pulse duration.
4. Number and type of electrodes.
5. Simplicity of the device.

Any resistance welding machine contains an electrical and mechanical part. The electrical part includes a welding current source, a control system for basic parameters and a contact block. The mechanical part must provide fastening of the workpieces to be welded, as well as the application of a compressive load.

Welding power source

The main element of the resistance spot welding machine is the welding current source, i.e. short current pulse. The most common current sources use energy storage and capacitor discharge. One of the simple circuits of such a source is based on the supply of direct current from the secondary winding of a transformer, onto the primary winding of which a capacitor is discharged (in Fig. 1 there is a diagram of the power source).

Figure 1. Power supply diagram.

The primary winding of the output transformer T2 is connected to the input electrical network so that one branch of the circuit passes through the diagonal of the rectifier bridge (diodes V5-V8). In this case, control is carried out through thyristor V9 connected to the “Impulse” start button by connecting it to the second diagonal of the bridge. Energy is stored in capacitor C1, located in the thyristor V9 circuit and connected to the diagonal of the bridge. The discharge of the capacitor through this circuit enters the primary winding of the output transformer T2. Capacitor C1 is charged from an auxiliary circuit, which is connected when the main circuit is turned off.

This welding pulse source works as follows. Capacitor C1 is charged while the output transformer T2 is turned off. When you press the “Impulse” start button, charging of the capacitor stops and it is discharged into an adjustable resistor R1 connected to the primary winding of transformer T2. The discharge parameters are controlled by thyristor V9. The duration of the welding pulse is regulated using a variable resistor R1, into which the discharge occurs. When the button is turned off, the capacitor charging process resumes.

Recommended parts for the circuit: capacitor C1 with a capacity of 1000 μF for operating voltage up to 25 V; thyristor PTL-50 or KU202, input transformer T1 with a power of 10 W for a voltage in the windings of 220/15 V. It is better to make the output transformer T2 with your own hands: the primary winding is PEV-2 wire with a diameter of 0.8 mm, 300 turns; secondary winding – copper busbar 20-25 mm², 10 turns. Output parameters of the device: current up to 500 A, pulse duration up to 0.1 s.

Increasing the power of the current source

Figure 2. Diagram of a high-power source: 1. schematic diagram; 2. winding of transformer T2; 3. starter connection diagram.

To increase the power of the welding pulse, you can make some changes to the device. The current is supplied through a contactless magnetic starter type MTT4K (operating current up to 80 A). 2 thyristors (Fig. 2), 2 KTs402 diodes and resistors R1-R2 are introduced into the control circuit. The response time is controlled by the RES time relay. A battery of capacitors C1-C6 of 6 pieces is recommended as energy storage devices (in Fig. 2 there is a diagram of a high-power source: 1) circuit diagram; 2) winding of transformer T2; 3) starter connection diagram).

It is recommended to install the following parts: electrolytic capacitors C1-C6 with a capacity of 47 μF, 100 μF and 470 μF (two of each type) for an operating voltage of 50 V; time relay RES42 or RES43 for a voltage of 20 V. Transformer T2 has a primary winding made of wire with a diameter of 1.5 mm, a secondary winding made of a copper bus with a cross-section of 60 mm² (number of turns - 4-7). The welding current of such a device is up to 1500 A.

Making an output transformer

One of the most important elements of equipment is the output welding transformer. Its production should begin with the selection of a type-setting core. A standard core with a total cross-section of at least 60 cm² should be used. The tie-down elements are made using an angle or strip and secured with bolts with a diameter of 8 mm. The primary winding is wound manually with PET or PETV wire onto one side of the core. The turns are spaced evenly along the length of the core. The ends of the winding are brought out onto the panel and secured in the connecting block. The secondary winding is made on the second side of the core from a copper busbar. The copper busbar is pre-insulated with fluoroplastic tape or fabric insulating tape. At the ends of the bus, brought out, holes are drilled for bolting the cable. An insulating layer is placed on top of both windings.

Contact block design

The simplest contact block device involves supplying current directly to the parts being welded. This method is used in butt welding. Alligator clips are used to ensure contact.

A more complex system involves direct connection of current only to the most massive part. The second contact is provided by a movable top electrode, which is fed into the welding zone manually. A welding gun can be recommended as such a contact. It is made from two identical textolite plates, cut in the shape of a pistol. Nuts are installed in the front part for screwing the copper electrode into them, and a start button is located in the central part. A cable is inserted into the device from above, which is connected to the electrode, and a wire from the circuit of the primary winding of the transformer, which is connected to the start button.

The plates are fastened together so as to securely fix the electrode mount.

Assembling the device

Figure 3. When assembling the welding machine, the input cable from the electrical network is attached to the contact block, which is located on the electrical plate.

The welding current source is placed in a metal housing. The electrical plateau is assembled on a PCB and fixed inside the source body, usually vertically. The output transformer is mounted on the base of the housing. A welding cable is bolted from above to the busbar of the secondary winding of the transformer, the second end of which is connected to the electrode in the contact gun. The input cable from the electrical network is attached to the contact block located on the electrical plate (Fig. 3).

Tools and auxiliaries that are necessary when making a resistance welding machine with your own hands:

• Bulgarian;
• electric drill;
• hacksaw for metal;
• file;
• chisel;
• hammer;
• pliers;
• screwdriver;
• vice;
• calipers;
• scissors;
• tap;
• die.

Making a resistance welding machine is not difficult. You can choose a very simple design, or you can make universal equipment.

Resistance welding is increasingly used not only in manufacturing plants, but also in home workshops and garages, where it can be successfully used for various metal-related work. Serial equipment for performing such a technological operation is quite expensive, but a device for contact welding can be made with your own hands from an old microwave.

One of the options for a resistance welding machine from a microwave oven

To manufacture equipment for contact welding at home, you will need the following components, accessories and tools:

• a transformer that can be removed from an old microwave oven (if you need a high-power device, then you will need two such transformers);
• thick copper wire or small diameter wiring harness;
• levers that will be used as clamps;
• lever of the required length;
• a reliable base on which the welding machine will be installed;
• clamping clamps;
• Screwdriver Set;
• cables and winding materials;
• electrodes made of copper, due to which welding will be performed.

Transformer assembly

The main element of any resistance welding machine is a transformer, which can be taken from an old but working microwave. In order for a homemade welding device to be able to connect steel sheets up to 1 mm thick, a transformer with a power of at least 1 kW is required. If you need a more powerful resistance welding machine, you will need two transformers.

Step-up transformer from a microwave oven

To make equipment for contact welding with your own hands, you need to take not the entire transformer from the microwave, but only its magnetic circuit and primary winding. The secondary winding is carefully removed from the transformer, and the shunts that are located on both sides are removed from it.

We cut off the secondary winding with a chisel (chisel) or saw it off with a hacksaw.

Shunts are removed

The new winding for the microwave transformer is made from stranded wire with a cross-section of at least 100 mm 2 (or a diameter of more than 1 cm). It will be enough to make 2-3 turns. If the wire has too thick insulation, you can remove it and replace it with fabric insulating tape. If two transformers are used at once, then the secondary winding for them is made common, but it is very important to correctly connect the leads from their primary windings.

The next stages of making resistance welding from a microwave with your own hands are the installation of controls, the manufacture and connection of electrodes, the installation of the internal part of the equipment in a reliable case, which can also be taken from broken household appliances.

Another great video on the topic:

Selecting Electrodes

The electrodes of equipment for spot welding perform several functions simultaneously: compression of the sheets being joined, supply of current to the welding zone, and subsequent heat removal. Important parameters when choosing an electrode are its shape, dimensions, etc. It is these parameters that directly determine how high-quality the welded joint will be. The geometric shape of the electrodes can be straight or curly, but preference is given to straight models, as they provide better access to the welding area.

When choosing electrodes for a microwave welding machine, you can simply refer to the corresponding GOST (14111-90), which already specifies all possible diameters of these elements (10, 13, 16, 20, 25, 32, 40 mm).

The diameter of the copper rods that will be used as electrodes must be greater than or equal to the diameter of the working wires. To avoid active oxidation of the electrodes during operation, they are connected to the working wires by soldering. The electrodes of a resistance welding device (including one made from a microwave) actively wear out during operation, so they must be regularly sharpened, giving them the shape of a sharpened pencil using a file.

Bottom electrode installed

How to operate a homemade welding machine

Despite the fact that resistance welding is a fairly simple technological operation, it must be properly controlled to achieve the required quality of the joint. It is for these purposes that a homemade microwave oven should be equipped with appropriate controls. The main ones are a switch and a lever, with the help of which the required compression force of the electrodes and connected parts is ensured.

The quality of the resulting connection directly depends on the compression force, so it is advisable to make the lever for the welding machine longer. It is very important that the equipment for resistance welding from the microwave is securely fixed to the surface of the workbench. Clamps are used for these purposes.

You can increase the force transmitted by the electrodes not only using a lever, but also using a lever-screw mechanism, which can also be equipped with a home-made device. It is most convenient to attach such a mechanism directly to the lever so that manipulating it does not take additional time. In addition, this arrangement of controls will free up the operator’s second hand, which can be used to hold the parts being connected.

A special feature of working with a resistance welding machine is that current can be supplied to the electrodes only when they are in a compressed state. If you turn on the current before compressing them, they will spark the moment they come into contact with the parts, and this will lead to their burning and rapid failure.

The switch, also related to the controls for contact welding (including those made from a microwave), must be installed in the primary winding circuit. If you neglect this recommendation and install it in the secondary winding circuit, through which a significant current flows, the switch will create additional resistance, which will lead to welding of the electrodes together.

For a homemade welding machine with a microwave transformer, you need to provide a simple cooling system, also made by yourself. An ordinary fan can be used as such a system. Using this device it will be possible to cool the transformer itself, electrodes and other conductive elements. Of course, such cooling will not be very effective, and you will still have to take regular breaks in work necessary to independently cool all heating elements of the equipment.

The welding process performed on a home-made machine is practically no different from a similar technological operation carried out on serial equipment. The first stage of this process is the compression of the parts, during which they undergo plastic deformation at the site of the future connection. In the second stage, current is supplied to the welding zone, passing through copper electrodes.

This stage is characterized by the formation of a liquid welding core, expansion of the weld pool and plastic deformation and sedimentation of the metal of the parts at the connection point. At this moment, molten metal begins to splash out of the weld pool. After the current supply to the connection zone is stopped, it begins to cool, which is accompanied by crystallization of the molten metal.

Spot welding machines are not used as often in everyday life as arc welding machines, but sometimes it is impossible to do without them. Considering that the cost of such equipment starts from $450-$470, the profitability of its purchase is questionable.

The way out of this situation is resistance spot welding with your own hands. But, before we tell you how to make such a device yourself, let's look at what spot welding is and the technology of its operation.

Briefly about spot welding

This type of welding is contact (thermomechanical). Note that this category also includes seam and butt welding, but it is not possible to implement them at home, since complex equipment will be needed for this purpose.

The welding process includes the following steps:

• the parts are combined in the required position;
• they are secured between the electrodes of the device, which press the parts;
• heating is performed, as a result of which, due to plastic deformation, the parts are firmly connected to each other.

A production spot welding machine (such as the one shown in the photo) is capable of performing up to 600 operations within a minute.

Process technology

To heat the parts to the required temperature, a short-term pulse of high-power electric current is applied to them. As a rule, the pulse lasts from 0.01 to 0.1 seconds (the time is selected based on the characteristics of the metal from which the parts are made).

When pulsed, the metal melts and a common liquid core forms between the parts; until it hardens, the welded surfaces must be held under pressure. Due to this, as it cools, the molten core crystallizes. A drawing illustrating the welding process is shown below.

Designations:

• A – electrodes;
• B – parts to be welded;
• C – welding core.

Pressure on the parts is necessary so that, when pulsed, a sealing belt is formed along the perimeter of the molten metal core, preventing the melt from flowing outside the zone where welding occurs.

To provide better conditions for crystallization of the melt, the pressure on the parts is gradually removed. If it is necessary to “forge” the welding site in order to eliminate inhomogeneities inside the seam, increase the pressure (do this at the final stage).

Please note that to ensure a reliable connection, as well as the quality of the seam, it is first necessary to treat the surfaces of the parts in the places where welding will take place. This is done to remove oxide film or corrosion.

When it is necessary to ensure reliable connection of parts with a thickness of 1 to 1.5 mm, capacitor welding is used. The principle of its operation is as follows:

• the capacitor block is charged with a small electric current;
• the capacitors are discharged through the parts being connected (the pulse strength is sufficient to ensure the required welding mode).

This type of welding is used in those areas of industry where it is necessary to connect miniature and subminiature components (radio engineering, electronics, etc.).

Speaking about spot welding technology, it should be noted that it can be used to connect dissimilar metals together.

Examples of homemade designs

There are many examples on the Internet of creating machines that produce spot welding. Here are some of the most successful designs. Below is a diagram of a simple spot welder.

For implementation we will need the following radio components:

• R – variable resistance with a nominal value of 100 Ohms;
• C – capacitor designed for a voltage of at least 25 V with a capacity of 1000 μF;
• VD1 – thyristor KU202, the letter index can be K, L, M or N, you can also use PTL-50, but in this case the capacitance “C” must be reduced to 1000 μF;
• VD2-VD5 – diodes D232A, foreign analog – S4M;
• VD6-VD9 – D226B diodes, they can be replaced with a foreign analogue 1N4007;
• F – 5 A fuse.

It is necessary to make a digression to tell how to make the TR1 transformer. It is made on the basis of Sh40 iron, with a set thickness of 70 mm. For the primary winding you will need PEV2 wire Ø0.8 mm. The number of turns in the winding is 300.

To make a secondary winding, you will need a Ø4 mm stranded copper wire. It can be replaced with a tire, provided that its cross-section is at least 20 mm 2. The number of turns of the secondary winding is 10.

Video: do-it-yourself resistance welding

As for TR2, any of the low-power transformers (from 5 to 10 W) will be suitable for it. In this case, winding II, used to connect the backlight lamp “H”, should have an output voltage within 5-6 V, and winding III – 15 V.

The power of the manufactured device will be relatively low, ranging from 300 to 500 A, maximum pulse time up to 0.1 sec (provided that the ratings “R” and “C” are the same as in the diagram shown). This is quite enough for welding steel wire Ø0.3 mm or sheet metal if its thickness does not exceed 0.2 mm.

Let us present a diagram of a more powerful device, in which the welding electric current of the pulse will be in the range from 1.5 kA to 2 kA.

We list the components used in the circuit:

• resistance ratings: R1-1.0 kOhm, R2-4.7 kOhm, R3-1.1 kOhm;
• capacitances in the circuit: C1-1.0 µF, C2-0.25 µF. Moreover, C1 must be designed for a voltage of at least 630 V;
• VD1-VD4 diodes - D226B diodes, replacement with a foreign analogue 1N4007 is allowed, instead of diodes you can install a diode bridge, for example, KTs405A;
• thyristor VD6 - KU202N, it must be placed on a radiator with an area of ​​at least 8 cm2;
• VD6 – D237B;
• F – 10 A fuse;
• K1 is any magnetic starter that has three pairs of working contacts, and the winding is designed for ~220 V, for example, you can install PME071 MVUHLZ AC3.

Now we’ll tell you how to make transformer TR1. The LATR-9 autotransformer, such as shown in the photograph, is used as a basis.

The winding in this autotransformer has 266 turns, it is made with copper wire Ø1.0 mm, we will use it as the primary. We carefully disassemble the structure so as not to damage the winding. We dismantle the shaft and the movable roller contact attached to it.

Next, we need to isolate the contact track; for this purpose, we clean it from dust, degrease it and varnish it. When it dries further, we insulate the entire winding using varnished cloth.

As a secondary winding we use copper wire with a cross-sectional area of ​​at least 80 mm 2. It is important that the insulation of this wire is heat resistant. When all conditions are met, we make a winding of three turns.

Setting up the assembled device comes down to calibrating the scale of the variable resistor that regulates the pulse time.

We recommend that before starting welding, you experimentally establish the optimal time for the pulse. If the duration is excessive, the parts will be burned, and if it is less than necessary, the strength of the connection will be unreliable.

As already written above, the device is capable of delivering a welding electric current of up to 2000 A, which allows you to weld steel wire Ø3 mm or sheet steel, the thickness of which does not exceed 1.1 mm.

The easiest to manufacture are AC resistance spot welding machines with unregulated current. The welding process is controlled by changing the duration of the electrical pulse - using a time relay or manually using a switch.

Before considering the designs of homemade devices for resistance spot welding, we should recall the Lenz-Joule law: when an electric current passes through a conductor, the amount of heat generated in the conductor is directly proportional to the square of the current, the resistance of the conductor and the time during which the electric current flowed through the conductor ( Q=I 2 R t). This means that at 1000A current, about 10,000 times more energy is lost on poorly made connections and thin wires than at 10A current. Therefore, the quality of the electrical circuit cannot be neglected.

Transformer. The main component of any equipment for resistance spot welding is a power transformer with a high transformation ratio (to provide high welding current). Such a transformer can be made from a transformer from a powerful microwave oven (the power of the transformer should be about 1 kW or higher) feeding the magnetron.

These transformers are distinguished by their availability and high power. Such a transformer is enough for a precision welding machine capable of welding steel sheets 1 mm thick. If you need a more powerful spot welding machine, you can use two (or more) transformers (how to organize this is described below).

In a microwave oven, the magnetron requires a very high voltage (about 4000V) to operate. Therefore, the transformer feeding the magnetron does not step down, but increases. Its primary winding has fewer turns than the secondary, and the thickness of the winding wire is greater.

The output of such transformers is up to 2000V (a double voltage is supplied to the magnetron), so you should not check the performance of the transformer by connecting it to the network and measuring the voltage at the output.

Such a transformer requires a magnetic core and a primary winding (the one with fewer turns and a thicker wire). The secondary winding is cut off with a hacksaw or chopped off with a chisel (if the magnetic circuit is securely welded and not glued), knocked out with a rod or drilled out and picked out. The need for drilling arises when the winding is packed very tightly into the window and an attempt to knock it out can lead to destruction of the magnetic circuit.

When removing the secondary winding, care must be taken not to damage the primary winding.

In addition to two windings, shunts that limit the current can be built into the transformer; they must also be removed.

After removing unnecessary elements from the transformer, a new secondary winding is wound. To provide a large current close to 1000A, a thick copper wire with a cross-sectional area of ​​more than 100 mm 2 (wire with a diameter of more than 1 cm) is required. This can be either a single stranded wire or a bundle of several wires of small diameter. If the wire insulation is thick and prevents you from making a sufficient number of turns, then it can be removed and the wire wrapped with fabric insulating tape. The length of the wire should be as short as possible so as not to create additional resistance.

2-3 turns are made. The output should be about 2V, this will be enough. If you manage to cram more turns into the transformer windows, then the output voltage will be greater, therefore the current will be longer (in comparison with fewer turns of wire of the same diameter) and the power of the device.

If there are two identical transformers, then they can be combined into one, more powerful current source. This may be required when there are two transformers with insufficient power or when you want to make your own spot welding machine to work with thicker metal.

For example, in the case of insufficiently powerful transformers, each of the 0.5 kW transformers has an input voltage of 220V, the output voltage is 2V at nominal current 250A (the value is taken as an example, let the short-term welding current be 500A). Connecting namesake conclusions of the primary and secondary windings, we get a device in which, at the same voltage value (2V) nominal the output current value will be 500A (the welding current will almost double, and there will be more losses due to resistances).

At the same time, the connections in the circuit of secondary windings shown in the diagram must be on electrodes, that is, in the case of two transformers with a power of 0.5 kW there will be two identical wires with a diameter of 1 cm, the ends of which are connected to the electrodes.

If you make a mistake in connecting the terminals of the primary or secondary windings, there will be a short circuit.

If there are two sufficiently powerful transformers and you need to increase the voltage, and the dimensions of the magnetic circuit window do not allow you to make the required number of turns with a thick wire on one transformer, then the secondary windings of the two transformers are connected in series (one wire is pulled through two transformers), with the same number of turns on each transformer . The direction of the turns must be consistent so that there is no antiphase and, as a result, the output voltage is close to zero (you can experiment with thin wires first).

Typically, in transformers, winding terminals of the same name are always marked. If for some reason they are unknown, then they can be determined by performing a simple experiment, the diagram of which is shown below.

Here, the input voltage is supplied to the series-connected primary windings of two identical transformers, and an alternating voltage voltmeter is connected at the output formed by the series connection of the secondary windings. Depending on the direction in which the windings are turned on, there can be two cases: the voltmeter shows some voltage or the output voltage is zero. The first case indicates that in both the primary and secondary circuits the opposite terminals of the corresponding windings are interconnected. In fact, the voltage on each of the primary windings is equal to half the input and is transformed in the secondary windings with the same transformation ratios. When the secondary windings are turned on as indicated, the voltages on them are summed up and the voltmeter gives twice the voltage value of each winding. A zero voltmeter reading indicates that equal voltages on the secondary windings of transformers connected in series have opposite signs and, therefore, any pair of windings is connected by terminals of the same name. In this case, by changing, for example, the sequence of connecting the terminals of the primary windings as shown in Figure (b), we will obtain at the output twice the value of the output voltage of each of the secondary windings and we can assume that the windings of the transformer are connected different names conclusions. Obviously, the same result can be obtained by changing the sequence of connecting the terminals of the secondary windings.

To make a more powerful spot welding machine with your own hands, you can connect more transformers in the same way, if only the network allows it. A transformer that is too powerful will cause a large voltage drop in the network, causing fuses to trip, light bulbs to flicker, neighbors to complain, etc. Therefore, the power of homemade spot welding machines is usually limited to values ​​that provide a welding current of 1000-2000A. The lack of current is compensated by increasing the welding cycle time.

Electrodes. Copper rods (rods) are used as electrodes. The thicker the electrode, the better; it is desirable that the diameter of the electrode is not less than the diameter of the wire. Tips from powerful soldering irons are suitable for low-power devices.

The electrodes must be sharpened periodically, because they lose their shape. Over time, they wear down completely and require replacement.

As already written, the length of the wire running from the transformer to the electrodes should be minimal. There should also be a minimum of connections, because There is a loss of power on every connection. Ideally, copper lugs are placed on both ends of the wire, through which the wire is connected to the electrodes.

The tips must be soldered to the wire (the wire cores must also be soldered). The fact is that over time (possibly at the very first start), oxidation of copper occurs at the contacts, leading to an increase in resistance and a large loss of power, which is why the device may stop welding. Plus, when crimping tips, the contact area is smaller than when soldering, which also increases the contact resistance.

Due to the large diameter of the wire and tip for it, it is not easy to solder them, but sold tin-plated solder tips can make this task easier.

Unsoldered connections between tips and electrodes also create additional resistance and oxidize, but since The electrodes must be removable; it is inconvenient to unsolder the old ones and solder in the new ones each time you replace them. Moreover, this connection is much easier to clean from oxides than the end of a stranded wire crimped with a ferrule.

Controls. The only controls may be a lever and a switch.

The compression force between the electrodes must be sufficient to ensure contact of the parts being welded with the electrodes, and the thicker the sheets being welded, the greater the compression force must be. On industrial devices, this force is measured in tens and hundreds of kilograms, so the lever should be made longer and stronger, and the base of the device should be more massive and can be attached to the table with clamps.

A large clamping force for homemade spot welding machines can be created not only with a lever clamp, but also with a lever-screw clamp (a screw tie between the lever and the base). Other methods are possible, requiring different equipment.

The switch must be installed in the primary winding circuit, because there is a very large current in the secondary winding circuit and the switch will create additional resistance, in addition, the contacts in a regular switch can be tightly welded.

In the case of a lever clamping mechanism, the switch should be mounted on the lever, then with one hand you can press the lever and turn on the current. The second hand will remain free to hold the parts being welded.

Exploitation. It is necessary to turn the welding current on and off only when the electrodes are compressed, otherwise intense sparking occurs, leading to burning of the electrodes.

It is advisable to use forced cooling of the device using a fan. In the absence of the latter, you need to constantly monitor the temperature of the transformer, conductors, electrodes and take breaks to prevent them from overheating.

The quality of welding depends on the experience gained, which comes down mainly to maintaining the required duration of the current pulse based on visual observation (by color) of the weld point. More information about performing spot welding is written in the article Contact spot welding.

Video:

When using the content of this site, you need to put active links to this site, visible to users and search robots.