Inverter: DC or AC? Features of AC welding DC welding machine

20 years ago, at the request of a friend, I built him a reliable welder to work on a 220-volt network. Before this, he had problems with his neighbors due to a voltage drop: an economical mode with current regulation was required.

After studying the topic in reference books and discussing the issue with colleagues, I prepared electrical diagram control on thyristors, mounted it.

In this article, based on personal experience, I tell you how I assembled and configured the welding machine DC with your own hands based on a homemade toroidal transformer. It came out in the form of a small instruction.

I still have the diagram and working sketches, but I can’t provide photographs: there were no digital devices then, and my friend moved.

Versatile capabilities and tasks performed

A friend needed a machine for welding and cutting pipes, angles, sheets of different thicknesses with the ability to work with 3÷5 mm electrodes. Welding inverters were not known at that time.

We settled on the DC design, as it is more universal and provides high-quality seams.

Thyristors removed the negative half-wave, creating a pulsating current, but did not smooth out the peaks to an ideal state.

The welding output current control circuit allows you to adjust its value from small values for welding up to 160-200 amperes required when cutting with electrodes. She:

made on a board from thick getinax;
covered with a dielectric casing;
mounted on the housing with the output of the adjusting potentiometer handle.

The weight and dimensions of the welding machine were smaller compared to the factory model. We placed it on a small cart with wheels. To change jobs, one person rolled it freely without much effort.

The power cord was connected through an extension cord to the connector of the input electrical panel, and the welding hoses were simply wound around the body.

Simple design of DC welding machine

Based on the installation principle, the following parts can be distinguished:

homemade transformer for welding;
its power supply circuit is from network 220;
output welding hoses;
power unit of a thyristor current regulator with an electronic control circuit from a pulse winding.

Pulse winding III is located in power zone II and is connected through capacitor C. The amplitude and duration of the pulses depend on the ratio of the number of turns in the capacitor.

How to make the most convenient transformer for welding: practical tips

Theoretically, you can use any model of transformer to power the welding machine. The main requirements for it:

provide arc ignition voltage at idle speed;
reliably withstand the load current during welding without overheating the insulation from prolonged operation;
meet electrical safety requirements.

In practice, I have come across different designs of homemade or factory-made transformers. However, they all require electrical engineering calculations.

I have been using a simplified technique for a long time, which allows me to create fairly reliable transformer designs of medium accuracy class. This is quite enough for household purposes and power supplies for amateur radio devices.

It is described on my website in the article This is an average technology. It does not require clarification of the grades and characteristics of electrical steel. We usually don’t know them and cannot take them into account.

Features of core manufacturing

Craftsmen make magnetic wires from electrical steel of various profiles: rectangular, toroidal, double rectangular. They even wind coils of wire around the stators of burnt-out powerful asynchronous electric motors.

We had the opportunity to use decommissioned high-voltage equipment with dismantled current and voltage transformers. They took strips of electrical steel from them and made two donut rings out of them. The cross-sectional area of each was calculated to be 47.3 cm 2 .

They were insulated with varnished cloth and secured with cotton tape, forming a figure of a reclining figure eight.

They began to wind the wire on top of the reinforced insulating layer.

Secrets of the power winding device

The wire for any circuit must have good, strong insulation, designed to withstand long work when heated. Otherwise, it will simply burn during welding. We proceeded from what was at hand.

We received a wire with varnish insulation, covered with a fabric sheath on top. Its diameter - 1.71 mm is small, but the metal is copper.

Since there was simply no other wire, they began to make the power winding out of it with two parallel lines: W1 and W’1 with the same number turns - 210.

The core donuts were mounted tightly: this way they have smaller dimensions and weight. However, the flow area for the winding wire is also limited. Installation is difficult. Therefore, each power half-winding was separated into its own magnetic circuit rings.

In this way we:

doubled the cross-section of the power winding wire;
saved space inside the donuts to accommodate the power winding.

Wire alignment

You can get a tight winding only from a well-aligned core. When we removed the wire from the old transformer, it turned out to be bent.

We figured out the required length in our minds. Of course it wasn't enough. Each winding had to be made from two parts and spliced with a screw clamp directly on the donut.

The wire was stretched along its entire length on the street. We picked up the pliers. They clamped the opposite ends and pulled with force in different directions. The vein turned out to be well aligned. They twisted it into a ring with a diameter of about a meter.

Technology of winding wire on a torus

For the power winding, we used the rim or wheel winding method, when a large-diameter ring is made of wire and wound inside the torus by rotating one turn at a time.

The same principle is used when putting a winding ring on, for example, a key or keychain. After the wheel is inserted inside the donut, they begin to gradually unwind it, laying and fixing the wire.

This process was well demonstrated by Alexey Molodetsky in his video “Winding a torus on a rim.”

This work is difficult, painstaking, and requires perseverance and attention. The wire must be laid tightly, counted, the process of filling the internal cavity must be monitored, and the number of turns wound must be recorded.

How to wind a power winding

We found it for her copper wire suitable section - 21 mm 2. We estimated the length. It affects the number of turns, and the no-load voltage necessary for good ignition of the electric arc depends on them.

We made 48 turns with the middle terminal. In total, there were three ends on the donut:

middle - for direct connection of the “plus” to the welding electrode;
the extreme ones - to the thyristors and after them to ground.

Since the donuts are fastened together and the power windings are already mounted on them along the edges of the rings, the winding of the power circuit was carried out using the “shuttle” method. The aligned wire was folded like a snake and pushed through the holes of the donuts for each turn.

The middle point was unsoldered using a screw connection and insulated with varnished cloth.

Reliable welding current control circuit

The work involves three blocks:

stabilized voltage;
formation of high-frequency pulses;
separation of pulses into circuits of thyristor control electrodes.

Voltage stabilization

An additional transformer with an output voltage of about 30 V is connected from the power winding of the 220 volt transformer. It is rectified by a diode bridge based on D226D and stabilized by two zener diodes D814V.

In principle, any power supply with similar electrical characteristics of current and output voltage can work here.

Pulse block

The stabilized voltage is smoothed by capacitor C1 and supplied to pulse transformer through two bipolar transistors of direct and reverse polarity KT315 and KT203A.

Transistors generate pulses to the primary winding Tr2. This is a toroidal type pulse transformer. It is made of permalloy, although a ferrite ring can also be used.

Winding of three windings was carried out simultaneously with three pieces of wire with a diameter of 0.2 mm. Made 50 turns. The polarity of their inclusion matters. It is shown by dots in the diagram. The voltage on each output circuit is about 4 volts.

Windings II and III are included in the control circuit for power thyristors VS1, VS2. Their current is limited by resistors R7 and R8, and part of the harmonic is cut off by diodes VD7, VD8. Appearance We checked the pulses with an oscilloscope.

In this chain, the resistors must be selected for the voltage of the pulse generator so that its current reliably controls the operation of each thyristor.

The unlocking current is 200 mA, and the unlocking voltage is 3.5 volts.

There are many types of welding machines, among which the most famous are the following: mechanical welding devices using consumable electrodes; equipment for argon-arc welding with non-consumable electrodes; for welding using flux with automatically consumable electrodes. In addition, there are generators for welding, transformers, inverters and devices for resistance spot welding. There are specific electrodes for working with each type of metal.

In terms of its design, an apparatus for working with direct current is much more complex than an alternating current unit, since in it, to obtain an output DC voltage A rectifier with a diode or thyristor bridge is installed. However, the output power of the welding machine is significantly less than that consumed due to its drop on the rectifier itself.

In other words, its efficiency is low, and this is a serious drawback from the point of view of energy savings. However, thanks to the stable arc and the ability to work with various metals, it could be classified as professional equipment.

AC welding machine - what is its feature?

Much cheaper than the previous model AC welding machine, also working with consumable electrodes. It is excellent for working with ferrous metals, allowing them to be overlapped and butt welded.

If this welding machine is used, 220 volts is the operating voltage, but at no load it may vary depending on the electrodes used, which can be either calcium fluoride or rutile coated. The device is very easy to use and provides for smooth adjustment of the current strength, which depends on the electrode selected for operation.

This transformer welding machine can be successfully used both at home and in factories. Electric welding machines are designed to operate from a 220 or 380 volt network and are accordingly called single- or three-phase. Depending on this, the connection diagram of the welding wires changes.

A single-phase welding machine is connected by connecting one welding wire to the “phase”, another to the “neutral” connector and a third to the “zero” ground. Otherwise, a three-phase welding machine is connected. The two ends of the welding cable are connected to any two “phases”, and the third - to the protective “zero”.

It should be noted that if a 380-volt welding machine is used, then it is considered more powerful than one connected to a 220-volt network, but this is not the only way to increase productivity.

Inverters - increasing the power of the welding machine

Until now, we have considered welding machines that use a conventional power transformer as an input voltage converter. This is what determines the solid dimensions and heavy weight of this type of equipment. However, it is reliable and inexpensive.

But there are other types of devices in which so-called inverters– semiconductor amplifiers. Small dimensions and weight have made them perhaps the most popular type of welding units.

With an efficiency level reaching 85%, the device works with different metals, guaranteeing high speed, quality and accuracy of welding. Inverter devices have different powers and can be connected to networks of both 220 and 380 volts.

Using alternating current, it is only possible to weld ordinary low-carbon steel (except for welding with an oscillator). In practice, there are many cases of welding parts made of cast iron, medium and high carbon steel, non-ferrous metals, and alloy steel. A constant current is required here. The fact is that electrodes for the above metals burn steadily mainly at direct current. In addition, the use of an arc of direct or reverse polarity provides additional technological advantages.

Professional welding of pressure vessels is also performed using direct current.

Diagram of a homemade DC welding machine

Transformer Tr 1 is a regular welding transformer, without any modifications. It is better if it has a rigid characteristic, that is, the secondary winding is wound on top of the primary. Diodes D 1 – D 4 – any, designed for a current of at least 100 A.

Diode radiators are selected in such an area that the heating of the diodes during operation does not exceed 100°C. A fan can be used for additional cooling.

Capacitor C1 is a composite of oxide capacitors with a total capacity of at least 40,000 μF. Capacitors can be used of any brand with a capacity of 100 μF each, including them in parallel. The operating voltage is at least 100 V. If such capacitors overheat during operation, then their operating voltage should be at least 150 V. It is possible to use capacitors of other ratings.

If you plan to work only at high currents, then you don’t need to install capacitors at all. Choke Dr 1 is the usual secondary winding of a welding transformer. It is desirable that the core be made of rectangular plates. No bias current flows through it. If a toroidal core is used, then it is necessary to cut through the magnetic gap in it with a hacksaw.

Resistor R 1 is a wire resistor. You can use steel wire with a diameter of 6 - 8 mm and a length of several meters. The length depends on the secondary voltage of your transformer and the current you want to draw. The longer the wire, the less current. For convenience, it is better to wind it in the form of a spiral.

The resulting welding rectifier allows for straight and reverse polarity welding.

Welding with straight polarity - “minus” is applied to the electrode, “plus” is applied to the product.

Reverse polarity welding - “plus” is applied to the electrode, “minus” is applied to the product (shown in Fig. 4. 1.).

If the transformer Tr 1 has its own current regulation, then it is best to set the maximum current on it, and extinguish the excess current with resistance R 1.

Welding cast iron

The practice of private welders has developed two reliable and effective methods of welding cast iron.

The first is used for welding products of a simple configuration, where cast iron can “stretch” after the cooling seam. It should be borne in mind that cast iron is a completely non-ductile metal, and each cooling seam causes transverse shrinkage of approximately 1 mm.

In this way, you can weld a fallen-off eye of a frame, a cast-iron body that has burst in half, and so on.

Before welding, the crack is cut using a V-shaped groove across the entire thickness of the metal.

You can weld the groove with any electrode, although the best results are obtained by welding with a UONI brand electrode (with any numbers) using direct current of reverse polarity.

Overlays should be welded in all possible places. The more there are, the stronger the welded joint. The linings should be welded along the current force.

Welded structures with overlays are often stronger than the original cast iron.

The second method is designed for products with complex configurations: cylinder blocks, crankcases, and so on. Most often it is used to eliminate leaks of various liquids.

Before welding, the crack is cleaned of dirt, oil, and rust.

For welding, a copper electrode of the Komsomolets brand with a diameter of 3–4 mm is used. Constant current of reverse polarity.

Before welding, the crack or patch is placed on spot tacks.

Welding is carried out with short, scattered seams. The first seam is made anywhere. Its length is no more than 3 cm.

Immediately after welding the seam, it is intensively hammered.

The cooling seam decreases in size, and forging, on the contrary, expands it. Forging takes about half a minute.

Then wait until the metal cools completely. Cooling is controlled by hand. If touching the seam does not cause pain, weld a second short seam of the same length.

The second and all subsequent seams are welded as far as possible from the previous ones. After welding each short seam there is forging and cooling.

The last to be welded are the closing sections between the short seams. The result is a continuous seam.

Determination of steel grade by spark

In repair practice there are quite a lot of cases of welding steels unknown by chemical composition. Without determining the composition of such steels, their high-quality welding is impossible.

There is a method for determining the carbon content in steel with an accuracy of ±0.05%. It is based on the contact of the metal being tested with a rotating emery wheel. By the shape of the sparks formed, one can judge both the percentage of carbon and the presence of alloying impurities.

The carbon in the separated metal particles burns, forming star-shaped flares. The asterisks characterize the carbon content of the steel being tested. The higher the carbon content in it, the more intensely the carbon particles burn and the greater the number of stars (Fig. 4. 7.).

It is advisable to carry out such a test on a carborundum wheel with a grain size of 35 - 46. The rotation speed is 25 - 30 m/sec. The room must be darkened.

1 – the spark looks like a light, long, straight line with two thickenings at the end, of which the first is light and the second is dark red. The entire beam of sparks is light and has an oblong shape;

2 – new light sparks begin to separate from the first thickening. The beam of sparks becomes shorter and wider than the previous one, but also light.

3 – the beam of sparks is shorter and wider. A whole sheaf of light yellow sparks is separated from the first thickening;

4 – at the ends of the sparks separating from the first thickening, brilliant white stars are observed;

5 – long sparks of a reddish color with characteristic separating stars are formed;

6 – long intermittent (dotted) spark of dark red color with a light thickening at the end;

7 – double intermittent (dotted) spark with light thickenings at the ends, thick and long - red, thin and short - dark red;

8 - the spark is the same as in point No. 7, with the only difference being that the sparks have a gap.

Training in the spark test method should begin with samples of well-known steel grades.

When using this method, it should be taken into account that hardened steel produces a shorter spark beam than unhardened steel.

The spark test must be taken at a depth of 1 - 2 mm from the surface, since there may be a decarbonized layer on the surface of the metal.

When non-ferrous metals and their alloys, in which there is no carbon, come into contact with the emery wheel, no sparks are produced.

Welding medium and high carbon steel

Medium-carbon steels are welded with electrodes with a low carbon content. The penetration depth should be small, so direct current of direct polarity is used. The current value is selected to be reduced.

All these measures reduce the carbon content in the weld metal and prevent the occurrence of cracks.

For welding, electrodes UONI-13/45 or UONI-13/55 are used.

Some products must be heated to a temperature of 250 - 300°C before welding. Full heating of the product is best; if this is not possible, then local heating is used gas burner or a cutter. Heating to a higher temperature is unacceptable, as it causes cracks to appear due to an increase in the penetration depth of the base metal and the resulting increase in carbon content in the weld metal.

After welding, the product is wrapped in thermal insulating material and allowed to cool slowly.

If necessary, heat treatment is carried out after welding: the product is heated to a dark cherry color and slow cooling is provided.

High carbon steel is the most difficult to weld. Welded structures are not made from it, but welding is used in repair production. For welding such steel, it is best to use the same methods as described earlier for welding cast iron.

Welding manganese steel

Manganese steel is used for parts with high wear resistance: dredge buckets, excavator bucket teeth, railway crosses, rock crusher necks, tractor tracks, and so on.

Electrodes TsL-2 or UONI-13nzh are used for welding.

The welding current is selected at the rate of 30 - 35A per 1 mm of electrode diameter.

Welding produces a large amount of gases. To facilitate their exit from the molten metal, surfacing should be carried out in wide beads and short sections, otherwise the weld will be porous.

Immediately after welding, forging is required.

To increase the hardness, strength, toughness and wear resistance of the surfacing, it is necessary to quench it with cold water after applying each bead, while it is still heated to red heat.

Welding Chromium Steel

Chromium steels are used as stainless and acid-resistant steels for the manufacture of equipment for the oil refining industry.

Welding of chromium steels must be performed with preheating to a temperature of 200 - 400°C.

When welding, a reduced current is used at the rate of 25 - 30 A per 1 mm of electrode diameter.

Electrodes TsL-17-63, SL-16, UONI-13/85 are used on direct current of reverse polarity.

After welding, the product is cooled in air to a temperature of 150 - 200°C, and then tempered.

Tempering is carried out by heating the product to a temperature of 720 - 750°C, holding it at this temperature for at least an hour and then slowly cooling it in air.

Welding tungsten and chrome tungsten steel

This steel is used to make cutting tools.

Using welding, a cutting tool can be made in two ways:

1) by welding finished high-speed steel plates onto a low-carbon steel holder;

2) surfacing high-speed steel onto low-carbon steel.

The finished plates are welded in the following ways:

1) using resistance welding;

2) using argon welding with a non-consumable electrode;

3) using gas soldering with high-temperature solder;

4) DC consumable electrode.

For surfacing, you can use waste high-speed steel: broken drills, cutters, countersinks, reamers, etc.

These wastes can be welded using gas or argon welding, as well as by making electrodes for electric arc welding.

After surfacing, the tool is annealed, mechanically processed, and then subjected to triple hardening and tempering.

Welding high alloy stainless steel

Stainless steel has found quite wide application in everyday life: various containers, heat exchangers, and water heaters are made from it. Used in private baths as heat-resistant.

You can distinguish such steel from ordinary steel by three characteristic features:

1) “stainless steel” has a light steel color;

2) when a permanent magnet is applied, it is not attracted, although there are exceptions;

3) when processed on an emery wheel, it produces few sparks (or none at all).

Stainless steel has an increased linear expansion coefficient and a reduced thermal conductivity coefficient.

The increased coefficient of linear expansion causes large deformations of the welded joint, up to the appearance of cracks. It is advisable to heat some welded stainless steel structures to a temperature of 100 - 300°C before welding.

Low thermal conductivity causes heat concentration and can lead to metal burning. Compared to welding conventional steel of the same thickness, when welding stainless steel, the current is reduced by 10 - 20%.

For welding, direct current of reverse polarity is used.

Electrodes of the OZL-8, OZL-14, ZIO-3, TsL-11, TsT-15-1 brands are used.

One of the main conditions when welding is maintaining a short arc, this provides better protection of the molten metal from oxygen and nitrogen in the air.

The corrosion resistance of seams increases with accelerated cooling. Therefore, immediately after welding, the seams are watered. Watering is permissible only for steel that does not crack after welding.

Welding of aluminum and its alloys

Welding with coated electrodes is used for aluminum and alloys with a thickness of more than 4 mm.

For welding technical aluminum, electrodes of the OZA-1 brand are used.

OZA-2 electrodes are used to weld casting defects.

Recently, OZA brand electrodes have been replaced by more advanced OZANA brand electrodes.

The coating of electrodes for aluminum welding strongly absorbs moisture. When storing such electrodes without moisture protection, the coating can literally drain from the rod. Therefore, such electrodes are stored in a plastic case with moisture absorption means. Before welding, they are additionally dried at a temperature of 70 – 100°C.

Before welding, aluminum parts are degreased with acetone and cleaned to a shine with a wire brush.

Welding is carried out using direct current of reverse polarity.

Welding current 25 - 32 A per 1 mm diameter of the electrode rod.

Before welding, the part is heated to a temperature of 250 - 400°C.

Welding must be performed continuously with one electrode, since the slag film on the part and the end of the electrode prevents the arc from re-igniting.

If possible, with reverse side linings are placed at the seam (see gas welding of aluminum).

Electric arc welding produces seams of average quality.

Welding of copper and its alloys

Pure copper lends itself well to welding, and it is recommended to weld it in two ways. The welding method depends on the thickness of the part.

When the thickness of the product is no more than 3 mm, it is best to use carbon electrode welding. Welding is performed with direct current of direct polarity with an arc length of 35 - 40 mm.

Electrical wire can be used as a filler material. Don't forget to remove the insulation before welding.

To improve the quality of the seam, a flux consisting of 95% calcined borax and 5% metal powdered magnesium is applied to the edges being welded and to the filler wire. You can use borax alone, but the results will be worse. If high quality welds are not required, flux is not used.

Safety precautions for electric arc welding

Electric arc welding has several factors harmful to the health of the welder: voltage electric current, electric arc radiation, gases, sparks and metal splashes, thermal heating, drafts.

The maximum permissible no-load voltage of a welding transformer is considered to be 80 V, and of a welding rectifier 100 V. In dry weather conditions, such voltage is practically not felt, but in humid conditions, a rather noticeable tingling of the hand begins. The same thing can be observed when the welder is on the metal part being welded, and even more so inside it.

When welding in wet weather, as well as standing on metal, regardless of the weather, you must use rubber gloves, a rubber mat, and rubber galoshes. Gloves, mats and galoshes should be made of dielectric rubber, that is, the kind that electricians use. Rubber products sold for household use are not electrically insulating.

To protect the welder from accidental breakdown of the transformer, protective grounding is used. The grounding device is described in Chapter 1.

To reduce the likelihood of electric shock, it is best to use transformers with low open circuit voltage.

Protection against arc radiation is a welder's suit, a mask with a set of glasses, and gloves. Always fasten the top collar of your suit, otherwise you will end up with a permanent “tie.”

The ultraviolet radiation of the arc is reliably attenuated by an air column of 10 m, so do not allow anyone closer than 10 m to the welding site (especially children!).

The coating of the electrodes contains gas-forming substances, so the coated electrodes smoke heavily. The only way to protect against smoke is forced ventilation. The design of such ventilation is described in Chapter 1.

Another unfavorable factor in a welder’s work is associated with ventilation – drafts. The welder's load during work is most often static, that is, the welder works almost motionless. In this case, the body does not self-heat, which can lead to hypothermia.

As the experience of many welders shows, no amount of hardening against drafts helps. More reliable protection is warm clothing, especially around the waist (the welder works bent over).

Warm clothing can also have a negative effect. When switching to dynamic load, the welder begins to sweat; sweat, together with a draft, causes a guaranteed cold.

The best option to avoid colds is to install a supply fan heater. It should heat the supply air to above-zero temperatures even in severe frost. If you prefer not to work in such frosts, then the fan power of 3 kW is sufficient.

Metal splashes are considered a rather unpleasant phenomenon. When they get on a suit or shoes, they cause smoldering of protective clothing or a fire if there are flammable substances nearby. Buy leather protective clothing and tarpaulin boots - and you will sufficiently protect your body.

When welding at high currents and arc cutting metal, the electrode holder, welding wires and welding helmet may overheat. Therefore, do not touch the metal parts of the mask with your face, and put a heat-insulating sleeve on the handle of the holder. Check all wire connections regularly as they may cause a fire.

The above rules also apply to other types of electric welding: argon, semi-automatic, contact.

Almost all last century welding work were produced on alternating current if gas welding was not used. This was due to the fact that simpler and inexpensive welding equipment was not available in industry and construction.

The AC welding machine was a powerful step-down transformer with a current regulator in the form of a movable secondary winding or additional taps in it. They were reliable simple devices, while being very heavy and bulky. But thanks to the development of semiconductor technology, it became possible to create a DC welding machine, which in terms of consumer properties is better than its “changeable” brother.

The use of direct current allows you to obtain a better quality seam due to the fact that the electric arc is stable. There are no zero crossings like an AC unit, so no splashing.

The ability to use direct and reverse polarity allows you to weld stainless steel and non-ferrous metals, that is, direct current electric arc welding has a wider range of applications, all other things being equal. When using inverters, the welding machine is significantly smaller in size and weight.

The disadvantages are the relatively high cost (compared to AC devices) and sensitivity to dust. You have to clean the indoor units often.

Devices on transformers

The first models of permanent welding machines were a development of alternating current devices. In addition to the welding transformer, a diode rectifier made according to a bridge circuit was mounted at the output of the secondary winding, then powerful capacitors were connected to reduce ripple and a choke to obtain a more stable arc.

From a single-phase or three-phase network, alternating voltage was supplied to the primary winding of the step-down transformer. At the output of the secondary, a voltage of about 70 V was obtained at idle, then it went to the rectifier and welding electrode.

When the electrode was shorted to ground and then separated for a short distance (approximately 5 mm), an electric arc occurred. The welder had to move the electrode along the future seam at the speed necessary to form a weld pool.

Inverters

They work according to the same principle welding inverters, which also belong to direct current devices. Transformations in them occur somewhat differently.

The input mains voltage of 220 V is immediately converted by the rectifier into direct current. Using a filter low frequencies the ripples are smoothed out, and the current, as a supply current, is supplied to the master oscillator, power bipolar or field-effect transistors.

The generator produces a signal with a frequency of 40 to 80 kHz. Changing the frequency with a variable resistor located on the front panel allows you to adjust the strength of the welding current. This frequency is supplied to the control inputs of power transistors, and the output results in a pulsed current of the same frequency.

For further conversion, it is passed through capacitors to produce high-frequency alternating current. It is then fed to a step-down transformer.

A reduced high-frequency voltage is removed from the secondary winding. Thanks to this, such bulky converters (low-frequency step-down transformers) are not required. in this case it turns out compact and ergonomic.

The resulting high-frequency current is rectified again by the diode bridge and turns into direct current. To reduce pulsations, capacitor banks are installed, and a choke is installed to soften the arc. Thanks to electronic circuit controls the strength of the welding current and voltage, there are no power dips and arc instability.

The welding current does not depend on changes in the mains voltage. The seam is of high quality. It is much easier for a welder to work with such a welding machine. Only, when using electric welding, it is necessary to comply with the requirements for filler wire.

Electrodes for welding should be used those that are recommended for this type of metal. The diameter must be selected based on the thickness of the material being welded.

Which electrodes to use

When selecting electrodes for welding parts with direct current, first of all you need to make sure that there are certificates of conformity.

They must be confirmed by relevant organizations such as the Center for Standardization and Metrology with the appropriate licenses. Next, you need to select electrodes taking into account the power of the welding machine, the thickness of the parts being welded and the type of metal. Among the many brands are the following:

UONI13/45 electrodes are suitable for DC welding of low-carbon and low-alloy steels. They are good for welding pressure vessels, thick-walled parts, and also for welding casting defects;
UONI 13/55 electrodes are also used to weld low-carbon and low-alloy steels. Used in steel structures;
Electrodes OZS-12 GOST 9467-75 are used to weld critical structures made of low-carbon steel. Welding is carried out in all positions except for the vertical seam;
OZS-4 can be welded on an oxidized surface with the same steels.

The brands listed above are the most versatile and easy to use. They can be ignited quickly and provide a stable arc maintained by a constant current.

For medium and high alloy steels, special electrodes are used. They have a composition close to the grade of steel being welded.

Before using the electrodes, you must make sure that they are dry and without chipped coating. Correct selection of grade, diameter, and welding current will ensure a high-quality weld. All the necessary data is available in the operating instructions for the welding machine and the passport for the electrodes.

Self-production

It makes sense to make a DC welding machine yourself if you have a spare semiconductor devices suitable for the denominations. When using a traditional transformer current conversion circuit, everything will be quite cheap.

If you decide to assemble an inverter device, then buying power transistors will cost a pretty penny; it’s easier to buy a ready-made inverter.

Rectifier

Direct welding current in homemade machines is usually calculated at 160-200 amperes. For this purpose, B200 rectifier diodes connected via a bridge circuit will be optimal.

You just need to take into account that the case is not isolated from the insides of the diode, that is, when voltage is applied to the terminals, the case will also be energized.

Since they get very hot during operation, they are installed on radiators. They must be isolated from each other, the body of the welding equipment and other elements of the circuit.

If you have diode bridge assemblies at your disposal, then this is even better, since the circuit will be easier to assemble. They have a forward current of about 35-50 A. If a more powerful bridge is required, then the assemblies can be paired and placed in parallel.

The reliability of such a connection is less than that of a single diode due to the variation in parameters, but if installed with a reserve, then everything will be great. Their cases are not energized, so they can be installed on one radiator.

Other components

A homemade transformer-type DC welding machine consists of a step-down transformer with a power of 7 kW and above, a rectifier bridge using diodes of type V200, VL200 or several bridge diode assemblies, a set electrolytic capacitors a total power of 30,000 uF and a choke. Aluminum radiators and a fan are used to cool the diodes.

It is recommended to make all contacts by soldering to reduce the transition resistance at the joints. The welding transformer will have different dimensions depending on the power and conversion frequency used. This must be taken into account when designing the housing or selecting it.

Welding cables must be connected to the device via a bolted connection. In this version, there are practically no adjustments for DC welding.

If you have an AC welding machine, then by adding a rectifier circuit you can get a DC device, but with AC voltage adjustments, which is also good.

Making an inverter-type welding machine can be done by people versed in electronics. There is not such a wide variation in parameters as in a transformer apparatus.

The circuits are quite complex for a novice radio amateur, but if you follow all the rules for soldering microcircuits and semiconductor devices, especially field effect transistors, you can make the device with the required parameters.