Schematic diagram of the transformer of the microwave oven and its application

It is a common aspiration for car enthusiasts or those who enjoy repairing things to own a superb welder. There are numerous welding devices available in the market, but not everyone can afford them. However, if one has the desire, what can be done? Instead of immediately discarding a broken microwave at home, it is possible to transform it into a working welding machine with a little time and effort.

Spot welding machine

In this article, we will explore the concept of a microwave transformer and its practical uses.

Transformer

The microwave oven features a transformer, which proves extremely useful for constructing a welding device. This crucial component comprises two regular copper wire coils that are wound around a core. It consists of a primary winding and a secondary winding. The two winding coils have varying numbers of wire turns. This is essential to ensure that there is voltage in the primary winding during connection, while the secondary winding experiences a current generated through induction, resulting in a lower voltage. As a result, the current strength should increase.

Removing the transformer

When constructing a DIY welding device, a transformer with an average power output of 750 watts is utilized. This particular electromagnetic tool is classified as a step-up device and enables the joining of metal sheets up to one millimeter thick. In order to supply power to the magnetron, which is a powerful electronic component found in any microwave oven, the transformer is engineered with a specific structure. The first winding of the transformer has fewer turns to accommodate a lower voltage requirement for the magnetron’s operation. Conversely, the secondary winding has more turns to produce a voltage of 2000 V. However, this voltage is subsequently doubled using a custom-built doubler, rendering any voltage measurement unnecessary.

Take out the microwave oven’s transformer with caution and delicacy. Avoid using a hammer or any other bulky items. Initially, it is essential to unscrew the foundation of this culinary device, and subsequently, remove all attachments. Then, proceed with the careful extraction of the transformer from its current location. From the internal mechanisms of the ultra-high-frequency oven (UHF), you will require a magnet conductor and the primary winding. The primary winding consists of a thicker wire and a lesser number of turns.

The secondary winding can be removed by using a hammer or a chisel. It is important to proceed with caution to avoid any damage to the primary winding. If any shunts are discovered during this process, which limit the current strength, they should be removed.

If the magnetic core is welded instead of glued, the secondary winding can be eliminated with a carpentry tool such as a chisel.

An ordinary hacksaw can be used as a substitute for a chisel. If the magnet wire is tightly packed in the window, it is necessary to cut the wires and then remove them by drilling. It is important to be cautious during this process to prevent deformation of the magnet wire.

Once the dismantling is complete, it is necessary to wind a new secondary winding. A wire with a diameter of one centimeter is recommended for this. If a wire of this diameter is not available, it can be purchased. The wire does not need to be multicore, a bundle of individual conductors can be used as well. The key requirement is that it has the appropriate diameter. After the secondary winding is installed, the transformed will be able to produce an output current of 1 kA.

If you require an enhanced power output for your welding apparatus, relying solely on a single electromagnetic device may prove insufficient. In such cases, the implementation of two devices becomes imperative.

Features of the transformer upgrade

To create the secondary winding, it is essential to wrap the core with two or three turns. This will result in an output voltage of 2 V and a short-term current of 0.8 kA, providing sufficient performance for the spot welding device.

However, winding this number of turns can pose difficulties if the wire has a thick insulation layer. Fortunately, this issue can be easily resolved. Simply remove the standard insulation and replace it with a layer of duct tape made of cotton fabric.

Repositioning the new secondary winding:

It is essential to limit the length of the wire used for the secondary winding to minimize its resistance and ensure that the current remains stable.

When welding metal sheets up to five millimeters thick, a more powerful device is required. To achieve this, two electromagnetic devices should be connected in a single circuit. It is crucial to strictly follow the established rules to avoid any issues, such as a short circuit, with the connection of the primary and secondary windings. To verify the correctness of the connection, a voltage measuring device must be used.

Once the two electromagnetic devices have been properly interconnected, it is crucial to determine the current value. Typically, when it comes to transformers designed for domestic spot welding machines, current limits are established, typically not exceeding 2 kA. If the reading surpasses this value, it can lead to disruptions in the power grid. For this reason, it is recommended to employ an ammeter.

Tips for connecting two devices

Let’s suppose we have two transformers with identical specifications:

• Power rating – 500 W;
• Input voltage – 220 V;
• Output voltage – 2 V;
• Current – 250 A.

If we connect them correctly, the current value will double, resulting in 0.5 kA.

However, this increase in current will also lead to an increase in short-term current, which comes with losses. These losses occur due to the high resistance of the electric circuit. To mitigate this issue, it is necessary to connect both ends of the secondary winding to the electrodes of the spot welding unit.

There may be a situation where, if you have two transformers with a high power output, the voltage is not sufficient to operate the device. In such cases, it is necessary to connect their secondary windings, ensuring that they have the same number of turns.

When connecting them, it is crucial to make sure that the directionality of the turns is consistent. Failure to meet this condition can result in a counter-phase, leading to an output voltage that is essentially zero.

Identification of homologous leads

It may happen that the winding leads of electromagnetic devices which are to be combined are not labeled. Therefore, it becomes necessary to identify the homologous ones. The primary and secondary windings should be connected in series. After that, a voltage is applied to the input, and the connection of the alternating voltage meter to the output should be established.

The meter can manifest itself in different ways, depending on the direction of the connection.

The measuring device can detect the following:

• Show voltage.
• Not detect voltage in the circuit.

If the measuring instrument shows a reading, it means that there are opposing terminals in the circuit. This connection has been made incorrectly, so the following phenomena can be observed here:

• When the primary windings are exposed to a certain voltage, it decreases to less than half of its original value.
• Conversely, the secondary windings experience an increase in voltage.

As a result, the meter display will show a total voltage that is twice the input voltage.

If the measuring device shows a reading of zero, it means that the voltages produced by the secondary windings are equal in magnitude but have opposite signs. This implies that they are cancelling each other out. One of the pairs of windings is connected properly, with matching lead names.

Therefore, the voltmeter and its readings must be used to guide the correct connection.

Electrodes

When selecting electrodes, it is important to consider the diameter, which should match the wire diameter as they will be connected to it. Copper bars can be used for this purpose. If a low-powered device is being created, soldering iron stingers can be used instead.

During usage, the electrodes experience significant wear and tear, so it is necessary to regularly sharpen them and eventually replace them altogether.

Therefore, to connect the wire to the electrode, a copper tip is used and connected through soldering.

The tip and electrode are aligned using a bolted connection. This connection needs to be dependable, as any increase in resistance in the area of an unreliable contact will result in a loss of power for the device. To avoid this issue, it is necessary to create holes in both the electrode and the tip, with the same diameter.

To achieve the best results, it is recommended to opt for copper bolts, as they offer minimal electrical resistance.

Mounting the housing

The housing for the panel can be constructed using wood. To install the switch and power wire, it will be necessary to create holes on the back of the panel.

Once this is done, the next steps include sanding, priming, and painting the panel. Afterward, the assembly process begins. Additionally, two copper wires are required, which need to be cut down to a length of two and a half centimeters. These copper wires will act as electrodes.

The next step involves mounting the switch, followed by fastening the transformer to the wood using regular self-tapping screws. To enhance safety and convenience, it is important to install a microswitch, which can be attached to the upper lever. Lastly, remember to insulate all of the connections.