Homemade Micro AC Welding Machine

In the process of amateur electronic production and maintenance, it is inevitable to encounter welding battery electrodes or thin steel plates, and to ensure the smooth completion of this task, electric welding machines are indispensable. Electric welding machines can usually be divided into two types: DC welding machines and AC welding machines. The AC welding machine introduced here is composed of a step-down transformer, a current regulator and a heat dissipation system, as well as welding wires, handles and other accessories. When welding, there is no need to use welding rods. Just use the two workpieces to be welded as the two electrodes of the circuit, and use the high temperature generated at the contact resistance to melt the metal instantly, so that the workpieces are firmly welded together. Since the purchase of finished electric welding machines is expensive, if you think your hands-on ability is not too weak, you can also give full play to the DIY spirit like the author to make a practical miniature AC welding machine.

The working principle of the circuit

is shown in Figure 1. B2 is a step-down transformer. It is also the core component of the electric welding machine. The AB2 rectifier bridge, unidirectional thyristor SCR, single junction transistor UJT, resistors R2, R3, R4, R5, capacitor C2 and potentiometer RP constitute a stepless regulator of welding current. The DC ammeter A is used to indirectly indicate the welding current. It and the LED form a power indication circuit. The small transformer B1, the rectifier bridge AB1, the capacitor C1 and the fan M form a heat dissipation system.

It can be seen from the figure that the device circuit is very simple. If it is complex, it can only be regarded as a current regulator. It uses the negative resistance characteristics of the single junction transistor to form a relaxation oscillator as the trigger circuit of the unidirectional thyristor. Since the power supply of the single junction transistor relaxation oscillator is taken from the full-wave pulsating DC voltage output by the bridge rectifier circuit. When the thyristor is not turned on, the capacitor C2 of the relaxation oscillator is charged through R2, R5 and RP, and the voltage VC2 across the capacitor rises exponentially. When it reaches the peak voltage VP of the single junction transistor. The single junction transistor UJT is suddenly turned on, and the base resistance RB1 decreases sharply. The capacitor C2 quickly discharges to the resistor R4 through the PN junction, causing a positive jump in the voltage Vg across R4. A steep pulse rising edge is formed. As the capacitor C2 discharges, VC2 decreases exponentially. When it is lower than the valley voltage V, the single junction transistor is cut off.

The output at both ends of R4 is a peak trigger pulse. This turns on the thyristor SCR. AC current flows through the primary winding of B2, and the voltage drop across the thyristor becomes very small, forcing the relaxation oscillator to stop working. When the AC voltage passes through zero, the thyristor is forced to turn off. The relaxation oscillator is powered again, and capacitor C2 starts charging again, and the above process is repeated over and over again. Adjusting the potentiometer RP can change the charging time of capacitor C2, that is, change the oscillation period of the relaxation oscillator. Naturally, it also changes the moment when the relaxation oscillator sends the first trigger pulse each time the AC voltage passes through zero. Correspondingly, the conduction control angle of the thyristor SCR is changed, causing the voltage applied across the primary winding of B2 to change. Ultimately, the purpose of regulating and controlling the secondary output current is achieved.

Device selection and testing

The step-down transformer B2 uses the filament transformer of the FU-720F power amplifier tube of the waste color TV transmitter. The primary AC voltage is 220V. The secondary AC voltage is 4 V. The stable output current can reach 80A. If you can't find such a suitable low-voltage and high-current transformer, you can also make it yourself. First, find a 220V AC power transformer with a power of more than 300W, and remove the original secondary winding coil. Use a copper cable with a diameter of more than 0.5 square centimeters to wind 6 to 10 turns on the transformer to ensure that the output voltage is about 4V. The single junction transistor uses BT33F, and the unidirectional thyristor uses CR10AM. Before actual production, you must test the quality and distinguish the pin electrodes. For the single junction transistor. First, determine the emitter e, set the resistance range of the pointer multimeter to RX 1k, and use two test pens to measure that the forward and reverse resistances between any two electrodes are equal (about 2~10kΩ), then the two electrodes are b1 and b2, and the remaining electrode is the emitter e. Then distinguish the first base b1 and the second base b2, connect the black test pen to the E pole, and use the red test pen to contact the other two electrodes in turn, and measure the forward resistance values ​​respectively. Due to the structure of the tube, the second base b2 is close to the PN junction, so the forward resistance between the emitter e and b2 should be slightly smaller than the forward resistance between e and b1, ranging from several to more than ten kΩ. Therefore, when the resistance value is small, the electrode connected to the red test pen is b2. When the resistance value is large, the electrode connected to the red test pen is b1. However, even if b1 and b2 are reversed. Under normal circumstances, the tube will not be damaged. It will only affect the amplitude of the output pulse; if it is found that the output pulse amplitude is small. Just swap the two bases. The appearance of a unidirectional thyristor is similar to that of a high-power triode. When distinguishing its three electrode pins, namely anode (a), cathode (k) and control electrode (g), use a pointer multimeter with R×10 gear. If the resistance value is not connected to the other two pins (positive and negative resistance values ​​are more than several hundred kilo-ohms), it is pole A. Then measure the resistance value of the remaining two pins. When the resistance value is small (about tens or hundreds of ohms), the black test lead is connected to pole G. The other pin is pole K. If the test result does not match the above situation, it means that the component is broken. DC ammeter A can be replaced by a milliammeter that is easy to buy and connected to a long wire. The wire is equivalent to a shunt resistor with a small resistance. The specific length should be determined after appropriate trimming according to the actual display situation. The cooling fan uses a common DC 12V computer fan, and the secondary of transformer B1 can take 10V. The resistor should be selected above 2W, and the FU should be a 250V, 4A fuse. Considering the high current of the secondary output, the welding torch wire should be made of copper core wire with a sufficient cross section to ensure that it will not get hot due to overload during use. Finally, please note that the assembled circuit should be placed in a suitable metal casing. In addition to ensuring good ventilation, the entire circuit should be well insulated from the casing, and the casing should also be reliably grounded.

Precautions for use

Connect the three-core plug to the 220V mains, and close the power switch K. The power indicator LED lights up. At the same time, the cooling fan starts to work. Turn the potentiometer RP knob to adjust the appropriate welding current, and then you can start welding. Due to the limitation of welding current, the diameter of the metal workpiece cannot be greater than 3mm or the thickness cannot be greater than 0.5mm. When welding, keep the workpiece stable. You can use a vise or other clamps to fix one welding handle on one workpiece, and the other welding handle contacts the other workpiece. After the metal melts, disconnect one welding handle electrode. In addition, you can also punch holes in thin metal plates, or clamp a welding handle to a metal workpiece, and use the other welding handle electrode as a pen to engrave or draw on the workpiece.

Operators of electric welders should understand and master the knowledge of safe use of electricity. The insulation of the line part with mains power should be reliable. The welding wire should be connected to the electric welder and welding clamp with bolts, nuts, washers and tightened. The inside of the electric welder should also be checked regularly. If the connection is loose or desoldering, it should be tightened and welded in time. If the wire is broken, it should be replaced or handled in time; when the welding work is completed or suspended, the power supply should be cut off immediately, and the power supply must be cut off when moving the electric welder; because a large number of sparks are ejected during electric welding and the temperature of the weldment is also extremely high, for safety reasons, stay away from flammable and explosive items during welding, and pay attention to rain protection when working outdoors; to reduce the strong ultraviolet radiation hazards generated during welding. Continuous working time should not be too long, and try to wear protective clothing and gloves made of canvas.