Management of high-frequency microwelding. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / welding equipment

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High-frequency micro welding is designed for welding metals, plastics, soldering copper wires, etc. Distinctive features of the use of high-frequency current are a high-quality seam, stable arc burning, reduced power consumption and smooth regulation of the welding current.

The current regulation in traditional welding machines is performed by a ballast rheostat, which serves to obtain the "falling" load characteristic necessary for welding. A significant part of the electricity in such a system is spent on heating the rheostat. The second significant drawback of classical welding is the need for increased voltage for the occurrence of stable arc ignition.

The use of an inverter source with field-effect transistors as electronic keys makes it possible to reduce the welding voltage while improving other indicators. The electronic circuit of the microwelding device automatically forms the load characteristic of the required type due to voltage and current feedback.

Feedback adjustment allows you to set any slope of the load characteristic. The scheme provides for automatic monitoring of the temperature of the semiconductor converter and timely reduction of the load current with a protection circuit against overheating of key transistors, automatic control of the welding wire feed rate depending on the load reduces its consumption. The operation of the electronic regulator is based on the conversion of direct voltage into pulsed voltage with duty cycle control using a key inverter.

The device (Fig. 1) consists of:

• rectangular pulse generator on the analog timer chip DA2;
• emitter follower on transistor VT1;
• inverter on field-effect transistors VT2...VT4;
• welding wire feed speed controller on elements DA5, VT5;
• transformer power supply with a powerful diode bridge T1, VD5, VD6.

(click to enlarge)

The master oscillator is assembled on the DA2 timer chip. The power supply of the generator is stabilized by the DA3 chip. In the lowest position of the resistor R2 engine, the pulse duration at the output 3 of the DA2 generator is maximum, as is the value of the welding current, in the upper position it is minimal. The power limit is determined by the transformer T1 used and the maximum drain current of the group of field-effect transistors VT2 ... VT4. The magnitude of the current pulse that occurs during resistance welding of metals can reach tens of amperes. Therefore, transistors are connected in parallel with mounting on a common radiator. The outputs of the drains and sources of the transistors are connected to common buses, the output connections are made with a stranded wire with a cross section of at least 6 mm2.

Stabilization of the output voltage is implemented by a negative feedback circuit. Voltage. The OS is removed from the welding circuit and fed to the control input of the DA1 parallel stabilizer included in the control circuit (input 5) of the DA2 timer. With an increase in the output voltage, the control voltage at input 1 DA1 increases, it opens more strongly and shunts input 5 DA2, which leads to a reduction in the duration of the generator output pulse and a decrease in the electric welding voltage. When the output voltage decreases, the reverse process occurs, i.e. voltage stabilization mode is created on the load. The characteristic required for welding is set by resistor R6 by adjusting the voltage. OS coming to DA1.

The voltage drop across the shunt resistance RS1, proportional to the load current, is fed through the divider R14 to the control input of the parallel stabilizer DA4 included in the base circuit of the VT1 switch. When overloaded, the voltage on RS1 increases, DA4 opens more and shunts the base circuit VT1. The transistor closes, and the passage of pulses from the multivibrator to the gates of transistors VT2.VT4 stops, which makes it possible to limit the short circuit current in the welding circuit.

Field-effect transistors VT2 ... VT4 operate in the key mode and open when pulses arrive at the gates. To speed up their closing after the end of the pulse, the gate circuits are closed to ground through the internal transistor of the DA2 timer. pulse diode VD3 eliminates spontaneous opening of transistors.

The temperature of field-effect transistors at operating current should not exceed the rating. Their temperature is controlled by the thermistor RK1 installed on the radiator. An increase in temperature leads to a decrease in the resistance of the thermistor, an increase in the voltage at pin 1 of DA1, its greater opening, a decrease in the frequency of the DA2 generator and a corresponding decrease in output power.

To eliminate oxidative processes during welding with uncoated bimetallic wire, inert gas is supplied to the welding site using valve K1 installed on the pipeline.

Filter C7-L1-C8 eliminates noise under load and prevents voltage drop between pulses, preventing arc break. To control the presence of the output voltage, the LED HL1 is used.

Most of the elements of the control unit are placed on a printed circuit board with dimensions of 104x65 mm (Fig. 2). The printed circuit board and the power transformer are located in a metal case in separate compartments. The current, speed and characteristics regulators with switching elements and an ammeter are located on the front panel of the device, the fan (if installed) is on the rear wall.

The device uses a power transformer type OSO-0,4 or TS320. The transformer is disassembled, all secondary windings are removed and new windings are wound, with a bundle of several winding wires (for better filling of the frame) with a total cross section of 3 mm2. The number of turns is determined by the dimensions of the frame (before filling). The windings are connected in series. Diodes VD5 and VD6 are installed on a separate board. They are equipped with "flag" radiators measuring 50x100 mm. Power circuits, indicated in the diagram by thickened lines, are made of stranded wire in vinyl insulation with a cross section of at least 4 mm2. When welding with a welding wire of 0,6 mm (in a semi-automatic device), it is fed into the welding place using a mechanism consisting of a feed electric motor and a broach mechanism. The SB1 "Start" button is located on the wire and inert gas supply hose.

The power supply circuit of the M1 feed motor consists of a speed controller on an analog stabilizer DA5 and a current amplifier on a VT5 transistor. Contact welding requires round copper-graphite electrodes 1 ... 3 mm with a pointed end for ease of welding and a clamping device.

If possible, the circuit should be supplemented with a fan from the computer's power supply by connecting it to a 12V circuit. Inert gas supply valve - industrial, RA1 ammeter (with an external shunt of 75 mV and a scale of 50 ... 100 A) - type M4200. Fixed resistors - type C2-29, variables - SPO-0,5; SDR. Possible substitutions of elements are given in table 1, and in table 2 - suitable types of field-effect transistors.

Choke L1 is made on a ferrite ring (2000NM) with a diameter of 42 mm. The winding consists of 30...40 turns of stranded wire with a cross section of 4 mm2.

The adjustment of the microwelding device consists in the initial setting of the wire feed speed with the resistor R10, the characteristics of the microwelding current - R6, the current adjustment - R2 and the protection against sticking of the electrode - R14. After a short operation, the circuit elements should be checked for heating; at temperatures above 80 ° C, it is necessary to increase the area of ​​​​radiators.

The SA1 start button turns on the K1 valve and the generator on the DA2 chip, while there is initially no voltage on the welding electrode. The appearance of welding voltage with a delay depending on the charge time of the capacitor C3 causes the subsequent rotation of the wire feed motor M1 at a speed depending on the position of the resistor R10 slider.

In order to avoid eye burns from the ultraviolet radiation of the welding arc, protective welding goggles with a UV filter are used during work.

Authors: V.Konovalov, A.Vanteev, Creative laboratory "Automation and telemechanics", Irkutsk.

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