DIY arc 200 repair

Hello everyone. I am with you again, a welder repairman. So today we received another failed welding inverter. Among our repairmen, such devices are called three-story buildings.

Declared malfunction: Does not produce welding current. Sparks and does not cook.

By the way, you can see three floors of the board inside,

the first is a board with conductors and soft start.

the second is the rectifier, choke, and power trance.

the third is mosfet transistors, a duty room and a control board.

Since the cause of the breakdown is stated to be low current and does not cook, we will check the OS by current. These three-story buildings have a sore spot for the current.

The CA3140 microcircuit is responsible for controlling the current in this welder.

And if we have something wrong in the current control chain, two LEDs light up. In my case, these LEDs were on.

Further punching in the control board revealed a faulty CA3140. Conclusions 2 and 3 rang among themselves at 4 ohms.

Further, my welder stupidly turned off in the cold, that is, the welding flew away completely out, not a single sign of life. At room temperature, it restored its working capacity, but as soon as I cooled it down, it refused to work. The malfunctions were a little chaotic, so I had to run from home to the street and vice versa to catch GLUCK and analyze the reasons.

By malfunction, it could be said that I did not have + 300V from the rectifier board and capacitors (the first bottom board). Therefore, when I once again caught a glitch, I threw the multimeter probes onto the two power lines of the welder. And he was surprised. There, instead of 300v, it was only 100v. Hmm, strange.

I took out the lower board and washed it. And he began to look at what was wrong.

I was attracted by a black coating under the relay, as if something was fucking there.

I unsolder it. By the way, when I was soldering, I was confused by the fact that the pin from the relyushka was visible in the penny, and the soldering iron did not feel it. As it turned out later, the output from the relay was short, or rather it was not really there at all. And because of this, the welding did not start.

The main element of the simplest welding machine is a transformer operating at a frequency of 50 Hz and having a power of several kW. Therefore, its weight is tens of kilograms, which is not very convenient.

With the advent of powerful high-voltage transistors and diodes, welding inverters... Their main advantages: small dimensions, smooth adjustment of the welding current, overload protection. The weight of a welding inverter with a current of up to 250 Amperes is only a few kilograms.

Principle of operation welding inverter is clear from the following block diagram:

An alternating mains voltage of 220 V is supplied to a transformer-free rectifier and a filter (1), which forms a constant voltage of 310 V. This voltage feeds a powerful output stage (2). Pulses with a frequency of 40-70 kHz from a generator (3) are fed to the input of this powerful output stage. The amplified pulses are fed to a pulse transformer (4) and then to a powerful rectifier (5) to which the welding terminals are connected. The control and overload protection unit (6) regulates the welding current and protects.

Because inverter operates at frequencies of 40-70 kHz and higher, and not at a frequency of 50 Hz, like a conventional welder, the dimensions and weight of its pulse transformer are ten times less than that of a conventional 50 Hz welding transformer. And the presence of an electronic control circuit allows you to smoothly regulate the welding current and provide effective overload protection.

Let's look at a specific example.

Inverter stopped cooking.The fan is running, the indicator is on, and the arc does not appear.

This type of inverter is quite common. This model is called "Gerrard MMA 200»

We managed to find a circuit of the MMA 250 inverter, which turned out to be very similar and helped significantly in the repair. Its main difference from the desired scheme MMA 200:

• The output stage has 3 field-effect transistors, connected in parallel, and the MMA 200 - by 2.
• Output pulse transformer 3, and at MMA 200 - only 2.

The rest of the scheme is identical.

At the beginning of the article, a description of the structural diagram of the welding inverter is given. It is clear from this description that welding inverter, this is a powerful switching power supply with an open-circuit voltage of about 55 V, which is necessary for the occurrence of a welding arc, as well as an adjustable welding current, in this case, up to 200 A. The pulse generator is made on a U2 microcircuit of the SG3525AN type, which has two outputs for control of subsequent amplifiers. The generator U2 itself is controlled through an operational amplifier U1 of the CA 3140 type. This circuit regulates the duty cycle of the generator pulses and thus the value of the output current set by the current control resistor brought out to the front panel.

From the output of the generator, the pulses are fed to a preamplifier made of bipolar transistors Q6 - Q9 and field workers Q22 - Q24 operating on a transformer T3. This transformer has 4 output windings which, through the formers, supply pulses to 4 arms of the output stage assembled in a bridge circuit. In each shoulder there are two or three powerful field workers in parallel. In the MMA 200 scheme - two each, in the MMA - 250 scheme - three each. In my case, the MMA-200 has two field-effect transistors of the K2837 (2SK2837) type.

From the output stage, powerful pulses are fed to the rectifier through transformers T5, T6. The rectifier consists of two (MMA 200) or three (MMA 250) full-wave midpoint rectifier circuits. Their outputs are connected in parallel.

A feedback signal is supplied from the rectifier output through connectors X35 and X26.

Also, the feedback signal from the output stage through the current transformer T1 is fed to the overload protection circuit, made on the thyristor Q3 and transistors Q4 and Q5.

The output stage is powered by a mains voltage rectifier assembled on a VD70 diode bridge, C77-C79 capacitors and forming a voltage of 310 V.

To power low-voltage circuits, a separate switching power supply is used, made on transistors Q25, Q26 and transformer T2. This power supply generates a voltage of +25 V, from which +12 V is additionally formed through U10.

Let's go back to the repair. After opening the case, a visual inspection revealed a burnt capacitor 4.7 μF at 250 V.

This is one of the capacitors through which the output transformers are connected to the output stage on the field workers.

The capacitor has been replaced and the inverter is working. All voltages are normal. After a few days, the inverter stopped working again.

A detailed examination revealed two broken resistors in the gate circuit of the output transistors. Their nominal value is 6.8 ohms, in fact they are in the cliff.

All eight output field effect transistors were tested. As mentioned above, they are included two in each shoulder. Two shoulders, i.e. four field workers, out of order, their leads are short-circuited. With such a defect, high voltage from the drain circuits enters the gate circuits. Therefore, the input circuits were tested. Defective elements were also found there. This is a zener diode and a diode in the pulse shaping circuit at the inputs of the output transistors.

The check was carried out without soldering the parts by comparing the resistances between the same points of all four pulse shapers.

All other circuits were also tested up to the output terminals.

When checking the weekend field workers, all of them were soldered. The faulty ones, as mentioned above, turned out to be 4.

The first turn-on was done without any powerful field-effect transistors at all. With this turn on, the serviceability of all power supplies 310 V, 25 V, 12 V was checked. They are normal.

Voltage test points on the diagram:

Checking the 25V voltage on the board:

Checking the 12V voltage on the board:

After that, the pulses at the outputs of the pulse generator and at the outputs of the shapers were checked.

Pulses at the output of the shapers, in front of the powerful field-effect transistors:

Then all the rectifier diodes were checked for leakage. Since they are connected in parallel and a resistor is connected to the output, the leakage resistance was about 10 kΩ. When checking each individual diode, the leakage is more than 1 mΩ.

Further, it was decided to assemble the output stage on four field-effect transistors, placing not two, but one transistor in each arm. Firstly, the risk of failure of the output transistors, although it is minimized by checking all other circuits and the operation of power supplies, still remains after such a malfunction. In addition, it can be assumed that if there are two transistors in the arm, then the output current is up to 200 A (MMA 200), if there are three transistors, then the output current is up to 250 A, and if there is one transistor each, then the current may well reach 80 A. This means that when installing one transistor in the shoulder, you can cook with electrodes up to 2 mm.

It was decided to make the first control short-term switching on in the XX mode through a 2.2 kW boiler. This can minimize the consequences of an accident if, nevertheless, some kind of malfunction was missed. In this case, the voltage at the terminals was measured:

Everything works fine. Only the feedback and protection circuits were not tested. But the signals of these circuits appear only when there is a significant output current.

Since the switching on was normal, the output voltage is also within the normal range, we remove the series-connected boiler and turn on the welding directly to the network. Check the output voltage again. It is slightly higher and within 55 V. This is quite normal.

We try to cook for a short time, while observing the operation of the feedback circuit. The result of the operation of the feedback circuit will be a change in the duration of the generator pulses, which we will observe at the inputs of the transistors of the output stages.

When the load current changes, they change. This means the circuit is working correctly.

But the pulses in the presence of a welding arc. It can be seen that their duration has changed:

Missing output transistors can be purchased and replaced.

The article material is duplicated on video:

ARC-200 welder Chinese. The scheme is 90% the same as the SAI-200.
malfunction: cooks, the current is adjustable, you can burn half of the 4Ki electrode. but when the electrode is torn off, the protection is triggered, after that it starts to work constantly at any current. Check the snubers, diode drivers, the protection was rude - no use.
The block diagram is as follows:

Can anyone come across this?

Replacing the top board eliminated the cause

your block diagram has the wrong welding output voltage. 28 volts do not exist with these devices. Usually 56-72 volts

I would like to find the reason if it is in the board. Usually 50-80 at XX, and when naked. 200A can and 28v What is written on the diagram, just infa taken from the nameplate of the inverter. Here is a photo

Yes, the layout is different, just everything was blinded on one board, except for the control board, but the circuit is basically the same.

Sketched the diagram, maybe someone will come in handy.

[quote = ”vasa”] I advise you to solder everything

If it does not help, carefully check the harness near CA3140, SG3525

Then try to replace CA3140, SG3525 [/ quote]
Everything that is poorly soldered in appearance is soldered, replaced, just in case, CA3140; KA3525 has a good response to the load, there is no point in replacing it.

And how did the device work before the breakdown?

Make sure that there are no pulsations in the power supply of the control unit.

Become a 9-pin PWM oscilloscope and check the absence of "jumps" in the OS signal at various current assignments

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