Hola peeps.
I'm placing it in this topic, because from what I can see, it is essentially a modified SMPS, but please move if there is a 'correct' forum section.
Recently, I bought an essentially brand new, but defective, 155 amp inverter welder.
The unit would turn on fine, with the digital display and fan doing their thing, however when attempting to strike an arc, the rod would merely spark or just get stuck onto the steel.
Now I know that many (if not most) in here would simply not have bought it, but I am still learning this trade and had never even looked inside one of these before, so for the equivalent of 25€, it is a good opportunity to learn.
Other than not striking arcs, the only other noticeable oddity was that after switching the neon illuminated mains switch off, the neon would remain on until the large capacitor(s) had discharged. And the plug pins would also give a sharp shock
Normally, the lack of arc symptoms could be simply a dirty or poor connection on the main leads, and there was actually some play in the two large bars which connect the pcb to the hefty 16mm cable terinals on the front panel, although tightening made no difference.
So without removing any parts, I looked for any obvious damge and basic tested the obvious for shorts... The large caps, the two IGBT's, the PCT, main relay, the two output diodes, etc. There is no fuse protection. One resistor next the the 7805 has almost no markings left on it.
After several attempts at welding down in the garage, I put the open unit on the bench to have a probe around and take some voltage and frequency readings.
When the welder had been turned on a few minutes, but before I had done any readings, there was a puff of smoke from the area of the relay and the unit switched off. Including the previously ever-glowing neon on/off switch.
Now, I haven't yet understood how this welder should be working, but with my somewhat limited understanding of SMPS design... IF it is based on the switching supply system, then the PWM controller not functioning correctly would limit the main current output, and any arcing capability.
Anyway, this puff of smoke event, took out the two 45 amp IGBT's and their two SS14 smd diodes.
The only chip in the region of the smoke is an LNK626 'ffline switcher' which I don't really understand what it is doing, but it isn't the PWM controller as that is an SG3525 which is located towards the end of the board,
So today, I put the unit back on the bench with the intention of drawing out some kind of circuit diagram in order to see if a better understanding could be gained.
What stood out immediately is that the two large electrolytic filter caps are connected before the bridge rectifier. This is a new one on me...
Even more unusual in my world of limited electronics understanding, is that one of the bridge rectifier AC terminals has no AC going to it, nor is the terminal connected to any other part of the PCB
I have drawn a very basic sketch of the first part for people to have a look at.
What is going on with the bridge rectifier, why are the caps before the bridge, and does anyone have any suggestions as to what else to check?
Thanks in advance
I'm placing it in this topic, because from what I can see, it is essentially a modified SMPS, but please move if there is a 'correct' forum section.
Recently, I bought an essentially brand new, but defective, 155 amp inverter welder.
The unit would turn on fine, with the digital display and fan doing their thing, however when attempting to strike an arc, the rod would merely spark or just get stuck onto the steel.
Now I know that many (if not most) in here would simply not have bought it, but I am still learning this trade and had never even looked inside one of these before, so for the equivalent of 25€, it is a good opportunity to learn.
Other than not striking arcs, the only other noticeable oddity was that after switching the neon illuminated mains switch off, the neon would remain on until the large capacitor(s) had discharged. And the plug pins would also give a sharp shock
Normally, the lack of arc symptoms could be simply a dirty or poor connection on the main leads, and there was actually some play in the two large bars which connect the pcb to the hefty 16mm cable terinals on the front panel, although tightening made no difference.
So without removing any parts, I looked for any obvious damge and basic tested the obvious for shorts... The large caps, the two IGBT's, the PCT, main relay, the two output diodes, etc. There is no fuse protection. One resistor next the the 7805 has almost no markings left on it.
After several attempts at welding down in the garage, I put the open unit on the bench to have a probe around and take some voltage and frequency readings.
When the welder had been turned on a few minutes, but before I had done any readings, there was a puff of smoke from the area of the relay and the unit switched off. Including the previously ever-glowing neon on/off switch.
Now, I haven't yet understood how this welder should be working, but with my somewhat limited understanding of SMPS design... IF it is based on the switching supply system, then the PWM controller not functioning correctly would limit the main current output, and any arcing capability.
Anyway, this puff of smoke event, took out the two 45 amp IGBT's and their two SS14 smd diodes.
The only chip in the region of the smoke is an LNK626 'ffline switcher' which I don't really understand what it is doing, but it isn't the PWM controller as that is an SG3525 which is located towards the end of the board,
So today, I put the unit back on the bench with the intention of drawing out some kind of circuit diagram in order to see if a better understanding could be gained.
What stood out immediately is that the two large electrolytic filter caps are connected before the bridge rectifier. This is a new one on me...
Even more unusual in my world of limited electronics understanding, is that one of the bridge rectifier AC terminals has no AC going to it, nor is the terminal connected to any other part of the PCB
I have drawn a very basic sketch of the first part for people to have a look at.
What is going on with the bridge rectifier, why are the caps before the bridge, and does anyone have any suggestions as to what else to check?
Thanks in advance
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