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ZX7-200 inverter welder teardown
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ealex:
hello

I'll have to start learning to weld and I decided to "splurge" on a bangood inverter welder.
I know it's not a machining forum, but the electronics part is interesting/
If I have to learn to weld, I can learn on a bad machine, to make things more interesting :)
Following Dave's advice, the first thing I did, after documenting the shipping damage (bent corners), was to take it apart.

The build is quite clean and the PCB's have a thin lacquer coating. They state it can reach 200A but I don't know if they are normal amps, or the other kind.

The front of the unit:


The main PCB has 2 IGHB's (600V, 40A), a big rectifier bridge, a 400V 680uF capacitor, a small flyback transformer for an auxiliary PSU, a gate drive transformer, a current sense transformer and a current shunt (I think), the main power toroidal transformer and the output rectifier.
There is no secondary inductor, and there is no room to add one.



The unit is NOT earthed, and there is no earth wire in the power cable.
Should I replace the cable with a 2 wire one and connect  the case to earth ? Or is it safer to leave it as it is ? I could not see any connection between the case and heatsinks / PCB's


The controller section is build as a separate  module, with an SG3525 controller, a bunch of transistors that seem related to the gate drive transformer, 2 op-amps (i think they are current sense amplifiers and fan control), some passives and several LED's.



It looks like it's a variation of http://danyk.cz/svar_en.html described here:


I don't know how well it works - I've seen some clips and it seemed ok, I'll buy the required accessories and protection gear these days, and try to stick some scrap together.
ealex:
and a schematic, does not exactly match my unit but seems close
ealex:
an idea: maybe I can modify it to use as an spot welder ?
I can set up the current limit via the normal feedback path and maybe I can take control of the shutdown pin with an external MCU that can be powered from the internal aux. supply.

from the datasheet (https://www.onsemi.com/pub/Collateral/SG3525A-D.PDF):
"
Shutdown Options (See Block Diagram, page 2)
Since both the compensation and soft−start terminals (Pins 9 and 8) have current source pull−ups, either can readily accept a pull−down signal which only has to sink a maximum of 100uA to turn off the outputs. This is subject to the added requirement of discharging whatever external capacitance may be attached to these pins. An alternate approach is the use of the shutdown circuitry of Pin 10 which has been improved to enhance the available shutdown options. Activating this circuit by applying a positive signal on Pin 10 performs two functions: the PWM latch is immediately set providing the fastest turn−off signal to the outputs; and a 150uA current sink begins to discharge the external soft−start capacitor. If the shutdown command is short, the PWM signal is terminated without significant discharge of the soft−start capacitor, thus, allowing, for example, a convenient implementation of pulse−by−pulse current limiting. Holding Pin 10 high for a longer duration, however, will ultimately discharge this external capacitor, recycling slow turn−on upon release.
"

A small oled screen + rotary encoder and a tiny STM should be enough.
I'll have to see what's attached to that pin - maybe just something basic, so I can override it easily,
FreddieChopin:

--- Quote from: ealex on September 05, 2019, 09:28:06 am ---If I have to learn to weld, I can learn on a bad machine, to make things more interesting :)

--- End quote ---

In other words you'll spend your precious time cocking about, doing poor welds and wasting material.

Here's a tip: get a proper, brand name welding inverter. Lincoln Electric 170S is okay, so is Kemppi Minarc Evo 180.

Also, on those cheap import inverters amps are faker than Pope's words. When set to "200A" it'll deliver about 80-90 amps. Enough for 2.5 or 3.2mm 7018 electrode.
ealex:
i'm aware about the fake amps and it's quality

i don't plan to make any critical welds with this, and i don't plan of using rods thicker than 2.5mm (i'm going to get cables, 2mm rods and protection equipment tomorrow)

i have access to better machines, but this can be thrown in my back-pack and start playing when I can get access to a proper space - i don't have a proper workshop and no chance of one in the near future.

the first thing I want to build is a quick to assemble mobile stand for my craft beer cooler / dispenser, as there is no solution available over here - some 600x1000mm frames made from 30 or 40mm square tubing, pre-drilled and with nuts welded in place, so I can pull 4 of them out of the car and make a sturdy 600x1200mm table in 15 minute.

regarding the schematic: the shutdown pin is connected to GND => it's mine to use if I can lift it off the board.
this makes it usable as a spot-welder, where the analog part will handle the current limiting and a basic digital section will control the weld pulse length.

the spot welder part was the main reason i got that unit, after looking for schematics, etc.
(my e-push-bike needs a new battery and I have some fresh US18650VTC6 cells waiting to be welded.
long term goal: there is no battery rebuild service around here, and electric push-bikes or electric bikes are getting popular, as this city is way to overcrowded - if i can be the first one on the market + provide a new BMS that's similar to what xiaomi is using; that's a new post, as I already have a prototype PCB and all the required parts to make a simple 4-5S pack, but that's another topic)

i've connected an external PSU to the aux supply and I can see the drive waveform on the primary of the gate drive transformer.

the next part will have to wait for tomorrow - now there are 2 30cm kittens sleeping on my lap, and it's time to follow them and get some sleep before i wake up with 2 set of paws pulling my beard.

the entire thing is more of a toy - i wanted to see how something like this is designed and built.
if it will be able to make a decent weld - even better.

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