ZX7-200 inverter welder teardown - Page 1
Forum section: electronics / projects

author: ealex
author_rank: Frequent Contributor
timestamp: September 05, 2019, 09:28:06 am
content: helloI'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 interestingFollowing 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'sThe 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 ofhttp://danyk.cz/svar_en.htmldescribed 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.

author: ealex
author_rank: Frequent Contributor
timestamp: September 05, 2019, 09:36:22 am
content: and a schematic, does not exactly match my unit but seems close

author: ealex
author_rank: Frequent Contributor
timestamp: September 05, 2019, 01:01:27 pm
content: 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 andhave 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,

author: FreddieChopin
author_rank: Regular Contributor
timestamp: September 05, 2019, 04:27:49 pm
content: Quote from: ealex on September 05, 2019, 09:28:06 amIn 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.

author: ealex
author_rank: Frequent Contributor
timestamp: September 05, 2019, 08:22:04 pm
content: i'm aware about the fake amps and it's qualityi 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.

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 05, 2019, 08:38:13 pm
content: Let me assure you that even with a very nice Miller inverter welder I make just as spectacularly awful welds as I did with a cheap-o model from Harbor Freight (a delightful importer of cheap Chinese tools here in the US). So, having a better welder isn't that big a deal.That said, I looked at the schematic and was not at all surprised that there is no output choke present - this thing apparently relies on the stray inductance of the cables to both smooth out the current waveform and limit the single-cycle peak current, which is just all kinds of wrong.

author: ealex
author_rank: Frequent Contributor
timestamp: September 06, 2019, 05:34:29 am
content: I'll use long cables for the normal welding toolsbut i think I can add an inductor just outside of the + connector if I can use it as a spot welder.

author: ealex
author_rank: Frequent Contributor
timestamp: September 24, 2019, 07:57:13 pm
content: that thing can actually weld, at least on thin stuff, with 2mm electrodes. ( it's was my first time welding, so I made something that must hold my weight - easy to test )i managed to use it for several hours without any undesired smokei just got the parts needed to try to convert it into a spot welderi'll update this thread when I'll start working on it - especially if it's a spectacular failure(i'll do the first test outside to minimize the things that can burn, if I'll make a 18650 cell blow up)

author: mikerj
author_rank: Super Contributor
timestamp: September 24, 2019, 08:31:32 pm
content: Quote from: ealex on September 05, 2019, 09:28:06 amA youngish guy at work bought the same unit and had it delivered to work for convenience. When it arrived we stuck it on the PAT tester just out of interest and discovered is was not earthed, so we we found a bit of heavy duty three core flex and fitted it. Personally I wouldn't use one without an earth connection, but then again I really dislike shocks

author: strawberry
author_rank: Super Contributor
timestamp: September 25, 2019, 07:04:54 pm
content: duty cycle (cooling) is the biggest difference between Miller kinda and general purpose chinese arc welderthere is no fake amps but printed on improper amps scale

author: friedolino
author_rank: Newbie
timestamp: November 25, 2019, 02:57:26 pm
content: Quote from: ealex on September 24, 2019, 07:57:13 pmim interested in this mod, too. would like to know if this device is suited to spot welder 0.1mm sheets of stainless steel. Which electrodes do you use?

author: ealex
author_rank: Frequent Contributor
timestamp: November 25, 2019, 03:12:49 pm
content: hellothe project is currently on hold - i've been busy with some other things.pretty much everything is done, I just need to connect everything and test it.for electrodes i plan to use some thick copper wire that's connected to the output cables with some stainless steel screws

author: friedolino
author_rank: Newbie
timestamp: November 25, 2019, 11:37:31 pm
content: do you have more ressources / references for spot weling with arc welding machine?I found only this video:

author: ealex
author_rank: Frequent Contributor
timestamp: November 26, 2019, 08:17:24 am
content: no, i don't have any extra info.and the bad part is that I don't have where to get any - the entire area is dead from the machining point of view - i'm just experimenting / learning everything as I go.(dead - took me 3 months to find someone with a lathe, to make a part that needed 30 minutes of work)

author: acruse60
author_rank: Newbie
timestamp: January 26, 2020, 02:46:42 am
content: does the cooling fan start running when power is switched on?

author: ealex
author_rank: Frequent Contributor
timestamp: January 26, 2020, 03:16:00 pm
content: yes, it starts as soon as you power the unitit's connected across the power supply that powers the control circuit.

author: acruse60
author_rank: Newbie
timestamp: January 26, 2020, 11:33:34 pm
content: Well I must have a faulty unit. have only ran three rods. Fan does not run even when I deliberately worked the welder hard enough to cut out on overtemperature.Do you have a close up picture of the circuit board connection for the fan?

author: ealex
author_rank: Frequent Contributor
timestamp: January 27, 2020, 07:45:20 am
content: I can't take a picture now, but from this photo:https://www.eevblog.com/forum/projects/zx7-200-inverter-welder-teardown/?action=dlattach;attach=827472;imagethe fan connector is in the top-right part, left of the mounting screw, the 2 pin connector - you can see one trace on the bottom side going all the way to the auxiliary PSU