Mini Spot Welder from Aliexpress

[hojnikb]

So there seem to be plenty of those kWeld clones on aliexpress including this https://www.aliexpress.com/item/1005001492794726.html?spm=a2g0s.9042311.0.0.53154c4dCZc8CF(link just for reference)

It seems to be pretty common knowledge that these have a high chance of blowing up mosfets due to poor design. Someone seem to found a fix

But it looks like there is already a new revision floating around and this is the one i got
https://imgur.com/gallery/JtHWr2n

Capacitor fix seems less obvious to me. Anyone played around with these newer ones and knows a fix ?

PS: If there's a similar topic already, please merge/delete.

[Fraser]

I just received one of these and will be looking at the best position for the additional processor capacitor, if required. Note that a number of components appear to have moved from the processor PCB down to the MOSFET PCB. They are located under the large electrolytic capacitor. The issue of processor voltage drop may have been dealt with in this new version but I will check.

Fraser

[Fraser]

Initial inspection of the Mk2 design suggests that the processor supply rail issue has been addressed. The large 10000uF capacitor is connected across the power supply input terminals with a diode in series with the positive rail. I am thinking that this capacitor is now providing the power to the processor and will easily hold up the processor power rail during the welding pulse. The components that have moved from the processor PCB down to the MOSFET PCB are the MOSFET gate drive components that include bias resistors and, what look to be, three transistors.

It would be interesting to see the schematic for the MK1 version of the unit but cannot find much on the internet. From the video you referenced, it appears that the large 10000uf capacitor was on the wrong side of the diode that is in series with the supply to the processor. A simple design error I think and one that looks to have been corrected. A 8 pin IC and what looks to be an option isolator or 4 pin TVS have disappeared from the Mk2 design.

Fraser

[Fraser]

Pictures of the unit that I have just received as a reference for others. I will reverse engineer the design.

The processor has had its ID removed by laser etching but I think there are pictures of the MK1 with the chips ID visible.

Fraser

[Fraser]

Pictures continued.....

[Fraser]

I already own a KWELD kit but bought this out of curiosity. At £20 it appears reasonable value for money if it does not blow up !

The issue of the voltage drop at the processor and MOSFET bias inputs on the Mk1 unit is well known and requires the adding of a 1000uF capacitor to the processor supply rail of that version. The PCB tracks that carry the heavy current remain somewhat light duty for the requirement so I would still add some nice thick copper wire to those traces to lower their resistance. The probes and cables supplied with the kit are decent enough to not require modification unless the user wants to reduce weld current path resistance to the absolute minimum.

All in all not a bad attempt at a MOSFET switched light duty spot welder. Just do not overload it ! The battery supply specification is more critical than that of the KWELD. You have to have a power source providing 9V to 12V with enough current delivery and low internal resistance to create a good weld, but not so powerful that it delivers too much energy to the circuit and kills the MOSFETS ! The recommended power sources are :

12V Lead Acid Car Battery of capacity 25Ah to 40Ah
3S 45C to 60C Lithium Battery of capacity 4000mAh to 6000mAh
12V Lithium battery of capacity 30Ah to 35Ah  capacity.

The recommended welding material is stated as 0.1mm to 0.12mm thickness.

Fraser

[Fraser]

I just found a Youtube video that shows the PCB's of the MK1 unit. I have attached some screen grabs that are of interest. Note that the large 10000uF capacitor is connected directly across the power supply input terminals and I presumed that this was some energy boost idea to deliver the capacitors energy into the weld along with the energy from the power supply battery. The problem is, there was no, or an inadequate, 'hold-up' capacitor on the supply rail to the processor and the MOSFETS suffered poor drive to the gate in some cases. The Mk2 version uses the 10000uF capacitor as the 'hold-up' capacitor for the processor and MOSFET bias circuit. There is now a diode between the capacitor and the positive rail to prevent the weld event drawing energy out of the capacitor.  The whole control and MOSFET bias circuit is now powered from a supply rail that is serviced by the 10000uF capacitor 

The video showing the Mk1 model teardown is to be found here and is the source of the attached Mk1 pictures.



The Mk2 looks to be a refined design.

Fraser

[Fraser]

Mk1 pictures continued....

[Fraser]

Mk1 Vs Mk2 pictures for comparison

[Fraser]

A couple of interesting Youtube videos including comment on the Mk2 version in the second video......





Fraser

[Fraser]

For information, the Mk2 unit is no longer fitted with the 8 pin MCP1403 Power MOSFET driver. The other 4 pin component that is no longer on the Mk2 controller PCB remains unidentified at the moment.

Fraser

[Fraser]

Another point to note. My Welder is the Mk2.1 version as I choose to call it. The video that details the issue with some SMT resistors bodged onto the bottom of the MOSFET PCB shows the Mk2 model but my unit has a revised MOSFET PCB design that accommodates the two SMT resistors along side the driver transistor stage. 

Fraser

[mastershake]

Another point to note. My Welder is the Mk2.1 version as I choose to call it. The video that details the issue with some SMT resistors bodged onto the bottom of the MOSFET PCB shows the Mk2 model but my unit has a revised MOSFET PCB design that accommodates the two SMT resistors along side the driver transistor stage. 

Fraser

which seller did you get the mk2.1 unit from? ive been curious to play around with one of these for a while. thanks

[Fraser]

Having researched these units a little, it would appear that I have received the latest / best version of the Mini-Welder that has addressed most of the issues that were found with earlier versions. Mine has the large 10000uF capacitor correctly positioned in the circuit to power the processor, it has a revised MOSFET drive that uses 3 transistors in place of the MCP1403 (a pity to lose that IC though) and the biasing resistors that were bodged onto the Mk2 have been integrated into the Mk2.1 PCB.

From the provided pictures it would appear that the OP has the Mk2.0 version of the Mini-Welder so he needs to look at the bodged resistor installation on the bottom of the MOSFET PCB and improve its installation if found wanting in any way. No capacitor modification is needed.

The design is by no means perfect, even in the Mk2.1 version. It is worth looking closely at the KWELD design to see how such a device should be made. I will be looking at the following enhancements to the Mini Welder in the near future.

1. Fitting a 300A in-line automotive fuse in the supply cable to the Mini Welder. This is sensible when dealing with a high current source and a potential catastrophic failure in the welding operation.

2. Increase the cross section of the negative power rails to and from the MOSFETS using thick copper wire or copper bus bars. The lower resistance will improve performance whilst reducing heating if the negative rail PCB traces. Use only one terminal for the two positive connections to the PCB to avoid needing to upgrade the positive rail cross sectional area.

3. Add some transient protection and bypass capacitors to the MOSFET PCB and also to the processor supply rail on the processor PCB. When you dump a lot of current through even short conductors with fast switching times you can get some nasty transients forming. I may also add another electrolytic capacitor to the processors supply rail on the processor PCB as a ‘belt & braces’ approach to avoid processor instability during the welding operation.

4. I will be using a heavy duty Radio Control type power connector pair in the power supply cable to the Mini-Welder. This will facilitate easy disconnection of power if the need should arise. I will also be using quick connect clamps on the terminals of the car battery that I will be using. I do not trust this Mini-Welder enough to use a LiPo pack as its power source !

[Fraser]

My Mini-Welder came from a UK warehouse but is undoubtably a Chinese Seller drop shipping in the UK. Look for the same auction pictures from USA or China based sellers on eBay.com. There is no guarantee that the stock holding in a USA warehouse is the Mk2.1 however.

https://www.ebay.co.uk/itm/Portable-Mini-Spot-Welder-Machine-18650-Battery-Various-Welding-Power-DIY-Supply/313380699576?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

Fraser

[Fraser]

Reverse engineering of the Mk2.1 schematic completed  I will use my notes to draw a neater schematic before posting. I work in ‘pencil on paper’ so no clever computer generated schematics from me, but useable 

I am now looking to see if I can identify the processor but it’s identity is not essential as its pin functions are now documented and its firmware code is locked inside anyway.

As suspected, the Mini Welder does not address the issue of high transient voltages occurring on the input or output side of the MOSFET switch array. As such the input regulator fir the processor is vulnerable and the MOSETS are also potentially exposed to high voltage transients. These transients are caused by the fast switching if large currents passing through cables that are effectively low value inductors. I will be adding some transient protection and processor supply rail decoupling to my unit.

Fraser

[Fraser]

Luca has just reviewed the latest ‘Mk2.1’ version of the welder. Note that the ‘Mkx.x’ name is just my way of identifying the different models. In the video Alicia mentions that R4 on the processor board needs to be changed to a lower value to fully drive the MOSFETS into saturation under all circumstances and avoid MOSFET destruction.
The value needs to be changed to 470 Ohms. Luca also identifies the processor on the unit, actually a microcontroller by ST. It is apparently an STM8F103.

https://youtu.be/0A3sDXM8jQQ

I have decided to create the schematic of the processor board using VISIO or KiCad. I will upload it here ASAP. The MOSFET board has already been reverse engineered and a schematic created. My Mk2.1 unit reverse engineering confirms that the Mk2.0 schematic available on the internet may be used for the Mk2.1 as well.

I will add the MOSFET board schematic to this thread later

Fraser

[Fraser]

The schematic of the Mk2.0/2.1 MOSFET board may be found here:

 http://hieroben.de/wp-content/uploads/2021/01/MiniSpotWelderCircuit.pdf

I attach it to this post as well. I am not the author of this schematic.

My processor board schematic is to follow.

Fraser