DoCreate portable spot welder damaged

[spleenharvester]

Hope someone can help me, this is the second time I have killed one of these and I wasn't able to fix the last one. This is a portable Farad capacitor-based welder: https://www.aliexpress.com/item/1005004682391629.html it has worked very well for me, but the DC jack failed. I replaced the DC jack, but did not wait for the capacitors to discharge, and in the process accidentally shorted the green COM wire (that joins the middle of the capacitors) to the -12V terminal. After that there was a burning smell. Now the machine turns on and runs but the capacitors do not charge up.

The only fault I could find was that F1 was blown, but bridging it did not fix the fault. Exactly the same fault as my previous one. The small 12V step-down module outputs +5.49V with no load, but when connected to the machine it drops to +5.21V, which is the voltage the capacitors are currently at. This is as far as I have gotten.

Any ideas?

[spleenharvester]

With the capacitors disconnected and the step-down module disconnected, the output of the step-down module measures +5.49V, but with the capacitors disconnected and step-down module connected the output terminals drop to +0.89V... so something on the PCB is sinking current but I can't find anything heating up

If I inject the +5.49V from a bench PSU the green LEDs light up, and something is pulling 0.7A, the two 4ohm resistors get extremely hot but I can't seem to trace what they are feeding

[stj]

check all the diodes

[spleenharvester]

If I have tested it correctly it looks like the phototransistor (EL817) is shorted, have ordered some replacements

[spleenharvester]

Spent the day tracing the schematic and uploaded it to ChatGPT. We eventually figured out Q5 (SS8550) is faulty (B-C open circuit, C-E stuck on) and after removing it I'm no longer getting the 0.7A current draw through the two resistors (marked R99 and R100 on my schematic). A resistor on the rear (marked R98 on the schematic) was visibly burned but still appeared to be OK, though I will be replacing it anyway. Will keep you all updated

[sam_sam_sam]

Ok there are several things you need to be aware about these battery tab welders first of all get your self board stand-off kit with different lengths and raise the buck converter so you have enough room to put heat sink on them this is a must do modifications that need to be done

Second of all I would highly recommend that you change the buck converter to the version that has the voltage and current adjustment controls on it and set it to following settings and has the voltage and current meters on the module this makes things so much easier to do the following things that I mentioned below

Voltage no higher than 5.0 volt this is extremely important because otherwise the two resistors will heat up very quickly and get very hot if this is not followed and could damage the capacitor controller board

The current setting is not as sensitive to being set high but I caution you that if you go beyond 3 amp I would highly recommend that you put a heat sink on the switching ic chip because at 5 amp which is its maximum current capacity it will get very warm to almost be to hot

The 12 volt power supply to me is a joke what would be a better option would be to use a 10 amp 5 volt switching power supply for the best results

One final note if you adjust the voltage so that the green light just starts to light is fine just remember that if this will cause the resistors to get warm to the touch and you are wasting power by doing this and really dose not effect the battery tab welder at all

I am referring to the two high current resistors for the voltage bleed resistors not the ones for the mosfets

I have two of them that are very similar to one that you have and when I was testing this device I was very concerned about the the behavior of it that the two resistors were getting hot and that the voltage controller ic chip was not getting warm and it took a very long time to charge up and when it was finally up on charge the green light would be very bright and very hot and I thought that this could not be right and when I saw the voltage rating on the capacitors are 2.7 volts and they were pushing this maximum rating hey wait minute this not right the parameters are not set correctly I going to have to fix these issues

The reason I recommend changing the buck converter to one that has the voltage and current meter it makes adjustments for each pot very easy to do and at a glance you know what the status of the capacitors are at and when the battery tabbing welder is ready to go

One very important thing to remember is that when you are going to do any work on these devices make sure that the capacitors do not have any voltage on them the small display that is used to set the parameters of battery tab welder is not very big and the voltage meter responds very slowly to voltage increase or decrease that is another reason for changing the buck converter

I have not finished the table for the capacitors and to hold the switching power supply and to be able to raise the capacitors unit up high enough to easily read the screen and keep everything together for simplicity and ease to move from one location to another location without breaking anything

I hope this helps you with your battery tab welder and stop having these issues I still have to modify the second one that I have

The one I bought was a little bit cheaper but not by much but I like how it comes in a metal enclosure I wish mine was that way but I am not sure about weather or not the setup has different issues than the one I have bought

On the other battery tab welder I might just do away with the buck converter and just use a voltage and current adjustable controls switching power supply and set it for 6 to maybe 8 amps depending on weather or not the capacitors can handle this amount of current I will have to do some research and testing on this subject

[spleenharvester]

Cheers sam, all useful information, hopefully I can put it to use if I can get this thing working again! Am I correct to interpret your caps charge to 2.5V each then? On my machine the buck outputs +5.4V and the documentation all states the fully charged state is +5.4V. The only mods I did before killing the thing were to put taller posts on the buck converter so that I can put a heatsink on each of the FETs and then put a fifth heatsink on the buck converter FET itself, that seemed to keep it cool enough.

Also uploaded another schematic with some fixes. Everything is there now except for the buck converter, and a few missing values of ceramic caps/transistors/that one burned resistor

[sam_sam_sam]

Cheers sam, all useful information, hopefully I can put it to use if I can get this thing working again! Am I correct to interpret your caps charge to 2.5V each then? On my machine the buck outputs +5.4V and the documentation all states the fully charged state is +5.4V. The only mods I did before killing the thing were to put taller posts on the buck converter so that I can put a heatsink on each of the FETs and then put a fifth heatsink on the buck converter FET itself, that seemed to keep it cool enough.

Also uploaded another schematic with some fixes. Everything is there now except for the buck converter, and a few missing values of ceramic caps/transistors/that one burned resistor

The most important thing is what does the capacitors say on them as far as working voltage if it says 2.7 and it is set for 5.4 volts then you are at its maximum voltage this is not good for prolonged periods of time at this voltage it needs to be dropped to 5.00 volts or maybe a little bit more than this

The capacitors on mine say clearly 2.7 volts maximum and capacitors in general should never be ran at there maximum voltage in the first place the version I have the capacitor board starts to limit the voltage at just over 4.90 volts and when you get close to 5.1 volts the green led light starts coming on and the current limiting resistor starts getting warm and if you go to 5.4 volt they get down right hot so I adjusted mine to just short 5.0 volts to keep the current limiting resistor cool it does not matter that it voltage is set at a lower voltage


Do not bother with who ever built this circuit because they really did not consider the capacitor longevity if you want the capacitors to last drop the voltage down to 4.9 or as high as 5.00 and be done with it I used mine set to this voltage and it worked perfectly fine the 5.4 volt power supply is the buck converter now if it causes an issue with the controller just change the resistor network for the input to the controller I doubt that you would have to do this unless it does control the welder from turning on the mosfets then you would not have a choice about it unless you can modify the software to the controller

The only issue is that the charge indicator does not show fully charged conditions but this does not bother me at all because I realize that who ever designed this device did take in account that the maximum voltage should only be 5.0 volts and not higher than that

[spleenharvester]

Cool thanks that's useful to know. I think a few resistors would need to be modified downstream of the buck converter to make it work at 5V though I may be wrong.

Anyway will update whether or not I'm able to fix it. Also have attached a finalised traced schematic, various fixes and now includes the buck converter. Also any educated guesses as to what R20 is supposed to be are appreciated, when uploading the schematic to ChatGPT it estimates 2.0~2.7Kohm. The burned R20 was a 1206 package that had a code starting with 2 but the rest of the characters were illegible.

[sam_sam_sam]

Cool thanks that's useful to know. I think a few resistors would need to be modified downstream of the buck converter to make it work at 5V though I may be wrong.

Anyway will update whether or not I'm able to fix it. Also have attached a finalised traced schematic, various fixes and now includes the buck converter. Also any educated guesses as to what R20 is supposed to be are appreciated, when uploading the schematic to ChatGPT it estimates 2.0~2.7Kohm. The burned R20 was a 1206 package that had a code starting with 2 but the rest of the characters were illegible.

Thanks for the schematic reverse engineering the circuit I going to try to do this for the charging controller on my battery testing machine that is 24.0 at 5 amps buck converter that is also used in this capacitor tab welder I have been unable to find a buck converter current controller because on this buck converter circuit it only controls the current and does nothing to the voltage this is the reason I am great full for your work on this circuit diagram

[spleenharvester]

Replaced Q5 and R20 today, charging circuit works perfectly now when injecting 5.4V into the buck output :-) pulls 4A and nothing gets hot. Annoyingly the buck converter appears to be blown too, it outputs +5.5V but collapses to nothing when any load is placed on it, checked it with a 10ohm resistor too. Hopefully a new buck converter is all it needs.

[spleenharvester]

Replaced the buck converter. She lives :-) very odd fault before - when applying +12V the machine would power up from the buck converter fine but the buck converter put out <0.1A current, which made it look like the buck was OK initially and that there was another fault in the charging circuit. It wasn't until I tested injecting from a bench supply without the buck converter that I found the circuit was actually fine with Q5 and R20 replaced (I used a 2K2 resistor for R20 btw). I can also confirm the machine takes a maximum of 12V/4A when initially charging, which drops to 12V/2A when the supercaps are charged to +5.0V, then drops to 12V/0.5A by the time the machine has hit +5.2V. So if you don't mind waiting for the initial charge it looks like these machines can be kept charged during use with any old 12V/1A power supply.

sam_sam_sam it looks like you have the same buck converter as me - you can actually get versions of this buck converter that have varistors mounted instead of the two SMD resistors, which can modify both output voltage and current - this might make your life a bit easier than modifying the existing one

[sam_sam_sam]

If I were you I would back the voltage down to 5.0 for the maximum voltage
These buck converter have a tendency to shut down when you over current them higher than 5 amps
If this capacitor board starts bleeding voltage above 5.4 volts to keep them from being overcharged then you are just wasting power and putting a strain on the voltage bleed circuit

I set mine to 5 volts @ 3.5 amps just to keep it from trying to over current the buck converter but the reason I use the version that has the voltage and current meter module on as well is because I want to keep an eye on the voltage and current as it is changing the capacitors and when it almost charged the current starts dropping and if I do not want to wait for it to be completely charged I will at least wait until the current starts dropping then I would start welding with it