New Spot welder - board design questions - Page 1
Forum section: electronics / beginners

author: BinaryBits
author_rank: Contributor
timestamp: March 23, 2022, 08:49:17 am
content: So I recently purchased a cheaper Chinese spot welder to try out. I know that on some of these Chinese spot welders its recommended to do some mods that seems to be needed in order to make the spot welder not blow mosfets etc.So I wanted to ask if somebody who is knowledgable can spot some obvious design flaws on this spot welder and see if something has to be modded/added, if thats doable from photos? Or if somebody has some previous knowledge about this spot welder? I have seen this same model called "KKMoon 5000W", but mine is unbranded.I haven't found any real detailed info on this particular spot welder, and also, I'm not that good at electronics.Im also wondering about overcharge/over discharge protections, and automatic discharge down to storage voltage. I read somebody on the internet saying that this model didn't have an over discharge protection because his battery had gone down to 1.6V?So I'm also curious if it has these kind of protection mechanisms?It has 2 LIPO cells in series, so 8.4V fully charged, each cell is at around 2200mAh (the seller wrongly advertises it as having a 5000mAh battery). It has a charge port and an included basic 8.4V charger.Thanks in advance.

author: jonpaul
author_rank: Super Contributor
timestamp: March 23, 2022, 10:11:01 am
content: Need a schematic to critiqueIn general Chinese power electronics is poorly designed and manufactured and protectedOften unreliable and dangerousAvoidJon

author: jpanhalt
author_rank: Super Contributor
timestamp: March 23, 2022, 11:04:13 am
content: Here's a "local" discussion of similar welders:https://www.eevblog.com/forum/reviews/mini-spot-welder-from-aliexpress/?topicseenHere's a thread by tatus1969 with his original design:https://www.eevblog.com/forum/projects/guesses-on-what-i-am-attempting-here/msg1232857/#msg1232857Sorry, if you already have those links. They were not referenced in your original post.The classical welder of that type used capacitors to provide the jolt of high current. tatus1969's original design used lithium cells. I did not follow that thread closely, but as I recall, the choice of cells was somewhat critical, and that author did some additional work with supercaps to augment the lithium batteries. The thread is 30 pages, but it would be worth skimming. Even better, maybe tatus1969 will give an update. In any event, as jonpaul said, one might assume that the protections built into the tatus1969 design might be skimped in a Chinese copy.

author: BinaryBits
author_rank: Contributor
timestamp: April 03, 2022, 09:19:38 am
content: Thanks for the links, I shall promptly read those.

author: m3vuv
author_rank: Super Contributor
timestamp: April 03, 2022, 12:48:49 pm
content: well i had a similar thing,there was yt vids showing it,one was called a maple leaf,first weld using mine set fire to the battery i was trying to spot weld,all fets went sc,i was running it from a 40ah fully charged car battery!,i replaced the fets and it did the same!Then replaced the fets a second time,Then i did the cap mod to the mcu as documented in the yt vid and that fixed it,it was caused by the mcu input voltage sagging + the same with the mosfet driver also,basicaly as far as i can see the fets were operating in there linear reigion so instantly blew up with several hundreds of amps,since the mod its fine,only issue i have is the welp probes get too hot to hold after 10+ welds,i always wave my psu charging the battery at 14v tho when i use it,must say it works flawless after the cap mod.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 06, 2022, 05:21:48 am
content: They seem to have put some effort into at least making it look good with those big brass bars and clean soldering.One of the main reasons the MOSfets get blown is by a lack of gate voltage, so they are opened enough to conduct a lot of current, but not enough to lower their resistance, and this leads to a lot of power dissipation in the FET's.A good guard against this is for example some capacitor that gets charged to 12V to 18V and then is used to generate the gate voltage, and hold it high enough during the weld itself, because the batteries can sag significantly during the high current pulse. Without an schematic, the easiest way to check this is with an oscilloscope.

author: m3vuv
author_rank: Super Contributor
timestamp: April 06, 2022, 01:44:08 pm
content: Try tacking a feed from the main battery power,via a diode to say a 470uf cap ,cut the existing vcc too the mcu and feed it from the + from the cap,that should cure any issues,did that to mine after blowing up 10 fets!,it has worked fine ever since,i use a 40ah car battery with a charger on it when using the welder,the trick is having enough umphh to power the mosfet driver so it doesent go into linear mode but gets turned had fully on instantly if fixed mine.!.