DIY spot welder: transformer, caps discharge or car battery? - Page 1
Forum section: electronics / projects

author: reyntjensm
author_rank: Regular Contributor
timestamp: January 31, 2023, 11:54:46 pm
content: Hi,I want to build my own 18650 pack so i was in need for a spot welder. As aliexpress is full of cheap spotwelders, i preferred to make one by myself as it would be cheap and i wouldn't have to wait for 2 months to start with my project.After some intense research, say 3 mins on youtube i was already salvaging a transformer from an old microwave. With some 100mm2(2 turns on secondary) wire, arduino and a solid state relay i did a first test. I was not happy with the results. It was more a melting/burning connection instead of a clean spot weld. So i rewired the secondary with a pair of 16mm2(4 turns). Again i was not satisfied with the result. I need about 100-200ms for a good connection and i need to try it multiple times to get a good connection. At this point my material is already very hot and i'm worried it would heat up the cells too much.After some more research... i found out discharging a large bank of capacitors should give better results. Any suggestions on this idea? What about shorting out a car battery? I know they sell controllers for these applications on aliexpress.What is the definition of a good spot weld? Is there some simple math to have an idea about the amount of energy needed to do a spot weld? In my case i use 0.2mm nickle strips.

author: tautech
author_rank: Super Contributor
timestamp: February 01, 2023, 06:08:30 am
content: Suggest you study the development of Kweld:https://www.eevblog.com/forum/projects/guesses-on-what-i-am-attempting-here/

author: Doctorandus_P
author_rank: Super Contributor
timestamp: February 02, 2023, 11:00:22 am
content: Since you've already spent 3 minutes on this you probably already know everything and there is nothing more to add.

author: reyntjensm
author_rank: Regular Contributor
timestamp: February 02, 2023, 04:32:45 pm
content: Quote from: Doctorandus_P on February 02, 2023, 11:00:22 amThank you for your great feedback. I guess you are not really in too jokes. At least you got some spare time to waste

author: james_s
author_rank: Super Contributor
timestamp: February 02, 2023, 04:33:25 pm
content: Capacitive discharge is what is typically used for battery tab welding, and is what I used when I built my welder. A friend of mine needed something beefier so he got one of those K-weld things and said it works well.

author: reyntjensm
author_rank: Regular Contributor
timestamp: February 02, 2023, 04:38:22 pm
content: Did you use some mosfets to control the discharge?

author: Bud
author_rank: Super Contributor
timestamp: February 02, 2023, 05:08:27 pm
content: Unless the source battery connects via a mechanical relay or you use the welder often, i would avoid battery based welders. I have one which i use occasionally and the battery in it discharges through the mosfets when stored. I have to pull it from the shelf once in a while to keep the battery charged a bit.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 02, 2023, 05:38:05 pm
content: We use an older thermocouple welder (that also works great for many other things, like battery tabs) and the circuit is pretty simple. A small Variac and rectifier is used to charge up a high voltage electrolytic. I think it goes up to 300-400 volts. This is discharged through the primary of a big transformer. The secondary is a turn or two of heavy copper strap, which is also brought out through the front panel for connection to whatever hand tool or fixture is to be used. There's a limit resistor between the charging circuit and the capacitor so the charging circuit doesn't get shorted out during the weld process. The switch between the capacitor and the transformer is a mercury wetted relay, but today you'd probably use something different.

author: james_s
author_rank: Super Contributor
timestamp: February 02, 2023, 09:34:52 pm
content: Quote from: reyntjensm on February 02, 2023, 04:38:22 pmI used a big old stud mount SCR I found on ebay, the sort of thing that used to be used in big motor control inverters back in the 70s.

author: tautech
author_rank: Super Contributor
timestamp: February 02, 2023, 09:38:13 pm
content: Quote from: james_s on February 02, 2023, 09:34:52 pmHardly decent control James as once it's ON it's ON until the source is depleted.Kweld provides proper duration control.

author: james_s
author_rank: Super Contributor
timestamp: February 02, 2023, 09:50:17 pm
content: Quote from: tautech on February 02, 2023, 09:38:13 pmIt's perfect for this application because that is exactly the behavior you want for a capacitive discharge welder, which as I mentioned is the technique I used for mine. The energy of the weld is determined by the voltage the capacitor bank is charged to.

author: tautech
author_rank: Super Contributor
timestamp: February 02, 2023, 10:40:46 pm
content: Quote from: james_s on February 02, 2023, 09:50:17 pmExactly but a fact easily missed by some.