Spot Welders – DIY and Consumer
With hobbyists now repacking their own battery packs for RC and general electronics there are literally hundreds of low power resistance welders now available. What should we, as hobbyists be looking at?
Most small spot welders coming out a few years back were only purchased and used by companies servicing laptop, RC and cordless power tools. They are now an essential part to all DIYers toolkit. The question remains, which one to get, make or buy?
In my own research, one thing that became quickly evident is battery storage (NiFePO4) discharge devices seem to be the most popular, the MOSFET switch gates timed to give best welding current for specific nickel strip gauges. Along with this is the complaint of these devices not being reliable. I gather this is because the MOS devices are often destroyed in the welding process – through a timed switching sequence the MOSFET electrodes connect the welder electrodes to the battery. User abuse or poor quality welders? Some consumers (the electronics DIY community) state that the devices either use inferior MOS devices or too few in the switching process. Does this skew our judgement when looking for a spot welder? Almost definitely. We’ll look at two very popular welders and their specific difference, one being a battery/capacity discharge device, and the other using a high current transformer.
We’ll kick off with the kWeld resistance spot welder, manufactured by Keenlab in Germany. Reviews on the kWeld are 99.9% positive, making this possibly one of the best battery tag spot welders available globally.
Keenlab kWeld: https://www.keenlab.de/index.php/portfolio-item/kweld/
If one wants to see the process in the actual build from revision one have a look here: https://www.eevblog.com/forum/projects/guesses-on-what-i-am-attempting-here/
The specifications of this unit are impressive. I like the fact that the engineer who designed this welder included a safety fuse. Many of them do not have this, it’s critical I think. How robust is this unit – I found one negative review and Keenlab got back to the owner immediately. (Possibility of MOS faiure). The Eevblog forum is very useful in that the design engineer explains his rationale and we get input from other users.
Next: The Sunkko 709A spot welder
The Sunkko 709A teardown is featured here: https://www.eevblog.com/forum/reviews/sunkko-709a-spot-welder-teardown/
Both the kWeld and the 709A are in the same price range except the kWeld is supplied without the battery. (the designer will make recommendations on battery type – DO use the recommended battery).
Whereas the kWeld advantage of portability and probably higher energy dissipation to weld 0.4mm nickel steel plate the 709A is going to be more forgiving of user type. Why would this be?
Spot Welders – DIY and Consumer recommendations from the users.
An interesting observation coming from many of the more educated and experienced spot welder users around damaged MOS devices is the user not sticking to recommended battery types. The energy dissipation must be instantaneous and clean – using a low capacity power source may not only cause bad welds but will damage the switching transistor, usually very low R(on) MOS devices.
The DIYers choice.
Mostly using microwave oven transformers with the secondary windings removed. The secondary winding usually is wound next to the primary which makes it a fairly easy project to remove – usually a hacksaw is all that is required. But do this removal carefully.
There are various ways to switch the transformer, pulsed timing being the more popular.
There will be a follow up to this article, chiefly because I have built one of these and have quite a few of the controllers in stock. Unfortunately at the time of purchase the 100A triac version was not available. I purchased mine through AliExpress. Will share with you some very clean and great circuits to trigger these transformers in the follow up. Have a great day.