MOT Spot Welder

IMHO a MOT transformer will not make a good basis for a welder. The core is likely ferrite and designed for high frequency operation. If the primary of your transformer is going to be connected to 120VAC-60Hz, you probably want a laminated E-core or a wound tape core. Most welding transformers have turns ratios on the order of 50/1. There is a lot to consider when designing a transformer from scratch. I have a barn full of used welding transformers in MA. I would be happy to sell you as many as you want real cheap. You can also find them on eBay. Unless you are determined to construct your own (which I do for a living), I would leave it to the professionals, and concentrate on learning how to weld. Saturation is avoided by proper core cross section for the ampere-turns in the transformer. You always want to use a core material with the highest saturation capacity you can obtain. Once you have selected the core material, you must increase the core cross section to avoid saturation. Once the core saturates, you get no more output and can damage the transformer.

So the core is ferrite?- How strange EVERY core I have seen & rewound for whatever is laminated E iron!. IOB(irascible Old Bassid).

I think the MOT transformer has a 20-1 with 20 at the secondary... they are designed to step up the voltage. In all of the examples I have seen from people who've made these MOT spot welders...they remove the secondary windings and replace them from anywhere between 3 and 10 windings of high-amperage coated cable. I think they stepped the potential from 120v down to about 6v or so...

I don't plan to use the transformer as it is. I need to get one first...then I can figure out how many turns would get me 50:1 or so. I have been considering this or a capacitive discharge welder...and this seems like a viable option and much cheaper. Capacitors have gotten MUCH more expensive it seems... Anyways thank you very much for the input...I'll take as much advise into consideration as I can get before planning and building this welder. I've seen some pretty nice tab welds done with the MOT welder...but I might have to go with a CD welder design I've seen.

If you can describe the materials you want to weld (type of metal, thickness, and configuration), I can tell you approximately how many welding amps you will need. Then you will be in a position to review design options. Not having a specific objective is a sure-fire recipe for having to re-design and rebuild, possibly many times. Better yet, if you send me samples of materials you plan to weld, I will weld them for you and let you know what it takes.

It would be a nickel battery pack building tab, and the ends of a nickel-metal battery. You can find something similar here: http://mtobattery.com/store/page7.html ; I think I might want to go slightly thicker as my pack will drain at higher amperage rates. maybe .007-.008 at 3/8" wide.

The two electrodes of the welder will come down and touch the surface of the battery tab on top of the battery. Zap. Zap. I will probably spot each one twice to ensure a good connection. I will try to find the example welder with three turns.

Dear Welderman,

We purchase used and remanufactured transformers and I would be interested to learn about your inventory. If you have an interest in selling them, please contact us at Spot Weld, Inc. Our phone number is 651-646-1393 or toll free at 800-582-5229.

Thank you,

SWI