How to Reduce Output Current on DIY Microwave Transformer Welder (130A-230A, 50V)?

Hi guys,

I made myself a microwave transformer welder! I'm an amateur inventer with a provisional patent and hopefully more on the way, so I needed a welder in my tool box. Total cost ~$10 vs $300. and for rare use it is perfect. BUT I have a problem I didn't anticipate, it is way too powerful (130A to 230A) most DIY welders on instrustables are 40A. To produce the 230A, I substitute the 130A for another 230A transformer, but even at 130A I'm eating metal.

*Here are the specs:*
Transformers In Series: 50V
Welding: 130A @ 50V 6500W

Tested individually, the 2 transformers are:

22.5 Volt Transformer:
No Load: 7.16A @ 120V 60Hz 859W
Welding:
Primary: 41A @120V 4920W
Secondary: 210A @ 22.5V 4725W

27 Volt Transformer:
No Load: 3.58A @ 120V 60Hz 429.6W
Welding:
Primary: 34.5A @120V 4140W
Secondary: 130A @ 27V 3510W

Total load will be approx. 10KW @ 120V 60Hz and 80 amps.

I have looked everywhere, looked into ohms law, looked for replacement parts for similar welders, but I'm unsure of the proper ohms and wattage of a rheostat that would fit this application.

edit: I keep coming back at a rheostat rated at .6 ohms? That doesn't seem right. (at least I think this is suposed to be easy question)

Thank you in advance!

Cooper,

go to the Ohmite webb site, there is an engineering article there that describes in detail how to select the correct rheostat for your application

from your description, a 64 ohm or 75 ohm 150watt ceramic rheostat will probably take care of your problem

the 75 is available from Newark, the 64 ohm is available from any lincoln dist, it is an M-5090C

Hope this helps

This is just off the top of my head, but I think your specifications may be off. These don't look like 3KW transformers to me. If you are using a transformer designed for a lower voltage primary, or for more resistance in the secondary, I would think it would fail spectacularly.

My limited understanding of arc welding is that once an arc is established, the output is virtually shorted so the current is mostly limited by the primary voltage and resistance. I would guess that shorting the output of most transformers wouldn't be good for them. Of course maybe microwave transformers are different. However, most microwaves use standard 120V outlets so they can't be 30A or more.

In general if most similar welders are 40A there may be a reason. Any rheostat that reduces the power will have to generate heat equivalent to the power lost. In this case that would be a big and expensive rheostat.

Yeah, that was my though, exactly, when I looked at the photo -- those transformers don't look big enough to supply that kind of power.

But, apparently they're supplying enough power to cause Cooper problems in terms of welding.

Oops, I didn't hit reply on post a few hours ago...

Hey, thank you very much! You guys are great and prompt!

My garage outlet was double wired for an electric dryer, but has a 120V outlet.

Yes, I've found shorting and welding produce the same amps. So far I have just been doing power testing and minor welding and the transformers get up to about 90F after about 45 seconds, but that is max amperage which would be rarely used. Once the enclosure is finished, each transformer will have at least its own 120mm fan and maybe an old CPU heatsink too.

I checked again and it's drawing 62A @ 120V- 7440W.

I used 10KW as a baseline if I was using two 41A 120V transformers.

Thanks again!