Reducing Power output

[crackle]

Hi Folks,
I use a very old Cebora 140 MIG and have disaster after disaster welding thin steel sheet. 1.6 mm is ok for me but thinner is a molten mess. So I decided to have a go at adding a power diode in the negative output rail to see if a small voltage drop of about 0.7 volt dc would reduce the minimum power setting to a usable power level for me welding 1 mm steel sheet with the old Cebora. I realise that more proficient MIG welders some how weld even thinner than 1 mm and that modern MIGs with more dials and buttons than a 747 probably can weld very very thin metals.
So after adding the power diode consisting of 6 diodes (from the junk box) in parallel (35 amp 100 volt working) to get a sufficient power rating but not ideal by any means I gave it a try and the 1st shaky result is in the attached pic. The steel is 1 mm thick and the weld is approx 95 mm long. Obviously a lot more practice is required but for me I can now MIG weld 1 mm steel but it looks so yucky. Practice, practice, practice.

Cheers,
Crackle

 

[normspanners]

your diodes are not going to help very much,only limiting the current that they can pass the old cebora's suffered from the pot on the wire speed control not being reliable, better to change it to a good quality wire wound so that you can adjust the wire speed more precisly

 

[normspanners]

Just had a thought what voltage are you getting if it is high then you could use a 12V transformer to drop the mains down a little

 

[R Kraft]

Each 1 volt drop in voltage will lower the amperage by about 20amps.
Does Cebora use capacitors in the output ?

 

[crackle]

The old Cebora 140 does not have capacitors across the output rails. I'm sure the addition would provide a smoother weld finish but I suspect that the caps would increase the weld current thus defeating the purpose of reducing the welding power.

Below is the cct of the MIG:

Crackle

 

[mike 109444]

Might be worth making up a new inductor (choke on your diagram) this could e switched over with ori one for trial. You would want to reduce the inductance. Some larger welders have up to 3 different connectors on the earth lead and you select more inductance for thicker metal by unpluging the earth lead and pluging it into the higher inductance socket. Implying less inductance = less force in weld. You could try winding some wire round a cast iron tube (drain pipe?) as another member had done to one of his machines.
That and maybe some caps to smooth the reduced power.

 

[mike 109444]

PS let us know what you come up with for future reference.

 

[R Kraft]

If manufactured with them.a welder that size would have about 50-100,000uf of capacitors.
It would also raise the power out by about 25%.

A small capacitor would raise the open circuit voltage and perhaps help the arc start and maintain
You might try just clipping a 470uf capacitor into the thing, just as an experiment.
I wouldn't go any higher than 4700uf total.