Adding DC to AC welder

[Cliff Lawson]

Hi Guys:

The article in the Feb-March 2003 Machinist's Workshop doesn't include a choke.

Any comments about this? Would there be a difference in how it works?

[Forrest Addy]

You may or may not need a choke.

I put a honkin' full wave bridge rectifier on my Lincoln 180 buzz box years ago and it ran well from day one. However, it ran a bit better (smoother) when I flipped a couple turns of the ground lead around the anvil.

Putting a rectifier on an AC only welder is only a step or two abive changing a light bulb but it does make a difference in running some rods - and you can run the few you can't run on AC only.

As for where to find big full wave bridge rectifiers cheap, I would suggest an industrial salvage yard. You need 200 V PIV. welding does induce higher voltage transients.

Here's a website where you'll find a bit of discussion of large rectifiers:

http://www.dansworkshop.com/00-index...n%20heater.gif

[This message has been edited by Forrest Addy (edited 01-24-2003).]

[milmat1]

I am not familiar with the article you mentioned, However you really do need to clean up the d.c. At the output of the bridge rectifer you will have inverted the negative going wave, and rectified the positive going wave, therefore you have a pulse train ("ripple"). Rather than a straight line d.c.
A large enough choke will help to clean up the signal, but you need to introduce large amounts of capacatance. In electronic power supply circuits, a rule of thumb says 2000uf per ampere of current. This is because electronic circuits like pure d.c. to operate. Especially if being used neer radio equipment. In your case i would think that this would be overkill. But in any case i am sure your welds will show the difference with a pure d.c. signal.

[Thrud]

milmat1:

You don't need to worry about the ripple in welding generally. Although a flat dc supply produces superior weld quality. But in general it is just nit picking.

For power supplies I agree 100% when they feed electonics. In Audio tight regulation gives cleaner sound and I prefer a robust supply (2x more than required) in these apps.

[halfnut]

Forrest,

In the article it says to use 1000V piv diodes, I myself thought that much overkill.

Your 200V piv sounds about right to me, I like it.

I've got my Lincoln 175 Shield Arc Jr running on a converter, works great at lower amperage levels. But it sure would be nice to convert that old Forney over to DC, much quieter. But I won't part with the Lincoln.