Subject: DC Welding from an AC Welder

From: [email-masked] (Roger Ronald)
Date: 18 Dec 1994 14:33:15 GMT

I am interested in doing some welding of bronze (am building
a 33 foot sailboat). I have an AC welder but my book says to
use RPDC.

Has anyone built and used a bridge rectifer for an AC welder
to weld DC? Surplus diodes of enormous size are often available
pretty cheap. Maybe there is a commercial version of this idea?

Roger Ronald

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From: [email-masked] (Mike Rehmus)
Date: 18 Dec 1994 23:22:47 GMT

In <[email-masked]> [email-masked] (Roger Ronald) 

>pretty cheap. Maybe there is a commercial version of this idea?

There are commercial versions but they are built into the AC/DC welders 
themselves.  I believe that when you rectify the AC, you have to derate 
the welder by a factor of 2 which frequently leaves you without enough 
amperage to do the job.

Mike

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From: [email-masked] (David Jenkins)
Date: 20 Dec 1994 15:39:19 GMT

>From  article <[email-masked]>, by [email-masked] (David Jenkins):
> 
> I keep hearing references to "external TIG" units - what are these, how do
> they work and, last but not least, how much do they cost?  Can they be built?
> 

An afterthought - can someone explain how a TIG works?

I don't mean the gas bit - I presume that's similar to a MIG, and I
understand the tungsten tip in the torch.  I would like an explanation of
the voltages, currents and frequencies involved.

Any info gratefully received.   ;-)

-- 
David Jenkins						Not long now...
--------------------------%<-------------------------------------------
	 Cuius testiculos habes, habeas cardia et cerebellum.
			Terry Pratchett --- Small Gods.

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From: [email-masked] (Waldo R. Robinson)
Date: Mon, 19 Dec 1994 18:06:44 GMT

In article <[email-masked]>,
>the welder by a factor of 2 which frequently leaves you without enough 

Yes, you can build up a bridge rectifier to make your "buss box"
a d.c. machine and get the advantages of such though a single
phase machine will not be exactly the same as a rotary converter
machine or a poly-phase machine. You can also use it this rectified
power with the external TIG high frequency units. (I've been
using this combination for about 10 years.) There is no derating
of the a.c. welder capacity as long as you use a bridge rectifier
which is what one should do on a transformer anyway. The only
reason for the 2:1 derating would be if one used a single diode
which is theoretically possible but not a good approach. Be sure
to use diodes with adequate voltage and current ratings. The open
circuit volltage is considerably higher than the arc voltage and
the current rating should be higher than the maximum the welder is
capable of just in case one forgets and tries operating at such a
setting. I have my LINDE buszz-box set up with both the ac and dc

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From: [email-masked] (David Jenkins)
Date: 20 Dec 1994 09:58:41 GMT

>From  article <[email-masked]>, by [email-masked] (Waldo R. Robinson):
> 
> power with the external TIG high frequency units. 

I'm in the market for a welder - Most of my work will be with an ordinary
stick welder (I don't care what others think, I _like_ stick welding :-) 
- but I have an up-and-coming project where a TIG would be ideal.  This is 
for thin (0.5mm) sheet stainless steel.

In an ideal world, I would rather pay a good sum of money for a quality
stick welder with decent regulation and duty cycle, with a not-too-expensive
add-on for occasional fine work.  

A MIG I could afford would be too lightweight for my usual welding, and too
heavy-handed for the fine work I have in mind.  I've also found that
affordable MIGs are not easy to drive, with problems in the wire-drive
department.  I've used a professional size MIG (up to 200 amps, 100% duty
cycle and 1.2mm wire!) and, frankly, domestic units simply don't come
anywhere near the useability and build quality.

Any help, guidance, etc. gratefully accepted!

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From: [email-masked] (James Kirkpatrick)
Date: 22 Dec 94 16:25:51 MST

David Jenkins asks --

One short answer is from the FAQ:

17.  What's TIG and MIG?

     TIG - Tungsten Inert Gas

     A small torch with a tungsten electrode is used to make the arc inside
     an envelope of an inert gas, usually argon or some argon mixture.  A
     filler rod is manually introduced to complete the weld.  The resulting
     weld is very pretty and usually requires no further finish.  It is
     used mostly for welding sheets of mild steel, stainless steel or
     aluminum.  The better machines have a foot control and a high frequency
     arc starter.  Any sizable stick welder can be retro-fitted to do TIG
     welding, but without the foot control.

Jim

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From: [email-masked] (Gordon W J Sorensen)
Date: Thu, 22 Dec 1994 21:07:00 GMT

: From article <[email-masked]>, by [email-masked] (David Jenkins):
: > 
: > I keep hearing references to "external TIG" units - what are these, how do
: > they work and, last but not least, how much do they cost?  Can they be built?
: > 

: An afterthought - can someone explain how a TIG works?

: I don't mean the gas bit - I presume that's similar to a MIG, and I
: understand the tungsten tip in the torch.  I would like an explanation of
: the voltages, currents and frequencies involved.

: Any info gratefully received.   ;-)

: -- 
: David Jenkins						Not long now...
: --------------------------%<-------------------------------------------
: 	 Cuius testiculos habes, habeas cardia et cerebellum.
: 			Terry Pratchett --- Small Gods.

Hi David,

TIG is Tungsten Inert Gas welding.  Some people refer to it as a TIG torch.
Basically the tungsten is used as a non-consumable electrode for establishing
and maintaining an arc. The inert gas provides the shielding, and you provide
the filler material externally. It is very versitile allowing for the 
welding of many different materials, at the expensive of speed and 
convenience.  It requires more skill than MIG and is comparable to 
oxygen/acetlyne welding.  In fact the TIG torch operates essentially the
same as the oxy/act torch. The TIG torch is operated with DC, straight
and reverse polarity, 60 hz AC, and AC or DC with a high frequency voltage
super-imposed on them.  There are also power supplies that deliver
various forms of square wave current with different duty cycles, and
asymmetric wave forms. I haven't used these but apparently they help when
welding certain alloys.

The high frequecy is in the kilo-hertz range, 30 or so.  The high frequency 
is great for getting the arc started and has as scrubbing action on the 
molten puddle.  This is especially useful when welding aluminum.

Hope that helps a bit.

gordon s.

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From: [email-masked] (Tom Taylor)
Date: Tue, 20 Dec 1994 07:18:50 GMT

In article <[email-masked]>, [email-masked] (Mike

> the welder by a factor of 2 which frequently leaves you without enough 

Is the output from a DC welder half-wave or full-wave rectified?  I assume
that if you looked at the waveform on a scope it wouldn't be smooth at
all.  Does it matter?

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From: [email-masked] (Gary Helfrich)
Date: 25 Dec 1994 10:30:45 -0800

David Jenkins ([email-masked]) wrote:

Tig requires a power supply that is constant current.  Mig on the other 
hand requires constant voltage.  A good tig power source will have a very 
high (over 100 volts) open circuit voltage.  Any good stick welder can be 
used as a tig power source, but the current regulation is not as tight as 
a high quality tig power source.

Welding voltages are between 12 and 18 volts.  AC is used for aluminum 
and DC is used for steel, titanium, etc.  In most cases the ac frequency 
is 60 Hz. (Some inverter sources use a higher welding frequency)

Gary Helfrich
Arctos Machine

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From: [email-masked] (Henry Kroker)
Date: 27 Dec 1994 21:23:18 GMT

Apologies if this comes out screwed up.
I followed this newsgroup some time ago, and omly recently regained access.

I built a gizmo like the one you are after some time ago- it works great!
What I did, was obtain four 150Amp 60V? (can't remember numbers but will look
them up if you want) a heavy (approx 4 lb.) transformer some copper pipe, and
a bunch of 00 cable. 

I arranged the four diodes on a piece of plywood like a bridge (mine had
heatsinks) and connected them with copper pipe. I used short pieces of welding 
cable to attach it to the welders output.
The transformer was stripped of all it's wiring, and (it was a while back)
wound 4 turns of heavy cable as tightly as possible to the core. This is
attached in series with the positive side of the bridge.

I had a proffesional welder work with it for a while and he claimed it worked
as good as their 3 phase Lincoln? Not sure what that means but...
I suspect a solid AC welder makes a big differance.

Anyway I have LOTS of power, and the pipe and heatsinks were probably overkill,
but it runs cool as can be even at 160 AMPS (I rarely go over this). 
Watch out for lousy connections, particulariliy if your duty cycle is high.

I'm currently working on a spotwelder mod if anyones interested.

Henry