"Randal O'Brian" wrote:

"Jon Danniken" wrote:

Thanks, Clare, I appreciate it. A duty cycle control shouldn't be too
hard to put in there, if I'm feeding them anyway.

Brings up another issue though, I'm guessing the waveform will need to
swing from positive to negative, and not just dance around above the work
potential. Hmmm, I'll have to think about that one.

Jon

The commercial inverter welders I 've seen use a transistor H bridge
between the DC output and the work piece to get the AC output. The
technology is the same as a VFD but single-phase only. A microprocessor
would seem to make it fairly easy to vary both freq. and pulse width as
needed.


Thanks, Randall, I appreciate that. When I was looking at inverters the
other night (from Clare's suggestion), I was contemplating a two-mosfet
jobby, such as this:

http://engr.nmsu.edu/~etti/spring97/electronics/cmos/IMG00006.GIF

Since I haven't built an inverter of any type before, what would be the
advantage of going with an H bridge?

Thanks,

Jon



Not sure if it will help but I posted the schematic of my Hitachi

clare at snyder dot ontario dot canada> wrote:
On Sun, 4 May 2008 21:04:24 -0700, "Jon Danniken" wrote

Alrighty then, so a squarewave would be the easiest waveform to make,
would
that be a sufficient enough selection of waveforms, and what frequency
range
would be most useful?

Jon

Square wave AC with variable duty cycle is best. More negative cleans
better, more positive penetrates better, from what I remember.

Thanks, Clare, I appreciate it. A duty cycle control shouldn't be too hard
to put in there, if I'm feeding them anyway.

Brings up another issue though, I'm guessing the waveform will need to swing
from positive to negative, and not just dance around above the work
potential. Hmmm, I'll have to think about that one.

Jon

"Randal O'Brian" wrote:
"Jon Danniken" wrote:
Thanks, Clare, I appreciate it. A duty cycle control shouldn't be too
hard to put in there, if I'm feeding them anyway.

Brings up another issue though, I'm guessing the waveform will need to
swing from positive to negative, and not just dance around above the work
potential. Hmmm, I'll have to think about that one.

Jon
The commercial inverter welders I 've seen use a transistor H bridge
between the DC output and the work piece to get the AC output. The
technology is the same as a VFD but single-phase only. A microprocessor
would seem to make it fairly easy to vary both freq. and pulse width as
needed.

Thanks, Randall, I appreciate that. When I was looking at inverters the
other night (from Clare's suggestion), I was contemplating a two-mosfet
jobby, such as this:

http://engr.nmsu.edu/~etti/spring97/electronics/cmos/IMG00006.GIF

Since I haven't built an inverter of any type before, what would be the
advantage of going with an H bridge?

Thanks,

Jon


If you have a center tapped power transformer, you can get polarity

Jon Danniken wrote:

Thanks, Randall, I appreciate that. When I was looking at inverters the
other night (from Clare's suggestion), I was contemplating a two-mosfet
jobby, such as this:

http://engr.nmsu.edu/~etti/spring97/electronics/cmos/IMG00006.GIF

Since I haven't built an inverter of any type before, what would be the
advantage of going with an H bridge?


If you have a center tapped power transformer, you can get polarity
reversal and DC shift rather than just pulsing out of two switches.
Without the center tap, you need to swap the work and the electrode
polarities with 4 switches (H).

Not related to this conversation, but here is a pointer to a guy who built
a TIG machine from a buzz box I think:
http://www3.telus.net/public/a5a26316/TIG_Welder.html

"BobH" wrote:
Jon Danniken wrote:
Thanks, Randall, I appreciate that. When I was looking at inverters the
other night (from Clare's suggestion), I was contemplating a two-mosfet
jobby, such as this:

http://engr.nmsu.edu/~etti/spring97/electronics/cmos/IMG00006.GIF

Since I haven't built an inverter of any type before, what would be the
advantage of going with an H bridge?


If you have a center tapped power transformer, you can get polarity
reversal and DC shift rather than just pulsing out of two switches.
Without the center tap, you need to swap the work and the electrode
polarities with 4 switches (H).

Not related to this conversation, but here is a pointer to a guy who built
a TIG machine from a buzz box I think:
http://www3.telus.net/public/a5a26316/TIG_Welder.html

Gotcha, thanks Bob. I was wondering about how I was going to get the
polarity shifts, didn't think about swapping it out between the two leads.

Thanks for that tip! [NPI] :)

Jon

"Gunner Asch" wrote:
Most full function tig welders have this AC as an ajustable
feature...more bias one one for cleaning, the other way for more
penetration.

Personally I use more penetration by about 60/40 over cleaning most fo
the time, at about 60 hz, though I do have the option of going from
1pps to about 300.

Many plain tig welders get by just fine at 60 hz

Thanks, Gunner, I appreciate knowing your experience in this matter. BTW,
in your usage, do you tend to find having a foot control an important part
of TIGging aluminum?

Thanks,

Jon

Foot or fingertip control is WAY MONDO importanto for ANY tig

Foot or fingertip control is WAY MONDO importanto for ANY tig
welding. You can get by without one..but you wont like it

And least of all with aluminum, with its very wierd solid/liquidous
range. You have to feather your way into it..else you will wind up
with the base metal all over your workbench

An inverter welder is really a switching power supply with an unusual
control loop and a strange load. Even the H bridge is pushing into that
area. There are a number of good books on switching power supply design.
My favorites are by a guy named Marty Brown, "Practical Switching Power
Supply Design" and "Power Supply Cookbook". He talks about switching
section design as H bridge or half bridge and thermal aspects and lots of
good stuff. Powells Technical books in Portland, OR is where I got my
copies.

Also, I did not want to speak for Gunner, but your question about TIG'ing
aluminum and footpedals - I think you need the footpedal even more for
aluminum than steel, and I would hate to weld steel without one!