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Posted by BobH on May 5, 2008, 8:41 pm
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Jon Danniken wrote:
> "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
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
Good Luck,
BobH
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Posted by Jon Danniken on May 5, 2008, 9:01 pm
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"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
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Posted by BobH on May 5, 2008, 9:53 pm
Please log in for more thread options Jon Danniken wrote:
> "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
>
>
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!
Good Luck,
BobH
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Posted by Jon Danniken on May 6, 2008, 4:32 pm
Please log in for more thread options "BobH" wrote:
> 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!
Thanks, Bob, I appreciate knowing this. I'll look into getting one or both
of those books, can't have too much information.
Jon
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Posted by on May 5, 2008, 7:18 pm
Please log in for more thread options wrote:
> 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
Could you get the best of both worlds if the bias was changed back and
forth at a few hertz?
Spend a portion of a second 'more negative' then 'more positive' etc
etc.
Dave
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