"Ignoramus8187" wrote:
For your question regarding TIG welding, I would say that if you can
replicate capabilities of a Syncrowave 250 DX, you would do very
well. Miller has the specs and manual available online.

Thanks for that, Iggy. That manual has some good expalations of the TIG
process, as well as the schematic, which is very informative for me.
The schematic, however, is bringing up a question in my head. For DC
usage,
it is pretty straight forward; xfmr, SCR bridge, smoothing choke, and
that's
about it.

For AC, however, they are doing something I haven't come across before.

(here is the relevent section; work is green, electrode is RED, and
blue is the (+) and (-) output from the SCR bridge:

http://home.comcast.net/~danniken/Sy...DX_Page_60.png

if it wraps: http://tinyurl.com/48bzgn

Now, the work goes back to the transformer, no surprises there, but
the electrode does something odd. It goes to the part of the bridge
where AC normally goes, and the DC outputs of the bridge are
connected through the smoothing choke!

What the heck?

They also have this going to the DC output from a diode bridge, SR2 in
the schematic.

Now, I haven't seen this topology before, but in the absence of a
later
inverter, I have to assume they are using the SCR bridge (and possibly
the diode bridge) to modulate the AC waveform somehow.

Any ideas what this is doing, or even what it is called? Maybe they
are doing this to avoid having to have a transistor/mosfet inverter
after the rectifier, I dunno. Sure is compelling, though.


Jon,
If you look at the circuit as you have highlighted it with the switch in
the AC position, you noted that one side of the transformer goes out to
the work - call that ground.

The other side of the transformer drives the anode of one and cathode
of the other SCR in the upper pack. That voltage will be positive on
one half cycle of the AC line and negative on the other with respect
to our work ground.

On the beginning of the positive half cycle, you would turn on G2 of the
lower SCR pack. This connects one end of the smoothing inductor to the
electrode. Later in the cycle, at a time determined by the welding
current selected, you turn on G1 of the upper SCR Pack. This puts a
positive voltage on the far end of the smoothing inductor. The current
transformer (HD1)measures the current for the electronics to control the
SCR firing angle or delay into each half cycle.

On the beginning of the negative half cycle the SCR roles reverse.
Gate G1 on the lower pack gets turned on for the the half cycle and
gate G2 gets fired for the phase modulation angle to get the desired
current out with reversed polarity.

It is possible that the lower SCR gates are fired at some other time in
the half cycle rather than left on for the whole half cycle. The
Syncrowave is a square wave machine, but I suspect that those two SCR's
are only used for "routing" while the upper pair are the phase control
pair.

The bridge rectifier SR2 is probably used to provide current for the SCR
gate drives, but it might be used for phase reference too. Note that the
only place outputs of the bridge go is the control board. No big power
path here. Someone would have to take a scope to their Syncrowave to
know for sure.

Your surmise that they are using this to avoid having an inverter is
correct. SCR's are mature industrial technology. I think the basic
design has been around since the 70's.

The Syncrowave 250 is a really nice machine - I have been using one for
a couple of years now.

Good Luck,
BobH

"BobH"
Jon Danniken wrote:
(here is the relevent section; work is green, electrode is RED, and
blue is the (+) and (-) output from the SCR bridge:

http://home.comcast.net/~danniken/Sy...DX_Page_60.png

if it wraps: http://tinyurl.com/48bzgn

Now, the work goes back to the transformer, no surprises there, but
the electrode does something odd. It goes to the part of the bridge
where AC normally goes, and the DC outputs of the bridge are
connected through the smoothing choke!

What the heck?

They also have this going to the DC output from a diode bridge, SR2 in
the schematic.

Now, I haven't seen this topology before, but in the absence of a
later
inverter, I have to assume they are using the SCR bridge (and possibly
the diode bridge) to modulate the AC waveform somehow.

Any ideas what this is doing, or even what it is called? Maybe they
are doing this to avoid having to have a transistor/mosfet inverter
after the rectifier, I dunno. Sure is compelling, though.


Jon,
If you look at the circuit as you have highlighted it with the switch in
the AC position, you noted that one side of the transformer goes out to
the work - call that ground.

The other side of the transformer drives the anode of one and cathode
of the other SCR in the upper pack. That voltage will be positive on
one half cycle of the AC line and negative on the other with respect
to our work ground.

On the beginning of the positive half cycle, you would turn on G2 of the
lower SCR pack. This connects one end of the smoothing inductor to the
electrode. Later in the cycle, at a time determined by the welding
current selected, you turn on G1 of the upper SCR Pack. This puts a
positive voltage on the far end of the smoothing inductor. The current
transformer (HD1)measures the current for the electronics to control the
SCR firing angle or delay into each half cycle.

On the beginning of the negative half cycle the SCR roles reverse.
Gate G1 on the lower pack gets turned on for the the half cycle and
gate G2 gets fired for the phase modulation angle to get the desired
current out with reversed polarity.

It is possible that the lower SCR gates are fired at some other time in
the half cycle rather than left on for the whole half cycle. The
Syncrowave is a square wave machine, but I suspect that those two SCR's
are only used for "routing" while the upper pair are the phase control
pair.

The bridge rectifier SR2 is probably used to provide current for the SCR
gate drives, but it might be used for phase reference too. Note that the
only place outputs of the bridge go is the control board. No big power
path here. Someone would have to take a scope to their Syncrowave to
know for sure.

Your surmise that they are using this to avoid having an inverter is
correct. SCR's are mature industrial technology. I think the basic design
has been around since the 70's.

The Syncrowave 250 is a really nice machine - I have been using one for
a couple of years now.

Thanks, Bob, that makes complete sense to me now; very much appreciated.

The one thing left that I am puzzled about, though, is the SR2. The way
that I'm looking at it though is that it is getting AC from the control
board, and putting out DC into the SCR bridge/smoothing choke circuit?
Those diodes in SR2 are described in the manual as "RECTIFIER, INTEG BRIDGE
40. AMP 800V", which is a pretty stout fellow.

Thanks again,

Jon