Jon Danniken

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:

if it wraps: 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.

Thanks again,

Jon

"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.

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

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

1. AMP 800V", which is a pretty stout fellow.

Thanks again,

Jon