Jon Danniken

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/Synch250DX_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.
Good question. I saw the bridge and it did not look like power path stuff, so I assumed it was power to the board for gate drives or maybe phase sensing. After you comment, I looked again and you are correct, the drawing shows the bridge with the AC input connected to the board. I assume that the drawing is correct.
I am fishing here, because I don't have the board schematic to know for sure what is on the board end of that bridge. I suspect that they may be using it to turn off the SCR's before the power line zero crossing. As you say, a 40 Amp 800V bridge is pretty stout. That 40 Amp is probably continuous current, they would probably handle 5X that or more in pulse mode. If you dumped a capacitor into that bridge from the board that was charged to the instantaneous phase voltage, you could probably drive the current through the SCRs to zero for long enough to switch them off. Switching off the SCR before the power line zero crossing would supply the negative side of the square wave. I have not looked at the current waveform shape on my Syncrowave to confirm this kind of switching, but I can't think of anything else to use that bridge in that configuration.
For some reason, my newsfeed is not showing this article thread, so my replies are cut and pasted from Google. Sorry if the readability suffers.
Good Luck, BobH
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"BobH" wrote:

Hi Bob, no problems with readability on this end, don't know why it's not getting propagated on the server, though.
Thanks for your estimation as to the function of SR2; this being my first time playing with thyristors I am much appreciative of your experience. I didn't think of shutting off the SCR this way, but in the absence of GTO SCRs, it makes sense.
I do know that they are controlling the polarity percentages somehow, as well as the pulsing frequency, and with the lack of a transistor/FET bridge after the thyristor bridge, it seems to be a likely culprit.
It would sure be nice to have the controller board schematic though, wouldn't it. :)
When I get a few of my parts in I will be playing around with this thing and looking at waveforms on my scope, which will hopefully give me some more clues as to what is going on (as well as raise some more questions, of course. )
Thanks again,
Jon
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