AC Square Wave Balance Control for TIG Welding?

Thread Starter

Krisby

Joined Mar 13, 2020
4
Hello People!

This is my first Post here.
I have searched the internet in vain, trying to find out the electronics behind "Arc Balance Control", which is used on TIG welders for the welding of aluminium. I understand what it is and why it is used when TIG welding on aluminium, but I can't find any information about how the proportions of the negative to positive polarity of the AC square wave are actually adjusted. When the Tungsten electrode is energised with the negative phase of the square wave current, the arc penetrates the aluminium work, and when the electrode switches to the positive phase, any aluminium oxide is cleaned off the surface. Somehow, by turning an "AC Balance Control" knob from 20% to 80%, the proportion of electrode negative to electrode positive, that is to say the proportion of the negative to positive levels of the square wave, can be adjusted. My analog TIG welder has no adjustment for the frequency of the square wave, and I'm guessing it is about 60 Hz, and 50% of the time the wave is in the negative direction, and 50% of the time the wave is positive. So, within that given frequency, the proportion of the time spent in the negative wave and inversely in the positive wave, can be adjusted (balanced).

But, how is this proportional balance of the square wave controlled? What is the technology behind it? I can't find any information or circuit diagrams, so I am hoping that someone on this forum might please be able to shed some light on the subject, or direct me to some source or authority that can explain it to me.

One authority has suggested:

"Presumably it is a type of pulse width modulation operating on the AC waveform, with an adjustable duty cycle between the positive and negative portions. It may be implemented using a TRIAC with separate duty cycle setting for each polarity. It may be possible to create this waveform by stepping down the 50Hz mains waveform to a lower voltage, followed by a Triac-based duty cycle controller."

Hmm, a "Triac-based duty cycle controller"............. is that what the TIG Welding machines use to effect this "AC Balance Control"? If so, were can I get more information?

Previously, I have converted a mains-powered transformer MIG welder to run on DC from onboard 18650 Lithium batteries, and I would like to do the same kind of conversion (or build from scratch) for a TIG welder.

Hope some info is incoming.

Cheers,
Krisby.
 

shortbus

Joined Sep 30, 2009
7,756
There may not be a "one size fits all" answer. You will need to pick one you want to emulate and compare it to the brand/model you have. Many of the better welder companies post there schematics online, I know Miller does or did.
 

Thread Starter

Krisby

Joined Mar 13, 2020
4
Thank you shortbus for your response, but it appears now that I have a problem in being understood, because of my inability to frame my question well enough. I'm trying to find information about a certain type of specific circuit, but I don't know what it's name is, and so I don't know what to look for or what to ask for! My post consisted mainly of what I had emailed to an electronics magazine, and the authority I quoted was the response (that the circuit probably used triacs). The welding machine schematics I've looked at so far only show generalized circuits or block diagrams. If anyone can point me to a detailed schematic of a generic-type analog TIG 200P AC/DC, or any schematic that shows the circuit I'm looking for, that would be helpful.

Perhaps people are thrown off by my mention of "TIG Welding", but my question is not about welding, rather it is about a specific electronic circuit, and ALL ABOUT CIRCUITS, well, should be the sort of place to seek information about the circuit in question, providing I am able to correctly identify it by name, or describe it adequately.

My last expedition online leads me to believe what I am looking for is known as "variable polarity AC" or "asymmetric AC wave", so maybe I should start a fresh post with those descriptions. I've found a number of research papers on the subject, but they are concerned only with describing the effects of tweaking the subject square-wave AC welding current, not how the variable/asymmetric AC square-wave is generated and manipulated.

Cheers,
Krisby.
 

shortbus

Joined Sep 30, 2009
7,756
I know what you were asking about, but like I said each maker has it's own way of doing it, to prevent having to pay royalties for the patents of another company.

But will say I think you'll find they do it by taking the AC output and converting to DC then using a circuit to remake the Dc to a variable AC, not a true AC. The same principle they use in a "modified sine wave inverter", or a class D amplifier but without amplification. Can't see how a triac would work for it.
 

Thread Starter

Krisby

Joined Mar 13, 2020
4
Ah, yes, thank you again shortbus. I already understand the basics as you have described, there being many generic "block" type schematics for welders and inverters. I am talking about something different, sometimes described as variable polarity AC square wave or asymmetrical polarity AC square wave. It has to do with increasing the time of one polarity, let's say the positive, while simultaneously reducing the opposite polarity of one cycle of the square wave. This is done because at one polarity (I think negative) of the tungsten welding electrode the arc heats and penetrates the aluminium, and during the opposite polarity the aluminium is being cleaned of oxide. The ratio can be adjusted by turning a knob on the front of the welding machine, but I wish to understand the basic electronic principle that is involved in controlling and modifying an AC square wave.

There must be a generic circuit schematic somewhere showing this AC balance control, as it is used in many Chinese made welders. I did find a US Patent given to the Lincoln Electric Co from the 1990s which shows SCRs in an H bridge configuration being controlled by a microprocessor, but these days Mosfets or IGBTs have replaced the SCRs. Triacs have been used to control the waveforms, because they can be switched on or off, and are designed to be used with alternating current.

They say a picture is worth a thousand words, so I have included some diagrams from the US Patent mentioned above. In the drawing of the sine wave, you will see the letters "EN" and "EP" which stands for Electrode Negative and Electrode Positive respectively. In the bottom illustration you can see their modified square wave, the "S1,S2" and "S3, S4" representing the pairs of SCRs, indicating where they were "on" or "off", and the corresponding wave shapes. So, that is the type of schematic I am looking for, but of a more modern design of circuit. Cheers.......Krisby.
AC Circuit Control 2 001.jpg
 

Thread Starter

Krisby

Joined Mar 13, 2020
4
OBTW, regarding the circuit schematic I posted, the arrow labeled "a" is pointing to the arc gap. Number 10 is the tungsten electrode, and "12" is the workpiece. The welding current enters the circuit via "30" and "32" from the secondary windings (24) of the transformer (20). The SCRs in an H bridge configuration is indicated by arrow number "40". The microprocessor is numbered "100", and the arrows coming out of the bottom of it indicate the connections to the individual SCRs. All clear?
 
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