Re: An update on my TIG inverter project

Ignoramus19740 wrote:

> On Thu, 06 Oct 2005 00:34:52 +0100, Chris Jones
> <lugnut808@xxxxxxxxxxxxxxxx> wrote:
>> Ignoramus29341 wrote:
>>
>>> IMPORTANT, I am not trolling, and if I am asking stupid questions,
>>> that's due to my ignorance and malice.
>>>
>>> A while ago I asked for suggestions regarding making a square wave
>>> inverter to convert a DC TIG welder into an AC TIG welder.
>>>
>>> Many things happened since that time.
>>>
>>> 1. I bought a real DC TIG welder for $9.99. See it here.
>>>
>>> Auction:
>>> http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=561560149
>>>
>>> It is a 3 phase welder that I run off my homemade phase converter.
>>>
>>> My page with more pictures and my experience so far
>>>
>>> http://igor.chudov.com/projects/Welding/00-Hobart-CyberTig-Welder/
>>> http://igor.chudov.com/projects/Welding/
>>>
>>> 2. Spent many times its cost of $9.99 on cabling and various welding
>>> doodads and consumables.
>>>
>>> 3. I played with arc welding, trying to learn to weld (see above links).
>>>
>>>
>>> Now, I am a little closer to the aforementioned inverter project. I
>>> bought four Toshiba IGBT, mounted on a heatsink:
>>>
>>> Auction:
>>> http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7550212847
>>>
>>> Data ***:
>>> http://www.galco.com/TechDoc/TOSJ/MG200Q2YS40_DAT.pdf
>>>
>>> My plan is to follow what this guy did:
>>>
>>> http://www3.telus.net/public/a5a26316/TIG_Welder.html
>>>
>>> and to use two IR 2011 gate driver chips, appropriate caps and
>>> resistors, and to use a Wavetek 171 to drive the logic inputs of these
>>> chips. Wavetek 171 can make square waves of arbitrary frequency, set
>>> digitally, and adjust pulse width.
>>>
>>> Note that each IGBT is a complete half bridge, so I need just two, out
>>> of the four that I bought. My welder is a 200 A constant current
>>> welder, so it would not exceed the 200 A limit of the IGBT. I set
>>> welding current digitally, using a digital potentiometer on the
>>> control panel of the welder. Thusly, I could, supposedly, get away
>>> with using just two IGBTs, one for each half of the H bridge.
>>>
>>> The welder's welding voltage is 28V and OCV is 85V. The IGBTs are
>>> rated for 1,200 V.
>>>
>>> Since wavetek 171 can put out pulses of adjustable width and
>>> frequency, I can use it as the control of this inverter. Wavetek's
>>> output will be the input of the IR2011 chips.
>>>
>>> For power, I will try to use a computer power supply.
>>>
>>> I read various relevant application notes and schematics, by now.
>>>
>>> i
>> You remember a while back, I suggested you use a 555 timer instead of
>> your
>> Wavetek? Well I still recommend that. Just imagine that the first time
>> you switch it on, lots of smoke comes out of the Wavetek.
>
> I agree with you. I think that I will do my project in stages. (just
> like I do computer programming). First, I will do the power part, that
> is, IGBTs (which I received today), and the gate driver. I will drive
> the gate driver with a DC power supply, then with the wavetek and use
> a small battery or some such as input to the H bridge, instead of
> using the welder.
>
> As far as timers or other drivers for the gate driver, I want to
> regulate pulse width as % of the cycle (positive vs. negative as % of
> cycle).
>
> It could be done with two 555 timers, I guess, but I am not sure if it
> is easy to control them naturally, using two pots (one for frequency
> and another for percentage).
>
> I was looking at the MAX038 chip, which can make square wave, can be
> controlled by simple pots, etc. The only minus of it that I see, is
> that it requires different voltage than the gate driver IR2109. Gate
> driver needs 16 volts power and 3-5 volts logic input, whereas MAX038
> needs +/- 5V power and makes +-1v output.
>
> As you can see, I am quite confused.
I think the MAX038 has a SYNC output or something like that which might have
a higher swing, but I don't know if the duty cycle of that output is
controllable. Also needing -5V is a pain.

>
>> That is quite
>> likely to happen. I have let the smoke out of a Wavetek and it upset me.
>> Believe me, it is not hard work at all to build a circuit which will be
>> just as satisfactory as a Wavetek for your use, and much easier to
>> repair. Start with figure 4 in the following document:
>> http://cache.national.com/ds/LM/LM555.pdf
>
> Very nice. I saved that pdf.
>
>> Also look at:
>> http://www.electronics-tutorials.com/devices/555-light-dimmer.htm
>
> Thanks. I am looking at it right now. I can see how pulse width is
> controlled by 50k pot, but I have to figure out how to make the
> frequency separately controllable. I will read more on it.

The simplest way of changing the frequency that I can think of is to add a
rotary switch with s few different possible tuning capacitors. That isn't
very elegant but it's pretty simple.

>
>> and
>> http://home.cogeco.ca/~rpaisley4/LM555.html
>
> Yes, it is a great page, I read it a little bit before and will read
> it more. My concern is that the duty cycle cannot be very well
> adjusted (the range is not that big). Maybe I am wrong.
>
> I definitely agree with your larger premise, that I need to use a chip
> to drive the gate driver. What I am figuring out now is what chip to
> use, and how. I want to have two pots, Frequency and Cleaning cycle
> percentage (percentage of DCEP vs DCEN).

One option that I have used in the past is to make a triangle - wave
oscillator using an op-amp as an integrator (non-inverting input to a fixed
mid-rail voltage (such as resistive divider between the supply rails with
bypass cap), and a capacitor from op-amp output to inverting input, and a
resistor from the inverting input to the signal to be integrated which is
the output of a comparator mentioned next...), and a comparator wired to
have hysteresis (resistor from output to non-inverting input, and resistor
from non-inverting input to op-amp output, and fixed mid-rail DC voltage on
the inverting input) to set the upper and lower extremes of the triangle
wave. You can then vary the frequency of the triangle wave using one pot,
which would be in series with the resistor between the comparator output
and the op-amp inverting input. You can set up a second comparator with
the non-inverting input connected to the wiper of a second pot between the
supply rails, the other comparator input is connected to the triangle wave
as produced previously at the op-amp output. When the triangle wave is
above the threshold set by your second pot, then the output is high,
otherwise it is low. By adjusting the second pot you set the duty cycle.
I have used this kind of circuit before but don't have a schematic ready to
post. It is a bit of a tricky circuit to get working because of the
hysteresis calculations and also the many ways of getting the feedback in
the wrong sense. For that reason I would use the 555 as a first step.

>
>> Also, you really do need to put at least varistors across the welder
>> output and probably the load terminals too (I suggest the big sort which
>> you
>> connect with nuts and bolts). The inductance within the welder will
>> easily produce more than 1200V when the arc goes out, and I am not
>> talking about HF starting here, just the inductance of the transformer
>> coils.
>
> Yes, I will put something, but I think that the IGBT manual specified
> a "snubber circuit". Which is a resistor and a capacitor between
> source and drain. Table 5-3 on page 5-9 of the fuji manual.
I think you would need the varistor(s) too.

> I am not sure how to calculate or measure the inductance of the
> welder.
I don't know either, but I would expect that the main thing that limits the
output current of your welder when there is a short circuit would be the
inductance present before the rectifier of the welder. The thing that
might save you (if the HF doesn't get you) is that they probably had to put
some protection in the welder already to stop it blowing up its own
rectifier. Still I would suggest adding the varistors, they are not that
expensive, and once all of your IGBTs are blown, you might not be able to
get more at a good price and that will fit the circuit, bolt holes on the
heatsink, etc. Better not to blow them in the first place.

Chris

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