[dr pepper]

Thanks for the warning I did and they are resistive, I think most leads are nowadays to prevent emi.

Had a very interesting idea about the output choke using an mot, make the thing infinately variable, I seached the net for info and found this, most interesting, the primary coil of the mot could be rigged to a modified pc power supply primary transformer wimding to give the saturation dc supply.

http://aaawelder.com/reactor.html

[iEdd]

Good stuff. So the rigged MOT is for a variable choke on the output of your 200A alternator? Is the main current still controlled by the amount of DC field current applied to the alternator though?

Also, you said this was for MIG/TIG/Stick, but usually MIG has a constant voltage source (like 99% of power supplies), but TIG and stick use constant current sources (vary the voltage to maintain constant current as the resistance changes). I'm not sure how much difference it makes in practice, but I'm wondering if your setup might do TIG/stick really well, but do MIG not so well, or vice versa.

[dr pepper]

Yep I see what you mean, heres to explain.

My control board for the alty has both a current limit pot and a voltage regulation pot, so you can either adjust the pots to give constant current with adjustable max voltage, or constant voltage with adjustable max current, this means the power source is good for either mig constant voltage or tig/stick constant current.
I was thinking of having the output of the alty rectifier go 2 ways, one through a arc starter circuit as per this discussion and use that for tig mode through its own dinze socket on the front panel, and have another output go through a saturable core inductor which can connect to the mig torch and stick output, so whilst using mig and stick I'd have adjustable inductance, a range switch for the inductance might be a good idea as mig only needs a little reactance and stick needs a bit more.
Another feature I thought for the reg board is a current detect circuit, this could go through a switch so that the hf start could be off, permanantly on or auto ie switches off when the arc strikes, similar to a professional weld set.

[bill south]

Hey Guys;
Just a quick question for the Tig Design Experts! I have a cheapie Harbor Freight Tig welder that works fine. However, it has scratch start instead of automatic HF start. What do I need to add this feature to the HF welder? Does anyone make such an add on feature within the price range of a hobby cnc shop?

Many Thanks.
Bill

[dr pepper]

You can buy them, allthough the last one I looked at was more expensive than buying a cheaper weld set that allready had hf start.

[Orvillefpike]

Usually, on a car ignition system, there is a capacitor in parallel with the points. When the points are open, the current from the collapsing magnetic field flows through the capacitor. Why isn't there a capacitor across the the emitter and the collector of the transistor turning the coil on and off?

Thanks

[iEdd]

I think in a car, you are using a capacitor to suppress any transient spikes that occur from the mechanical sparking of a gap.

In the digital drivers, you have a collapsing magnetic field from the primary of the coil, which gives you a reverse EMF 'kick' of up to a few hundred volts. A diode in reverse across the transistor will short this to protect it. A capacitor wouldn't really help across the collector and emitter.

[Orvillefpike]

The capacitor, in a car ignition I was told, is used to control the rate of discharge of the collapsing of the magnetic field in the coil.

[dr pepper]

The condensor or capacitor is an important part of the car ignition and it can shape the envelope of the coils primary waveform.
And the circuit I looked at has nothing accross the coil, theres a diode across the source and drain of the mosfet transistor to protect it, but theres another diode in series with the tranistor which would be reverse biased and off when the coil is discharging, so I spose your both right, the circuit is not so good ans the coil will have really high reverse spikes accross it, great for someone fiddling in the basement making sparks, but not so great for reliability.
For my application I think I'm gonna fit a spark suppressor x rated cap and resistor module accross the lt of the coil (same as accross domestic appliance motors), which is a module containing a 0.1uF cap and a 100r resistor, just to keep the voltage on the primary within the capbilities of the coil and to stop uneccessary rf interference.

[iEdd]

In the RMCybernetics link I gave you, there is a cap+resistor across the coil. That's a good thing. A cap across the transistor would be a bad thing.

[dr pepper]

Normally I'd agree with that, the subject seems to be one for conjecture though.
Heres a link to a photo of my coil driver circuit completed, just above the snips to the right is the arc produced accross a couple of pieces of tig wire, the sound produced is quite loud.
If I cap the coil the spark goes nearly altogether, the reverse spikes are within the rating of the 1n4005 I used, an am radio nearby doesnt seem to be affected.
Maybe I'll just put a transzorb accross the coil at a little lower voltage rating than the 1n4005, it seems that the coil 'likes' being open circuit.

http://www.landyzone.co.uk/photopost...=&ppuser=32763

Theres a circuit around called a megasquirt, or more appropriately megasquirt and spark, its a home made engine management system, I'll see if I can get a schmatic of the coil firing circuit and see how they do it.

[dr pepper]

Ok then so I have made the ignition coil driver circuit, and the tank coupling coil and tank cap, the tank coil has 10 1 inch interference suppressor type ferrite cores on the inside, held together wilth heat shrink, theres a layer of heat shrink between each winding, the tank cap isnt a doorknob, I used 20 2kv 10nF caps in series to make 500pF at 40KV, the caps were only 15pence each, I changed my freq from 300k to 1meg, I heard on this thread further up that someone had used test gear to determine the resonant freq of the coil/cap, I have the gear so I tried this, simply by connecting a 10k resistor inline with the sig gen accross the tank coil/cap and connecting the scope at the same point, and according to the scopemeter I'm close, the coil and cap resoantes at 1.3 meg.

Heres some pictures.
Getting impatient now as I still dont have an alty, I'll try this arc starter with my buzzbox first though, to make sure it does as it should, th ign coil I used is a more modern twin spark job, its a small thing but appears to work well.


http://www.cnczone.com/gallery/data/...icture_147.jpg

http://www.cnczone.com/gallery/data/...icture_136.jpg

[Orvillefpike]

Dr. Pepper,

Are you driving the ignition coil at 120V or 240V?

[dr pepper]

12vdc via a variable pwm driver.

[Raban]

If anyone interested, here is' the beginning of an open source project for an inverter tig welder ac dc

http://meccanicaedintorni.morpel.it/...hp?f=75&t=3817

[kaighn80]

well i think it needed resurecting as i would love to build a tig welder so has anyone finished theirs yet and if so do you have a diagram?

[dr pepper]

Mine does work now, only its not as intended driven by a alternator, its powered from a stick welder, HF works ok, and variable power control works ok, I had a bit of troublwe with the tranny getting warm, you have to be carefull how you arrange the trigger circuit for the triacs, as if the triggering isnt symetrical, the resulting harmonics and DC nukes the tranny.
I'm working on a schematic.
The protection for the rectifier is no hard task, a power factor correction capacitor from a scrap fluorescent tube fitting does the job nicely.

[kaighn80]

cool im looking to build one from a dc stick welder as im not that interested in welding ally so steel and stainless only are there any differences between the two or will i build it exactly the same as if i was using an ac stick welder? sorry if this is a dumb question but i have no background in designing anything electronic, im purely a if it dont fit trim it if its too small recut kind of person, given a diagram i can pretty much build anything but i dont understand enough of this sort of thing to work it out for myself. previous projects include a mini crewcab pickup, mini roadster, mega squirt ecu, 3 axis cnc machine and a few bikes. if at all possible and you have it could i have a copy of the diagram for the tig youve built so as i could build my own please. thanks in advance.

[moking]

I would like to use a stick welder as the power source for a TIG converter. The problem is the Stick O/P is AC and the requirements for the TIG converter are DC minimum voltage 50V. I take it a standard Bridge rectifier of 200A at 400v would suffice and can I do without any smoothing or does the rectifier require some capacitance as protection on back EMF. All info would be most welcome, especially from you guys that have been there and got the T shirt so to speak.
Best Regards
Mo King UK Ex-Pat in Spain

[dr pepper]

You dont have to use smoothing caps, allthough if you do you'll have better pen, as the average DC voltage will be higher, but try without first.
If you use an arc starter on a DC set, then its important to use a capacitor accross the DC output of the rectifier to stop the RF comming back and toasting the rectifier, a 20uF motor start cpacitor, or a fluorescent light capacitor is fine.
Heres a link to a cool start box, this ones dead easy,


http://www.casano.com/projects/hfstart/index.html

To make a AC welder DC you need the rectifier as mentioned, and the capacitor, a fairly simple setup, with the DC stick welder all you need is the capacitor accross the output and the arc starter.