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Registered Member #396
Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176
Yes, another one of my topics regarding a device for superimposing a high voltage, high frequency current over a low (80v) voltage high current welding supply, similar to below:
I finally went about constructing one based on spark gap topology - stats are as follows:
HV Transformer: 4kv, 30ma NST w/safety gap and RC filter Main capacitor: .030uf 9kv rated polypropylene MMC HF Bypass capacitor: 5.6uf polypro cap and 3.9k ohm resistor Coupling coil: Primary - 8 turns insulated wire, Secondary - ~20 turns 4AWG wire
The core consists of 200 1/4"x1" ferrite rods stacked together to form a 1.25" diameter by 12" cylinder. This was then encased in polyester epoxy (the core was originally meant for a solid state design but I kept it).
The whole thing produces a very strong and loud arc - however the whole concept doesn't work - when the welding voltage is applied over the output spark gap (where the welding arc should be) the high voltage arc just snaps away and thats it. The only thing I can think of is that there isn't enough current being put out by my tesla type circuit. I had a 3.9ohm resistor in series with the LC circuit to dampen things somewhat but even when this is shorted it still doesn't start an arc (even when the electrode(s) are held very close but still not touching.
The only thing different in my design as opposed to others:
A) I have a much larger cap - if anything this should increase performance because it is slightly larger than resonant (although it would lower frequency . . .)
B)I have significantly more ferrite in my design than others. In a previous thread member " . . ." calculated the core at 50uh. The theory is that the core is heating (I can't easily obtain access to it to see - not that it would be that easy to tell anyway) and taking power away from the output. I'm not sure how to calculate the frequency of the system with the core or where ferrite starts to become lossy. The datasheet it here Although I'm not sure what rods they were (they were 30mm long though).
I'd really appreciate any ideas - I really just want to get this thing working and I've put a lot of time into it.
EDIT - heres a quick shot of my apparatus:
I also have a video of the arc if anyone thinks it would be helpful.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Hmm, I don't remember giving a definite number for the coil L, although I would say you secondary is probably closer to the 5-10uH range. (if you have a link to my past reasoning that would be great...)
In any case, I am am little curious as to what exactly you are trying to achieve with your 10uf capacitor and 10r resistor/switch across the main current cables. I get that the 10k resistor is a bleeder for the capacitor, and that the switch cancels out the 10r resistor (thus directly connecting the capacitor across the +/- cables) but I do not quite see what this achieves.
One thing to consider is that if you are using an AC power source the voltage is not very high most of the time. If everything is working correctly the peak of mains voltage should coincide with the spark gap firing, although if that got somehow phase shifted 90 degrees you are now sparking with 0v across the gap. Although come to think of it, that is probably a very unlikley problem.
More serious would be the inductance of the mess. You have a huge amount of inductance in your welder, which is going to try to keep the current from changing, so without some kind of capacitor to give it a boost you will have a hard time getting a good arc going while there is still the HF available. I believe that is what that 10uf cap and 10r resistor are supposed to take up (with 100v and 10r that is about 10a which seems about right for starting an arc...)
You also might try adding a small bypass cap directly across the upper/right terminal of t2 to the lower/right 'work' connection (try to make it low inductance) to help keep the RF out of that transformer. A few nF should be enough.
good luck!
BTW, I really like the idea presented in the 'further discussion' at the bottom of the second link, where you don't use a transformer for t2 but rather just have a single inductor that the welding current flows through. You use a rf bypass cap across your main transformer, and hook your resonant capacitor/sparkgap directly across the inductor. When the cap fires you develop the full voltage on the capacitor across the inductor, which goes strait through the rf bypass cap and our the gun. No core needed :)
Registered Member #396
Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176
... wrote ...
Hmm, I don't remember giving a definite number for the coil L, although I would say you secondary is probably closer to the 5-10uH range. (if you have a link to my past reasoning that would be great...)
No reasoning but you can see it here
... wrote ...
In any case, I am am little curious as to what exactly you are trying to achieve with your 10uf capacitor and 10r resistor/switch across the main current cables. I get that the 10k resistor is a bleeder for the capacitor, and that the switch cancels out the 10r resistor (thus directly connecting the capacitor across the +/- cables) but I do not quite see what this achieves.
Steve Conner explained it in a previous thread "The 10 ohm series resistor for AC operation is so that welders with square wave output won't blow the 10uF capacitor out with harmonic currents. (the 10uF cap is indeed to stop RF feeding back into the welder)"
I don't personally have that exact arrangement - mine is 5.6uf worth of poly capacitors and a 3.9k resistor (which probably doesn't serve much of a purpose). This is to route the HF high voltage to the arc and stop it from arcing over the welding transformers windings.
... wrote ...
More serious would be the inductance of the mess. You have a huge amount of inductance in your welder, which is going to try to keep the current from changing, so without some kind of capacitor to give it a boost you will have a hard time getting a good arc going while there is still the HF available. I believe that is what that 10uf cap and 10r resistor are supposed to take up (with 100v and 10r that is about 10a which seems about right for starting an arc...)
You also might try adding a small bypass cap directly across the upper/right terminal of t2 to the lower/right 'work' connection (try to make it low inductance) to help keep the RF out of that transformer. A few nF should be enough.
I already have one - thats essentially the same thing as the 10uf capacitor - if anything the larger cap will offer less reactance. The inductance of the welder shouldn't matter should it? The point of the capacitor is prevent any HF from entering it in the first place.
... wrote ...
One thing to consider is that if you are using an AC power source the voltage is not very high most of the time. If everything is working correctly the peak of mains voltage should coincide with the spark gap firing, although if that got somehow phase shifted 90 degrees you are now sparking with 0v across the gap. Although come to think of it, that is probably a very unlikley problem.
That makes sense although I've moved the tungsten spark gaps around a bit and this would have likely solved the problem at some point I assume. I could rectify the welding output . . .
... wrote ... BTW, I really like the idea presented in the 'further discussion' at the bottom of the second link, where you don't use a transformer for t2 but rather just have a single inductor that the welding current flows through. You use a rf bypass cap across your main transformer, and hook your resonant capacitor/sparkgap directly across the inductor. When the cap fires you develop the full voltage on the capacitor across the inductor, which goes strait through the rf bypass cap and our the gun. No core needed :)
Sorry - I should have posted that schematic as that is pretty much the exact schematic I used (I'm assuming you mean the hand drawn schematic? I don't see a further discussion part . . .)
I don't really understand the difference between a transformer and an inductor in this case - mine has some voltage gain but not much. The one in the second link also has a ferrite core.
Steve McConner wrote ...
Can you draw an arc from the secondary of T2 with nothing connected to it? Just fire the thing up and try to pull an arc between the two #4 leads.
Yes definitely - this arc is the same intensity as the one with the welder connected and the RC filter in place. It starts at about 7mm (depending on the primary spark gap width and if the resistor in the primary is used or shorted) and is surprisingly loud.
Registered Member #1721
Joined: Sat Sept 27 2008, 08:44PM
Location:
Posts: 136
Hello:
I built my own HF unit and TIG welder a few years ago. The circuit is a mini Tesla/Oudin coil driven by 15KV ceramic capactors with a double spark gap. I used a 60hz/4KV bug zapper transformer to drive it.
The secondary of the coil is very thick wire, and carries the full welding current. The primary is only a few turns, less than 5 if I remember correctly. It will start the arc at 1-2cm away from the work with argon.
So in a nutshell its just a mini-tesla coil with the secondary in series with the welding current. I ordered my coil as a "replacement" part from Lincoln Electric posing as a repair shop, sorry I do not remember the part number, but it was only about 45 bucks. I do believe it has a small ferrite inductor in the center to make it small. The unit still works great today after about 10 years of intermittent use.
You may also try calling some welding shops in your area, and ask them if they have any old "buzz" boxes laying around, since for them they are obsolete now and these contain everything you need. You could walk out with one for 20-50 bucks.
This may sound silly, but was the Argon flowing when you tested it? The reason is mine will have extreme difficulty striking a good arc with the HF, in air or CO2. (I just wanted to see what would happend on my MIG unit with HF with CO2)
One Note: Add a protective capacitor / resistor on the welder end, so that the HV HF does not fry your welders big rectifiers!!! Dont ask me how I know :)
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
wrote ...
BTW, I really like the idea presented in the 'further discussion' at the bottom of the second link, where you don't use a transformer for t2 but rather just have a single inductor that the welding current flows through. You use a rf bypass cap across your main transformer, and hook your resonant capacitor/sparkgap directly across the inductor. When the cap fires you develop the full voltage on the capacitor across the inductor, which goes strait through the rf bypass cap and our the gun. No core needed :)
Sorry - I should have posted that schematic as that is pretty much the exact schematic I used (I'm assuming you mean the hand drawn schematic? I don't see a further discussion part . . .)
I don't really understand the difference between a transformer and an inductor in this case - mine has some voltage gain but not much. The one in the second link also has a ferrite core.
The 'inductor' only has one winding, not two. Instead of having a separate primary/secondary they are one and the same.
In any case, I would still try to add a small (several nf is probably enough) capacitor with no resistor in series with the welder input. Even if you have a welder with a square wave (some sort of smps one?) a few nf won't hurt it, but will make sure that hf is going strait to the electrode.
Registered Member #396
Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176
... wrote ...
The 'inductor' only has one winding, not two. Instead of having a separate primary/secondary they are one and the same.
In any case, I would still try to add a small (several nf is probably enough) capacitor with no resistor in series with the welder input. Even if you have a welder with a square wave (some sort of smps one?) a few nf won't hurt it, but will make sure that hf is going strait to the electrode.
I have no clue as to what this few nf of capacitance is supposed to do. I already have capacitance in the HF routing part of the circuit. Sorry I didn't describe things completely in the first post - there's a lot to cover. Here is a diagram I personally drew of my own EXACT setup:
I do see what you mean by just an inductor - but I don't understand - wouldn't it just short the HF across itself . . .
LutzH wrote ...
This may sound silly, but was the Argon flowing when you tested it? The reason is mine will have extreme difficulty striking a good arc with the HF, in air or CO2. (I just wanted to see what would happend on my MIG unit with HF with CO2)
No, this wasn't under argon. I assumed that if I held the electrodes extremely close (but not touching) I would get something. I also tried with carbon rods which make it very easy to strike an arc (I'm assuming they ionize easily) but still no results with HF.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Ok, that setup makes a lot more sense with your comments.
I might try doing some experiments to see what voltage it takes before you can get it to 'strike'. I would just replace the welter transformer with a cap (few hundred to a thousand uF would give a nice loud bang) and see how much voltage you need to have stored in the cap before you can get it to discharge by firing the hf circuit. It possible that your coupling coil just isn't supplying enough juice to get things going...
Also, the inductor idea. An inductor tries to keep a constant current across it. If you put a small inductor in series with your main transformer (~10 turns) it will not have nearly enough inductance to do anything for 60Hz. But, if you try to discharge a capacitor across it it will try to keep the current a 0 (or close to it) by allowing a a huge voltage to develop across it (about as much voltage as is stored on the cap) and give you a nice high voltage peak to start the main arc going.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
If your welder outputs AC, and the cap in your arcstarter is so big that it only fires once per half-cycle: Maybe it's firing at just the wrong instants, as the welding voltage passes through zero. To get ignition, you want several firings clustered around the peak of the welding voltage.
So, try a smaller capacitor, and/or moving your spark gap contacts closer until it hisses and squawks instead of buzzing.
If that doesn't work, either your welder isn't putting out enough open-circuit voltage, or your welding technique is bad and you're holding the electrode too far from the work. I'm no expert in welding, but I believe it should be something like 1/4" or 1/8".
Registered Member #396
Joined: Wed Apr 19 2006, 12:55AM
Location: Pittsburgh, PA
Posts: 176
Ok, I tried the inductor idea using my current setup. I removed the primary wire from its connections on the MMC and the spark gap and attached new wires directly across the 4 gauge inductor I had used before. This made a strong output arc but still failed to start an arc.
I've been using carbon rods for an arc testing setup and the arc doesn't form at any distance (unless I physically touch the electrodes and draw one out).
I tried adjusting the tungsten spark gaps from very wide to very small - I did get to a point with a more squawk-like noise rather than the a harsh buzz but I have a feeling my capacitor is too large and the pulse rate too low. Maybe this is why everyone has such good success with smaller 2200pf caps. And I thought a bigger cap would only help . . .
I doubt at this point that the zero crossings are aligning because the arcstarter is 120v and the welder is run off of 240v split phase - and my gap adjusting should have taken care of that as well.
The open circuit voltage of the welder is a measured 77 volts - this seems consistent to what I've read about real TIG welders which use HF start. I've also tried ballasted 120v mains - nothing.
EDIT - Ok I just tried replacing the welder with a 500uf capacitor charged with a camera flash circuit (after removing the 3.9k resistor in the HF bypass) and got some interesting results. The high voltage arc discharged the capacitor at any voltage (it left between 2-12v in the cap). I couldn't charge the cap at all when the arc was on (although my power supply was wimpy). At higher voltages (including 80v which is the same as my welder) on the cap I could definitely hear a snap (the cap discharging) when the arc was initiated. I suppose the HF arc does have enough power to discharge the capacitor but just barely not enough to allow enough current from the welder to sustain an arc.
EDIT 2- SUCCESS!! I replaced the tank capacitor (MMC) with a significantly smaller 1.4nf (1400pf) MMC setup, which raised the spark gap firing rate significantly and now it will start a perfect arc with a stick welding electrode. An AC TIG arc using CO2 is still hard to establish but like LutzH said this is probably normal (it works better in air but then the tungsten electrode burns off quickly and overheats). I'm looking to buy an argon tank so that should solve all of the problems.
The shoddy, jerry-rigged cap bank (smaller one) I threw together just killed itself but when I get a proper polypro one rated for the voltage, I might put up some pictures of the assembled unit on the "Projects" section.
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HF arcstarter . . . again
