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Alternative high voltage coupling - sparkgap to solid state.

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Ash Small

Sat Feb 19 2011, 04:19AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Newton Brawn wrote ...

.Also 2uF in series with 10 ohm 2w CARBON resistor bridges the hi current work terminal to the ground. This is the way to rf grounding the buzzbox secondary.

..

Newton, can you please explain why this is required?

Thanks.

EDIT: I've looked at the circuit diagram that you mention, and it is quite a bit different to my circuit. My tank circuit looks completely different. I think I've described it above.

3l3ctrici7y

Sun Feb 20 2011, 06:16AM

Registered Member #1806 Joined: Sun Nov 09 2008, 04:58AM
Location: USA
Posts: 136

Hi Newton. Welcome :)

Ash, the diagram you looked at that Newton mentioned, is it one of the ones I posed, or a different one?
If different, could you post a link?

I never really made it clear what I did, so here is a drawing of it.
I intermittently had a resistor between the cap and the HF primary, as well as testing with and without the ferrite core (chunks of ferrite reasonably centered inside the former.)

It is interesting that Newton mentions an RF ground. The manual for the welder I finally broke down and ordered shows a screw on the back for RF grounding. It appears to be just screwed into the case, but I will have to look at it when I get the machine. The manual says NOT to connect that to the electrical ground... but rather a grounding rod in moist soil. It will be interesting to see where the electrical ground is connected and what the screw screws into.

I knew that Tesla coils needed an RF ground, but I assumed that it was because you wanted to play the capacitance of the top load off of that of the Earth through the RF ground. My assumption was that since the goal was to create an arc between the ends of the RF secondary, a ground would not be necessary. I suppose the difference is that with one side grounded, one end of the RF transformer will be held at ground potential and the other will swing wildly, and without a ground, both ends will swing wildly. I can see where even when the goal is to arc between the ends, having one at a constant potential could have a positive effect.

This all leads me to recall something interesting. I mentioned this before, but I will reiterate.
During one of the tests, I used the TIG torch to blow argon onto the short gap of the RF secondary. It immediately started arcing, both between the two ends of the secondary and to the tungsten in the torch. What makes it so interesting is that the torch cable was not connected to anything; it was just coiled up on the bench. My guess is that it has something to do with the capacitance in the torch cable relative to HF secondary. I am thinking a capacitor where one side is to your circuit and the other is to free space. This makes me wonder if my circuit would have worked better if I had grounded or at least had a large capacitance on one side of the secondary.

Newton, click the middle icon/button on the top right of your post and you can edit the post.

+12v    +-----+       1500-7400pF   ||O-----+
                       |     |     |----+------||------+  ||O     |
  +------+------+------+-----| GM  |    |              |  ||O     |
  |      |      |            | HEI |    \/             +-O||O     \/
  |     Gate    +--||--+-----|iggy | gap ~= .032"        O||O      .2"?
 555----Drive          |     |     |    /\             +-O||O     /\
  |     Chip---------igbt    |     |    |     6t/18t   |  ||O     |
  |      |             |     |     |----+--------------+  ||O     |
  +------+------------GND    +-----+                     ||O-----+

The capacitor across the iggy's primary says

225P400V
224K 0138

Maybe .22uF, or maybe 2.2uF and 400V.. I am not sure.. but that's what it says. Just ahead of the 225P, there is a small 2 with a circle around it. The whole thing is the same size as the 2 in 225P

Ash Small

Sun Feb 20 2011, 05:06PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

The circuit that Newton mentioned is here:

My tank circuit is the same as yours in the post above except that the capacitor and gap are switched, ie, my cap is across the coil and my gap is in series with the coil (air core transformer). This just seems to make more sense to me, the cap can be charged without the primary of the air core transformer being in series (remember, the whole point of the flyback/iggy is to charge the cap). Also I have a high value resistor between the output of the flyback and the tank circuit, 330 k Ohm, If I remember correctly.

I don't have an RF ground and it works fine. I just wondered if it was there for safety, or why it is there. My circuit doesn't need it in order to work.

3l3ctrici7y

Sun Feb 20 2011, 06:43PM

Registered Member #1806 Joined: Sun Nov 09 2008, 04:58AM
Location: USA
Posts: 136

Thanks.

Yea, that is one of the ones I posted. A while back, someone here was building off that diagram and I wrote a lengthy post explaining what all is wrong with it. Most obviously, the ground circuit of the mains is in series with the HF circuit.

Also a while back, I was reading about Tesla coils and the two topologies were mentioned, where the HV transformer was across the gap or the cap. Both work. With what I did, the disadvantage is that the primary is in series with the cap for charging (as you mentioned), and it is also shorted out when the gap fires. With having the HV transformer across the cap for direct charging, it is exposed to the high frequency AC that the cap and primary produce. I did it the way I did because that is what I have most often seen done.

I am glad to hear that the RF ground does not seem to be necessary; that is most consistent with what I would expect.

Ash Small

Sun Feb 20 2011, 08:42PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

It's only just occured to me the significance of what you said about the secondary arcing when you placed it under argon.

This means that your circuit works, but it doesn't seem to work when connected to the buzzbox/capacitor.

What value cap are you using? (mine is 200 nano farads), Maybe it's worth adding a choke as well?

EDIT: the resistor in my circuit is there to block the HF from getting back to the flyback (at least, that's the theory). I really can't remember the value, but it was 330 something. I think it was 330 k Ohm, but it could have been anything from 330 Ohms to 330 meggohms. It was made from about 12 inches of carbon cored automotive ignition lead (mayby 18 inches).

3l3ctrici7y

Mon Feb 21 2011, 01:36AM

Registered Member #1806 Joined: Sun Nov 09 2008, 04:58AM
Location: USA
Posts: 136

Ash, I don't quite follow what you're saying about it working but not when connected to the buzzbox/capacitor.

Please see my revised diagram in the above post. In the first version, I forgot to show the capacitor across the iggy's primary. I have fixed that mistake. As I noted, I am not sure what it is, but probably .22 uF or 2.2 uF. I had originally thought it was 200n or so, but that is probably way off.

The capacitor I was using in series with the spark gap stared out as being a 4700pF doorknob and after two of those burned out, I built my own capacitor, that burned out, and I rebuilt it more robustly and based on the dimensions, it should have been around 1500 pF, I did not roll it quite tight enough, so it could have been 750 pF or less.

Ash Small

Mon Feb 21 2011, 02:26AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

3l3ctrici7y wrote ...

Ash, I don't quite follow what you're saying about it working but not when connected to the buzzbox/capacitor.

You said the output of the HF coil arced under argon. This means that the circuit works. The problem seems to be when you connect the buzzbox and blocking capacitor, or did I misunderstand you?

3l3ctrici7y wrote ...

Please see my revised diagram in the above post. In the first version, I forgot to show the capacitor across the iggy's primary. I have fixed that mistake. As I noted, I am not sure what it is, but probably .22 uF or 2.2 uF. I had originally thought it was 200n or so, but that is probably way off.

I don't know enough about transistor driven iggy circuits to know if it's right or wrong. I'd expect it to go across the transistor, but I don't know. 0.22 uF is around 200 nF (1000nF=1uF).

3l3ctrici7y wrote ...

The capacitor I was using in series with the spark gap stared out as being a 4700pF doorknob and after two of those burned out, I built my own capacitor, that burned out, and I rebuilt it more robustly and based on the dimensions, it should have been around 1500 pF, I did not roll it quite tight enough, so it could have been 750 pF or less.

4700pF is 4.7 nF. Mine is 20 nF, not a lot bigger (4X). I think you really need to get a cheap DMM to measure the capacitance of your home made caps.

3l3ctrici7y

Mon Feb 21 2011, 04:54AM

Registered Member #1806 Joined: Sun Nov 09 2008, 04:58AM
Location: USA
Posts: 136

Oh, that's it :)
I never had it connected to anything. I just had some wire wrapped around the former in place of the welding cable as the secondary of the HF transformer. I had the two ends of the wire bent so that they were close. There were no sparks between the two ends of the wire when in air, but blew argon over the two ends of the wire, then it started sparking. At some points it did spark in air, but it was the time that I blew argon over it and it jumped to the tig torch that I thought was most interesting.

Placing the cap across the iggy is just what I seem to recall being recommended in something I read long ago. As I think about it now, I think the only real difference between having the cap across the switch or the iggy itself is whether or not the voltage source is in series or not. For a battery, it probably won't matter. I'm not sure about what the reactance formula would be for a battery, but I imagine it has very low reactance.

Great, my brain gave out on me, and I forgot the order of the metric prefixes. When I looked at the numbers and came up with .22uF, I thought wow that's A LOT more than the 200 nF that I had thought it was. Whoops! My bad.

Yea, I have been wanting to get a LCR meter for a while, but just never managed to pull the trigger. I agree with you about what Steve said, that the DC output of the FBT allows it to charge a larger capacitor; and that just cannot hurt in these situations.

One of the reasons I called it quits on this project was that I only occasionally got it to spark at all, and lacking any experience with commercial units, I don't know what sort of level of performance would be necessary to be reliable. Having seen that YouTube video, I thought that that might be what I would have to accomplish.

Since you're on a pay per bandwidth connection, I took two screen caps of the impressive HF output.
Corona, and Strong HF arc. The video.

Ash Small

Mon Feb 21 2011, 05:30AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

3l3ctrici7y wrote ...

Oh, that's it :)
I never had it connected to anything. I just had some wire wrapped around the former in place of the welding cable as the secondary of the HF transformer. I had the two ends of the wire bent so that they were close. There were no sparks between the two ends of the wire when in air, but blew argon over the two ends of the wire, then it started sparking. At some points it did spark in air, but it was the time that I blew argon over it and it jumped to the tig torch that I thought was most interesting.

.

Is it just occasional sparks you were getting, or an arc?

Some articles say you only need a spark to initiate the welding arc, but mine looks like arc in the second picture.

It seems the only real difference between your circuit and mine is the diodes and resistance between the HV (iggy) coil and the tank circuit, apart from the choke on the output of my buzzbox.

3l3ctrici7y

Mon Feb 21 2011, 06:04AM

Registered Member #1806 Joined: Sun Nov 09 2008, 04:58AM
Location: USA
Posts: 136

Sometimes it was an occasional spark here or there and other times it would spark/arc too rapidly to distinguish one from the next, but usually nothing at all.

With the apparent similarity between yours, mine, and the one I was basing mine on, and my failure to accomplish the goal; I think there must be some subtle but critical thing I was doing wrong.

As a perhaps interesting aside, here is a page about getting huge voltages out of an HEI coil by removing the iron E-core and treating it as a Tesla coil.

There is a definite mystique about simple traditional designs (spark gap TC), but I think if I ever revisit this HF arc starter, I will try to make it out of a ZVS and see if I can get favorable results. Maybe weld aluminum foil using the HF alone. That would be so awesome, but totally impossible.
uzzors2k said that with his ZVS the wires start to glow, so perhaps welding with the HF isn't so far fetched ;)

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