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Alternative high voltage coupling - sparkgap to solid state.

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LAN_403

3l3ctrici7y

Sat Feb 12 2011, 12:09AM

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

Oh, sorry; he used it for the coupling transformer. He has about 4 turns (I cannot see for sure) for the primary, and 9 for the secondary. Also included is a can style iggy, light dimmer, and motor run cap. A door knob across the iggy, and a single spark gap consisting of a pair of bolts across a small PVC tube. It looks like he is using a doorknob for the HF bypass capacitor.

Using a yoke was on my list of things to try, but a little past the bottom of the list, because I have no good place to get a yoke. Maybe once I have it working, I will try removing the ferrite and see how it does as a pure air core. Well not "pure" air.. but at least nothing-metallic-core.

Ash Small

Sat Feb 12 2011, 12:47AM

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

I'm wondering if the ferrite core isn't part of your problem. The inductance will be much greater than an air core xformer, and could be contributing to your problem of blowing caps (I'm not sure here, just a gut feeling)

I used a polypropylene former (see above)

I also used a 1:1 ratio for the primary/secondary. You don't need a huge voltage because the HF arc is under argon.

I was unable to measure much but I assume the output from the flyback is around 15kv, due to the length of the arcs obtained during testing. What voltage were your caps rated for?

Some iggy coils put out 30-40kV (ones from electronic ignition vehicles, depending on the operating voltage) Older ones put out around 20kV on 12 V input.

I've described my caps elsewhere on the forum, but basically, they are polyethylene rubble sacks and kitchen foil on plastic waterpipe formers.

3l3ctrici7y

Sat Feb 12 2011, 01:47AM

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

I have read various descriptions of HF arc starters, and most but not all used ferrite cores. I would be very happy if I could avoid it (one less part :) I was loosely copying this one because it seemed to be the most well thought out and detailed of those I had to look at.

I am planning on building another cap and I will try it with and without the ferrite and see what happens.

Currently my former is PVC, but I could hunt down some PP and try that.

Funny you mention the voltage necessary, I am currently just using some 16ga wire in place of the welding cable (for testing) and had the two ends of the secondary close. I was not getting any sparks, so I bring the TIG torch over and blow argon over the gap, and lo, it starts sparking.
What was really strange was that it also sparked to the TIG torch, which wasn't even hooked to anything, let alone any of the wires involved in the HF transformer. HF/HV is strange stuff.

The doorknob caps I purchased are supposedly rated for 20kV.

The iggy was originally driven by some sort of solid state ignition module. It will easily make strong arcs/sparks across 20-25mm, possibly more. But if shorten it to maybe 8-10mm and fiddle with the drive frequency and pulse width and the arc will get really hot. I am not clamping iggy's primary in any way. I am driving it with a 1200V IGBT, and have a 200nF (or so) capacitor across the primary. This capacitor makes a huge difference in output.

The capacitor I built was made out of a zip top kitchen freezer bag and some baking aluminium foil.
Two plates are maybe 7cm by 15cm. I was amazed how well it worked... until it blew :(
All I did was connect it across the spark gap, but the spark in the gap got fairly bright and loud (very comparable to using the doorknob). I was mightily impressed considering what I used to make the cap and how long it took. I guess next time around I will use a little more dielectric to keep it from blowing.

Ash Small

Sat Feb 12 2011, 02:27AM

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

I'm pretty certain the iggy is putting out a lot more than 20kV.

I was driving my flyback with a 2N3055.

Try some thicker dielectric for the caps.

Steve Conner

Sat Feb 12 2011, 09:06AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Presumably the secret of success is that Ash used a DC flyback, one with a built-in diode. This allows it to charge a much bigger capacitor than an ignition coil could, because it can pump up the charge over multiple cycles. If the ignition coil can't do it in one cycle, it won't do it at all.

Might be worth trying to rectify the ignition coil output with some microwave oven diodes, but be sure to get the polarity right, it generates much more voltage in one polarity than the other, and will just blow the diodes if you get it wrong.

A good capacitor also helps. You can't necessarily trust some random doorknob capacitor to be pulse rated, and home-made caps are notoriously hit-or-miss too.

Ash Small

Thu Feb 17 2011, 12:06AM

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

Steve McConner wrote ...

Presumably the secret of success is that Ash used a DC flyback, one with a built-in diode. This allows it to charge a much bigger capacitor than an ignition coil could, because it can pump up the charge over multiple cycles. If the ignition coil can't do it in one cycle, it won't do it at all.

.

That explains a lot Steve. When I built it ten years ago (before 4HV) the best explanation I could find was that the coil charges the cap on one cycle, then the circuit oscillates, then the coil charges the cap again. Your explanation makes a lot more sense (once the cap is sufficiently charged to jump the spark gap, the circuit oscillates, and the coil keeps the tank 'topped up', losses being in the gap, the coil,(primary to secondary coupling) and the capacitor.).

There were some errors in the circuit diagram that I used as a guide as well, took me weeks to sort those out.

3l3ctrici7y

Thu Feb 17 2011, 01:19AM

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

Thank you all for the replies. I am sorry for the long delay; I just now got back to the project.

I built another cap using 11 mil PET with two plates and an overlap area of about 25 square inches. According to my calculations, that should have yielded something around 1.5nF (1500pF). It worked quite well for pulse discharge across the spark gap even at a very high frequency. The output was hot and loud as compared to no capacitor. Regrettably, using this capacitor did not produce any HF output (sparks) at all; ferrite or no ferrite.

Am I giving up, or am I reading the writing on the wall about having been beaten? I am going to say that I've been beaten.... the Visigoths have arrived :(

Ash Small

Thu Feb 17 2011, 01:56AM

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

Follow Steve's advice and try adding some diodes....or try a DC flyback.

Newton Brawn

Sat Feb 19 2011, 04:07AM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Hi Ash, Hi 3l3 !

Im realy interested in what you have done. I have working in welding machines repair for some time.

The caps that shunt the buzzboz output high current terminals are normally paper-foil 400-600V. Polyester or polypropylrene METALIZED caps are not suitable. Some manufacturers use of 0.02uF. Other (Miller, Esab, etc) use caps up to 10uF.
0.02uF is enough to avoid the hv enter in the secondary coil or buzzbox diodes .
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.

Some time ago I have noticed in the 4HV.ORG forum that the schematic is wrong. The 10 turns primary coil to be in series with the spark gaps, See the post made by Newton Brawn on October 10 2010.

Other interesting point is that for proper arc start, just one pulse of 10 kV 0.5 microsecond is required. this pulse tobe at each 8.333ms, for a 60Hz buzbox. Using only one pulse close the zero voltage passage a lot of energy is saved, reducing the size and ratings of conponents.

The ignition coil may be replaced for homemade pulse tranformer. 24turns in primary 400 turns at secondary, ferrite core from flyback. insulation 0.2 polyester between layers. No more 20 turns per layer.

Glass capacitors 1000uuF, 10kV proper handymade are reliable.

I have more info to share, for next time

Regards

Newton

Newton Brawn

Sat Feb 19 2011, 04:16AM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Sorry for the copy and paste duplication in my reply. Someone can fix it?
Newton

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