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Herr Zapp

Mon Aug 30 2010, 01:41AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

iceowl -

Although it is very easy to wind a primary using "spring copper" weatherstripping and foam tape, electrically you may be much worse off than using copper tubing. Almost all copper alloy weatherstripping material is actually spring bronze. Even brass has only 28% of the electrical conductivity of copper, and spring bronze (typically phospher bronze) can have electrical conductivity of less than 10% of the conductivity of copper. Additionally, depending on the peak voltage in your tank circuit, the small radius of curvature at the edges of the strip (compared to the radius of copper tubing) can cause problems with corona and flashover.

Regarding your other primary, I had a very similar experience long ago when I tried to wind a small flat spiral primary using the "thread it through the holes" method, except mine was an ever worse design. This was a primary for a small "desktop" coil, that used a "fixed" primary inductance without an adjustable tap point. I used 3/8" thick ABS primary supports, and #12 AWG THHN (insulated) solid-copper household wire as the conductor, with six or seven turns total. Feeding the wire in from the outermost holes, I got about one turn installed and the friction between the wire insulation and the plastic supports became so great that it was just about impossible to feed any more wire in. Ultimately I used wire-pulling lubricant (a slippery, water-soluble gel used to lube bundles of wires as they are pulled through electrical conduit) to grease everything up, but it still was a nightmare as I could only advance the wire maybe 3/8" of an inch at a time, then had to incrementally work the wire forward all around the spiral. What should have taken 10 minutes ended up taking hours and two sets of hands, just because I didn't want to trash my carefully machined (and permanently glued together) baseplate and primary supports.

Live and learn ......

Herr Zapp

iceowl

Mon Aug 30 2010, 03:15AM

Registered Member #2834 Joined: Thu Apr 29 2010, 02:04PM
Location:
Posts: 22

Ah ha. You are absolutely correct. It is spring bronze - not "spring copper" as I mistakenly wrote. I'm tapped at the 8th turn.

Though honestly, I ran both resonators with the same power supply, and I got better results with the spring bronze primary. Though the secondary is 2" wider in diameter, and who knows which sorts of other differences there were.

As you wrote, I was honestly suspecting arc-over between turns with the turns so close. But that hasn't happened. Yet. Perhaps at higher powers. Or, perhaps the spring bronze is serving as a current limiter as well as providing inductance. And Terry Blake seems to be running much higher powers than I can summon with my bank of NSTs. So maybe there's something to it.

At any rate, it makes for a fun experiment!

Cheers,
Joe

Herr Zapp

Mon Aug 30 2010, 06:46AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

iceowl -

You mentioned that you have not experienced any flashover problems with your flat-strip primary. To help put this in perspective, can you provide the following info?

1. Voltage and current rating of your NST power supply;
2. Total spark gap width;
3. Width of the foam weatherstrip material (compared to the 1.5" width of the conductor);
4. Minimum turn-to-turn spacing of the flat conductor (e.g. was there much compression of the foam material?);
5. Are the edges of the conductor "rolled over" to eliminate sharp edges?

Thanks!
Herr Zapp

iceowl

Tue Aug 31 2010, 11:58PM

Registered Member #2834 Joined: Thu Apr 29 2010, 02:04PM
Location:
Posts: 22

Good Herr Dr Zapp -

The information you request (I can also post video of the coil running, though it is sideways due to having been captured on a Droid phone...)

1) Running 15/150 via 3 parallel NSTs 2x 15/60 + 1x15/30

2) Spark gap is a propeller SRSG at 1800 RPM, 120bps . Max width I have not measured recently and may have changed due to electrode erosion, and as I sit here I cannot recall what I gapped them to. I want to say that each side of the rotor is at around 1/16 to 1/8 inch, but don't quote me on that. I will have to remeasure.

3) Foam weatherstripping is 1" wide (or tall, depending on how you want to look at it) The spring bronze strip is 1.5" wide or tall. Exact same material specified on Terry Blake's website, acquired from exactly the same sources on line.

4) Minimum turn to turn spacing - nominal is 1.4" - not much compression at all except for the very first turn. I adjusted the spacing by clipping the heads off a number of nylon 1/4x20 bolts to about 1/4" and using the bolts to space where the material compressed by moving away the insulation and inserting them in place. The turns are not visibly compressed much below 1/4" beyond the first turn, and the winding was done horizontally, so gravity was not involved in tightening the turns. It's only marginally hand tight, and I was careful not to compress the foam - though one can never be exactly precise with such things.

5) As I sit here thinking about it, away from my home, at least one edge of the spring bronze is rolled over. I don't remember if both are, In any case, in the pitch dark I have seen absolutely zero corona. Though, the bright arcs and the light from the SRSG may wash that out.

Hope that is useful, and I will do some measurements when I get to the coil later.

Cheers,
Joe

iceowl

Wed Sept 01 2010, 04:15AM

Registered Member #2834 Joined: Thu Apr 29 2010, 02:04PM
Location:
Posts: 22

Update -

2) Total spark gap width, considering this is a rotary gap - so measuring both sides, is about 6mm total. Again it's 1800 rpm, 1 spinning electrode, 4 stationary electrodes for a break rate of 120BPS. I am also using a John Freau phase shifter to get the timing adjusted correctly. The gaps were set based on a lot of trial and error when many things were being adjusted - coil timing, primary tuning, coupling adjustments, rotary phase - so I will not swear to their being correct to the equations, but they work perfectly with the setup I have now so I have not endeavored to change them.

5) The spring bronze IS indeed rolled over on both ends. I am tapped at the 7.75th turn, as prescribed by JavaTC, so even though the primary has 12 turns, I'm only using the inner 8. DC resistance is in the realm of a couple tenths of an ohm.

Hope that's useful,
Joe

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