Hi Tim,Good job recognizing the lead length inductance effect. Your coil is one of those coil sizes where the lead length will effect the coil tune point very much. Your in for a challenge because it's small. But, all can be successful if you pay attention to those details.
The spark gap is important for any coil. Base the spark gap on the power and voltage. I personally prefer large copper pipe (about 4" in length) which can be purchased ready to go at most hardware stores. Just use 2 part epoxy to secure them in line and to the distance you need. 5 to 7 pipes is all that is needed. This large surface area allows for a high power static gap. Base the distance between pipes on the peak voltage of the transformer for the number of pipes you use. I use simple 2 part epoxy. I set the first pipe as perfect as possible with the epoxy. Then I set the second pipe in position and lay a feeler gauge between the first and second pipe until the epoxy cures. I do this with each pipe to ensure a parallel and exact distance (one pipe at a time, no need to hurry).
Once all the pipes are set, just add a good fan to suck through the pipes. Here's an example and this particular coil is running a 12/60 with half it's shunts removed. The gap has always performed super excellent. The power is rather high with the modified NST (almost 200mA after modification which is why the sparks are so good).
http://www.classictesla.com/photos/ba45/ba45.htmlThat coils running a good amount of power through that simple static gap. The gap is keeping up however. This particular coil burned up the bottom turns about a month ago. I removed the burned turns and simply reconnected to RF ground. I also painted the coil a blood red color (Little Devil I'll call it now). I ran it last night just for my own personal enjoyment. It produced nice long powerful sparks. One thing it doesn't do any longer is arc down to the primary. For the past year I simply layed a round plexi disc on top of the primary. That stopped all the primary hits. Amazing what a little acrylic can accomplish. It's basically a disc about 1/2" larger than the outer turn of the primary and the center hole is about even with the inner turn of the primary. A simple 1/8" thick acrylic disc laying directly on top of the primary. It makes a difference. Who needs a strike ring? Well, I don't any longer. They just don't go down there since the acrylic. I'm not quite sure how to theorize it, but they don't. Sparks stay horizontal and hit everything they can reach. I don't think it's ever gone down to the primary since the addition of the acrylic.
Take care, Bart Tim Meehan wrote:
Thanks for the advice! I heard so much bad stuff about the wood for the spark gap, so I ditched it and got a scrap piece of delrin and bored some holes in it for the spark gap (until I get a better one at the hardware store this weekend). I have 2 foil capacitors (I guess - they aren't electrolytic, and they aren't the pancake shaped ceramic ones ... they are about the size of a cigarette lighter). They are 0.01 microfarad 8kV rated, in series, so I figued that they were 0.005 microfarad 16kV or so. (Now that you guys mention it - I never checked the capacitance to see if it was punched through yet). The NST is 120V pri / 9000 V sec, 30mA, 270 VA. I am using a Variac to vary the input also. I went back to measure the secondary. It is 989 turns of 28AWG on a 1.5inch diameter phenolic tube. The height of the windings is about 12.5 inches and there is 1 inch of tube above and below the windings. JAVATC said that it should have a Q in my configuration near 200. I have a nice topload - a spun aluminum toroid from ( http://hometown.aol.com/futuret/page1.html) which measures roughly 2.5 inch minor diameter, 6 inch major diameter. I went back and ran with a ground connection to the secondary and got no sparks again, so I decided that I would scrap my primary and go back to the drawing board with JAVATC. When I ran it with the "tune my coil" box checked and a test helical primary I noticed something else that was probably messing me up. JAVATC said that with a 2-inch radius primary coil I should have 6.8 turns, and that the inductance of the primary coil was 5.7microhenry, and with 2 feet of primary leads (that were 0.857 microhenry) ... however I had a lot more than 2 feet of cable there - I had about 5 feet, and I coiled it up to prevent it from getting tangled up and in the way ... so the primary leads probably had at least as much inductance as the primary coil if not more. So, I'm probably going to be making some nicer secondary stands, and a better RF ground needs to be located ... Thanks for all the tips On Nov 16, 2007 11:05 AM, <Mddeming@xxxxxxx> wrote:Hi Tim, A complete answer to your questions will require more information than you provided. See comments interspersed below: In a message dated 11/16/07 1:56:03 A.M. Eastern Standard Time, btmeehan@xxxxxxxxx writes: "This is my first tesla coil, I built it without consulting any sensible source of information on the web." This is far more common than you'd think. "All I had was a healthy respect (fear) of high voltage and a neon sign transformer." A little fear and a lot of respect is healthy at this point. What voltage NST? What current rating? "I made a spark gap out of two screws and a u-shaped block of wood." NOT GOOD! At ~10 kV and above, wood, with any degree of humidity, is CONDUCTIVE!! Several heavy coats of varnish or polyurethane helps, but it's still better to use HDPE or similar plastic. For your gap, use 2 brass bolts head to head, or two round brass lamp finials, or two brass drawer pulls, or two round brass door knobs, etc. You can also use short sections of copper pipe in parallel but your sum of all gaps should not exceed 2kVrms/mm. Also use plenty of air flow, vacuum cleaner, fan, pressure hose, etc. "The choke coils are wound on a 3.5" long 3/8" diameter slugs of ferrite and they had a 1millihenry reading on the LCR meter at the office. I put them there to protect the NST, but I have since found out from members of this list that there are much better ways to do that." Ditch the choke coils, use a terry filter if you can, but at least use a safety gap. See the archives for details. "The secondary that I built had a 1.5" diameter, 10" tall and has a first resonance at about 500kHz. I used the frequency generator to drive the base of the secondary and read the peak from an oscilloscope probe used as an antenna." This is smaller diameter than most people build, but it's not bad for a first time. Without knowing what size wire and how many turns, it's not possible to analyze. "The primary is 15 turns in a flat coil with an inside diameter that is 1" away from the base of the secondary coil (3.5" diameter). The primary circuit and the NST are not grounded at all." Again, without knowing the spacing between turns, wire or tube size, or outer diameter, there's not enough to analyze. If this is fastened to a wooden board and/or your using metallic fasteners, you probably have a high resistance short circuit or two. "The capacitor is a 1 uF 20 kV rating that I borrowed from someone at the office. I did let them know that there was a fairly good chance it might not come back alive." What type - mylar, mica, foil, film, ceramic, etc?? NOTE: Your cap voltage rating should be not less than twice the output of your transformer. If you have a 12 or 15 kV NST you may have already punctured your cap. "I have a crude ASCII art drawing, (obviously this only works with fixed width font): +---CHOKE---+--||--+ | | | o | v | | NST PRI SEC | ^ | | | | | | +---CHOKE---+------+---------+" Ditch the chokes and add a safety gap. The general layout is OK. You should have a least a Variac with a switch and a fuse for safety and control as a minimum. You can add metering, safety lockouts, contactors, etc as resources permit, and as your lust for power grows. (And it WILL!) "It never sparked, however I have more enthusiasm than smarts for RF ... I also had the spark gap about 1/4" apart, and when it ran there were big cloudlike angry blue sparks ..." Depending on the size of your NST, that may or may not have been too wide, as mentioned above. You may indeed have blown the cap and are now feeding all your power into the gap. "I am guessing that I am breaking about 20 good design rules at this point, so I sought help before I shocked myself or blew up my secondary coil." You got here, and enlightenment does not care which door you came through or how long your journey was. "I am open to all criticism of the design, or pointers - I have found some of the online resources for design such as JAVATC, etc. and I'll probably start working on version two using some of the designs that I see on other pages ..." Copying designs, even good ones, without understanding what or why, could be VERY hazardous to your health. Ask many more questions, read more on the principles and safety, and make haste very slowly. Thanks again, Tim Welcome, Matt D. ************************************** See what's new at http://www.aol.com _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla_______________________________________________ Tesla mailing list Tesla@xxxxxxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla
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