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Registered Member #1408
Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679
Some time back I made a tiny SSTC. I have wanted to make a micro sized SGTC but it's giving me more problems than a small unit. The smaller I go the more critical things seem to become in an SGTC..... I like the challenge but I'm getting ready to throw in the towel. SGTC's just don't seem to work well after a certain size. Medium sized ones have so much to work with. I have been having a great deal of difficulty with the JavaTC, it's possibly not meant to drop down lower than 5K and downsizing. Has anyone made a micro SGTC? My break out point sees nothing even if I drive it with a bit more power.. My gap is functioning well; if it were a bigger coil, I'd see some darn thing but this little nasty is tough to get visual. The primary is really not a easily tuned unit as the design of a flat coil would have so few turns that the movement of such a sleight bit throws off my output. I just tried a standing primary but again, the tuning with small materiel is not easy. All the table-top micros I have seen were SS. I just can't get any streamers even after test after test.. Any advice with this? Is a micro SGTC too sensitive to work with?
secondary is 28 thin wire @430 turns in a 2" tube driven by AC @ 5K 20ma (but I can swap out anything) Tried flat and column and flat primary (column was 18 turns 12awg /21 turns 16a awg pancake primary was 6 turns 1/4" tube Top load was DIY metal take & tube affair: break out with a guitar string, needle, whatever. {I don't have a scope} secondary is small but I was able to overdrive a small secondary before to get started with a little breakout: no-go this time
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
quicksilver -
A spark-gap coil with a 2" dia secondary and powered by a 5/20 is not a "micro" coil, but similar coils have produced 12" to 15" and longer streamers. In fact, Gary Lau has had a "mini-coil" competition going for several years, based on a specific input power (4kv/20ma NST) lower than you are working with.
See: for design details on Gary's mini-coil, and a number of other mini-coils that have entered his competition.
JAVATTC will work just as well for designing a "mini-coil" as for any other sized coil; they all follow the same laws of physics.
I'm not sure why you are saying the primary will have too few turns to be easily tuned; select your tank capacitor value so your primary coil (flat spiral) has anywhere between 5 and 10 turns, and (assuming you have designed and constructed the coil so the primary and secondary circuits can be tuned to resonate at the same frequency) you should have no problem finding the appropriate tap point.
For best performance, your secondary coil will need between 800 and 1200 turns; for a 2" diameter coil form, you will need considerably finer wire, like #30 or #32 AWG.
The descriptions of the primary coils you have "tested" seem odd; an 18 turn helical primary and a 6 turn flat spiral? Were both of these primaries tested with the same value tank cap? What was the calculated inductance of each of these primary coils? What is your target primary inductance required to get the primary circuit to resonate at the same frequency as your secondary circuit?
If your spark gap is firing, and you have wired the coil correctly, you are probably just wildly out of tune. Go back and carefully enter all your parameters in JAVATC, and verify the resonant frequencies of your primary and secondary circuits. If tuned properly this coil WILL work, but you're unlikely to get spectacular performance with only 430 secondary turns.
Use JAVATC to analyze a modified design using a new secondary with closer to 1000 turns, and determine the required primary inductance and tank capacitor value.
Registered Member #1408
Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679
Thank you both for the input. I'm pretty sure it's wildly out of tune due to the secondary. The problem was trying to make parts fit the design rather than properly design the parts. I had completely forgot about the UK contest and all the info therein. There are some really incredible tiny TC's!
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Challenges to Micro sized SGTC
