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Registered Member #1796
Joined: Mon Nov 03 2008, 04:30AM
Location:
Posts: 2
Hi everyone,
I am currently testing my first tesla coil, a half bridge SSTC based on Steve Ward's MINI SSTC, although with a PLL driver.
It's been loads of fun, last night i finally got the PLL to lock on ( 375kHZ with a soda-can topload ). What I suspect I need to do now is play around with the primary winding. Its currently three turns of AWG 18 pretty tight around the 2ndary.
My question is, other than trial and error, whats the best way to figure out how many turns of primary to use, and how close to the secondary should it's diameter be?
Thanks a bunch, I plan to post some pics/video real soon.
Registered Member #1107
Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
My advice from past experience with that coil is to take some 6"pvc pipe and wind the 3 turn primary in your case with the turns spaced about 1/8in apart and then just move the primary up until you get flashovers and then back it down a bit.
Registered Member #1739
Joined: Fri Oct 03 2008, 10:05AM
Location: Moscow, Russia
Posts: 261
As for the coupling - do everything it takes to get it as high as possible. The primary should be tight and cover as much of the secondary as it can without an arcover. Check on the lower voltages first, watching for any corona forming up on the top turn as the voltage rises. The number of turns should be set up keeping the magnetizing current in mind. Check inductance or do calculations before you set the number of turns. Starting with a low number is very wrong - below some point the spark doesn't grow much, unlike the stress on the switches. Start with like 10, and watch the heat level after a minute of operation.
Registered Member #1796
Joined: Mon Nov 03 2008, 04:30AM
Location:
Posts: 2
So when I see pics of SSTC's with the primary well separated from secondary, this is done only to prevent flashover? I thought the idea was to have very loose coupling, to allow the secondary to freely resonate, otherwise the voltage developed would be limited by the turns ratio of primary to secondary (like a normal transformer).
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
I've been having trouble with coupling too, flash-overs always occur when it's about to work best. Primary tuning is more complicated than it seems. You need to balance the amount of turns to get the right magnetizing inductance, adjust how much of the secondary you couple, and what turns ratio the secondary is coupled with. 3 turns at 375kHz sounds like awfully little, but whatever works. I tend measure the inductance and see what the reactance is a little below fres, and estimate magnetizing current from that. Few tightly wound turns spaced over a large portion of the secondary give good voltage gain and power coupling, at the expense of flashovers and less secondary to resonant with, increasing fres. Lots of turns wound over a small portion of the secondary on a large spacer give little coupling and voltage gain.
If I'm too brief on this Richie Burnett has it all covered on his site.
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primary coil coupling -- how "tight""
Primary tuning is more complicated than it seems. You need to balance the amount of turns to get the right magnetizing inductance, adjust how much of the secondary you couple, and what turns ratio the secondary is coupled with. 3 turns at 375kHz sounds like awfully little, but whatever works. I tend measure the inductance and see what the reactance is a little below fres, and estimate magnetizing current from that. Few tightly wound turns spaced over a large portion of the secondary give good voltage gain and power coupling, at the expense of flashovers and less secondary to resonant with, increasing fres. Lots of turns wound over a small portion of the secondary on a large spacer give little coupling and voltage gain.