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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Every time the wire passes through the toroid hole, that's one turn. As far as the transformer ratio is concerned, it doesn't matter if the rest of the wire is tight against the core, or looped 5 times round the known universe.
So for a toroidal CT, simply poking the primary wire through the centre hole gives a x:1 ratio. And the one in the picture is x:2.
Of course the tightness of the windings affects other parameters of the transformer, like leakage inductance and stray flux, so when winding more than one turn they should be kept tight against the core and distributed evenly around the toroid. But the effective number of turns still goes up by 1 every time the wire passes through the hole, and nothing else you do changes it.
I've heard of coil winding companies that define one turn on a toroid differently, along the lines of Ash Small or Dr. Electrons' arguments. They deliver CTs with a ratio 1 different to what you expect. If you order a 100:1 you get a 99:1 or a 101:1. I don't think anyone ever notices, as other errors in the transformer construction add up to more than 1%.
Corollary: Unlike E-I types, toroid transformers can't have half turns.
Registered Member #3885
Joined: Sun May 15 2011, 12:47AM
Location: Newton, Massachusetts, United States
Posts: 94
Thanks for your help guys! Even though I didn't manage to get the DRSSTC working in time, one of my old SGTCs combined with my keyboard-modulated SSTC were good enough to please the crowd. Here is a quick video of some of what went on today, if anybody is interested.
Once the school year ends, I will have more time to experiment and hopefully return to this project. I already tried different CT primary side configurations based on your advice, but none of them have seemed to work. I think the next thing I'll try to do it rewind the CT on a smaller core with 30AWG wire instead of 26AWG.
Here are some more pictures (perhaps I will post something in the project thread):
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