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Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Marko -
Posted in vain ?? I don't think so.
Just because you didn't receive immediate response doesn't mean that no one is reviewing the information you provided. I think the time and effort you took to post this info was not wasted; the overall concept, pcba layout, and especially the mechanical arrangement of the heat sinks are all very interesting. Whether or not the experiment is successful in the end, documenting the progress of the experiment on 4HV leaves a permanent record that will likely be useful to others.
What is the "hub" in the center of your toroid? It looks like some kind of poured-in potting material; epoxy, or something similar? How did you secure the butt-joint of the toroid while the potting material cured?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Yeah, if you don't get any replies, it usually just means that nobody can see anything wrong with your project to bitch about! :P I think the heatsink design is very clever too...
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
As of now I have no clue whether base current feedback will work at all, in which case I'll need to trash all the circuitry and start over.
I don't think my primary is good neither. I don't have enough length of cable anywhere to make it bigger. If I leave it like this it will run 10-15amps of magnetizing current. Through 28A mosfet pairs.
so I'm guessing you won't want to go creating streamers
I'm definitely going to create streamers. And draw arcs.
I actually tried a similar approach about a year and a half ago with a center tapped CT, but eventually I ended up doing something completely different : I used an ordinary CT, with a potential divider on it made with a couple of resistors. I grounded the center point, and this resulted in 2 sine waves 180 out of phase. The whole idea was to not load the CT down, so I played around with the number of turns on the CT and the value of the resistors until I had 2 sine waves with a peak-peak no greater than 24v. These two sine waves went into 2 zero-crossing detectors (made with fast comparators) and the resulting signals were buffered and drove the MOSFETs through a transformer.
It is not good to load CT with resistors because it results in sinewave output.
I need something closely resembling square and that's why I use the diodes to clamp it.
1k resistors are there to prevent output of one secondary to be killed by the other once it's shorted through the clamp diode. (I can't allow reverse voltage into the driver input).
As for feedback - secondary base should be fine if the coil isn't even breaking out. Might get a more 'even' signal from the primary current though especially with that high coupling.
That is an interesting question - which I don't know to answer because I don't know too much about SSTC's.
I never seen anyone building a primary current feedback with SSTC's.
After reading Steve C. and Steve W. 's posts I just got more confused about those things.
There is a lot of inductive reactance on the primary. But, presence of the secondary somehow manages to force the current into sine wave shape, and force it to cross zero on itself too.
Do the diodes conduct in that case? I don't know.
It is possible to take feedback by using a separate winding near the primary instead of CT. So could just putting the CT across the primary do the same? I *don't* know.
What is the "hub" in the center of your toroid? It looks like some kind of poured-in potting material; epoxy, or something similar? How did you secure the butt-joint of the toroid while the potting material cured?
It is polyester resin, used with fiberglass wool as main construction material for plastic boats.
It is mixed with talcum powder to make it into putty which I used to fix the wooden part to the tube. after it cured I poured the liquid resin into the gap. Still on this toroid it leaked a bit as I haven't used enough catalyst to cure it quickly enough.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Primary current feedback doesn't work on untuned primary SSTCs, only DRSSTCs.
Why?
If I had very high coupling, like with a ferrite transformer, It would be the same if I put a CT on the primary or secondary, would it? I was pretty sure that it would work in that case.
But what happens when coupling is lowered... and why wouldn't it work then? Considering current still does zero cross on it's own and everything.... how would it be different from secondary feedback?
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Why?
Because the primaries natural frequency (determined by the Lpri and Cblock) is far too low. If Cblock doesnt exist, then the thing wont even oscillate at all .
Why doesnt the secondary force the primary to resonate in this case? The way i see it is because the coupling is always too low to make it work. The system has to have a full current reversal in order to make it oscillate (otherwise you never reverse the output). If you apply a step input to your primary coupled with secondary (and assuming no DC blocking cap, or its infinitely large) then you will get something that oscillates, but not a full swing like you do for a second order LC system (where you get a natural harmonic oscillation). Instead you will see a typical
I(t) = Ae^(L/r)t + Bsin(wt)
(might have my math a bit wrong here) type thing with the secondary resonance (B term) as a small sine wave riding on top. Only as K approaches 1 (as in, maybe .98 and up) will you get a a sine wave that reverses below the 0 point enough to get a stable oscillation.
The DRSSTC works perfectly with primary feedback because the primary itself is a second order system, and thus a step response (ignoring primary resistance/damping) results in a pure sinusoid output that has equal + and - amplitude. Its this natural reversal of primary current that allows the use of feedback to keep the system oscillating.
I hope this makes it crystal clear to you now why primary feedback doesnt work for the untuned primary SSTC. It also goes to show how step responses of an open loop system can be very useful for learning.
Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
wrote ... There is a lot of inductive reactance on the primary. But, presence of the secondary somehow manages to force the current into sine wave shape, and force it to cross zero on itself too.
Surely this can only be caused by the mutual inductance between primary / secondary, in which case could the primary feedback work in theory for an SSTC if the coupling were high enough? I would have thought it would be possible, but not without some slight phase lag on the feedback.
(or maybe I should just turn my computer off and go to the pub..!)
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Wireless powering SSTC
