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Registered Member #103
Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845
I was wondering... is there any reason why feedback cannot be taken directly from the primary, without using a current sensing transformer?
I had this idea from a (new?) primary feedback scheme I have been working on recently, which takes the output of a single CT, and puts it across a potential divider which also loads the CT slightly. The centre of the potential divider is tied to ground, so each side swings positive in relation to ground, out of phase of course. These two outputs then feed a dual high speed comparator, configured as two zero crossing detectors. The outputs from the comparators are therefore inverted and non inverted drive signals at exactly 50% duty cycle. I hope that makes sense, I have had it working quite well but I'm still perfecting the whole thing. I guess I should draw up a schematic but it is fairly simple really. My controller originally had a center tap on the CT, but once I had eliminated this I realised it could probably be hooked directly across the primary with a suitable potential divider network.
Anyway, if you can visualise what I've just described above, is there any reason why I cannot simply remove the CT altogether, and hook a divider directly across the primary? Of course the resistor values would need changing, and more would need adding to get a center point, but they can be of a high value due to my driver not needing a particularly high current input to swing its outputs. Has anyone in the history of coiling created a solid state coil with direct primary feedback, and is there any reason not to do it despite the added complexity?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The voltage across the primary is affected by a whole bunch of things. If it were a pure inductor, the voltage would just be proportional to the rate of change of current, so you could integrate it to derive a current signal. But there's also the reflected impedance of the resonator to worry about.
Registered Member #358
Joined: Sat Apr 01 2006, 06:13AM
Location: UCSB
Posts: 28
You gotta remember that your CT is in *series* with the primary, and you're proposing to put the divider in *parallel* with it, so no, it's not gonna work like that.
It would be possible if you put your sense resistors in series with the primary, but you'd have to be very careful about isolation, what you think is ground, stray inductances, and your signal is only going to be a few millivolts, unless you don't mind wasting a lot of power. Remember, the CT steps down current, and steps up voltage.
Or... You can cough up the $0.20 that the ferrite cores from goldmine cost, and spend 2 minutes winding a CT.
Actually, if you're against using a CT for some reason, use the "antenna" method. I put this in quotes because I'm not talking about a piece of wire, but a capacitive feedback system that works the same way. The antenna is really just a small capacitor from the toroid to the antenna, so it can give a tiny feedback current.
This forms a "capacitive current divider", and then we stick our resistor/diode bridge in series with the small capacitor. We can do the same thing with primary feedback and the MMC. You just have to fix one side of the MMC to a DC potential (ground, + rail, - rail, etc...), and then put a small capacitor between the MMC and your current sense resistor.
That capacitor would have to do the full MMC voltage unless you tap off at a lower string, but the value can still be pretty low, so it should be that bad.
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Direct primary feedback
