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SSTC weird resonant mode

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GeordieBoy

Mon Oct 05 2009, 05:59PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

The CT needs to be terminated into a low impedance for large signals, (ideally a short-circuit) in order for it to opperate correctly. If the burden does not present a very low impedance to the CT secondary, a significant voltage will be developed across the CT windings. This has at least these negative consequences:

1. A significant magnetising current will flow in the winding which detracts from the current being sensed. It effectively causes the CT to "under read" the current.

2. The CT may saturate if too many volt-seconds are impressed across the winding. Causes "gross under reading!"

3. The voltage developed across the CT when the current changes direction can be very high and may breakdown the secondary insulation of the CT.

4. A significant magnetising current flow will also cause a phase error in the current sense signal. This may or may not affect the operation of whatever the CT is part of.

-Richie,

teravolt

Tue Oct 06 2009, 01:59AM

Registered Member #195 Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111

then why do we use current tranformers for fead back. I had some frequency dubling in my DRSSTC and had to add some resistance in parallel with my CT to nock it down. why does a DRSSTC do 2F some times. maby a toride with less turns are more practial for feadback

Frosty90

Tue Oct 06 2009, 03:20AM

Registered Member #1617 Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139

When you enable the UCCs, one of the UCC outputs will go high, essentially kickstarting the oscillation. It would seem to me that your logic implementation of the enble function should do the same thing though. Maybe your coil is harder to start ue to the strange feedback circuit you're using.

Ahh yes I see now, because Steve uses one inverting and one non inverting IC. I use two ICs the same (I cant remember if they invert or not), and they are both low when the interuptor is low, and if there is no 'drive' signal, even with a high interuptor signal, they will both still be low, so it wont self start.

The CT needs to be terminated into a low impedance for large signals, (ideally a short-circuit) in order for it to opperate correctly. If the burden does not present a very low impedance to the CT secondary, a significant voltage will be developed across the CT windings. This has at least these negative consequences:

1. A significant magnetising current will flow in the winding which detracts from the current being sensed. It effectively causes the CT to "under read" the current.

2. The CT may saturate if too many volt-seconds are impressed across the winding. Causes "gross under reading!"

3. The voltage developed across the CT when the current changes direction can be very high and may breakdown the secondary insulation of the CT.

4. A significant magnetising current flow will also cause a phase error in the current sense signal. This may or may not affect the operation of whatever the CT is part of.

-Richie,

So is the zener/schotkey combo a good way to load the CT? My thinking is that it is good because it will clamp the signal to +/-5v always. The CT should look like aproximately a current source, so you should always get more or less a 5v square wave. Would it be better to load it with a resistor? But then the optimum resistor size would change with power level that your running at. I think that the diode load would be better for a wider range of singal strengths (proided of course you dont exceed the max dissipation of the diodes...)

In your feedback circuit, the clamping zeners will act as a very low impedance, making the series capacitor represent most of the CT loading if I'm not mistaken. Loading a CT with a capacitor is not really a good idea in this application

Hmm I dont think so, the capacitor only has a reactance of about 3 ohms at the resonant frequency, and assuming a large enough current from the CT, the zeners should clamp to about 5v. I really dont know what the base current of a coil like this would be, but say the CT provides 20ma, like a current source, then the with 5v across the zeners, thats effectivley 250 ohms, much bigger than the impedance of the cap. I found that I needed the cap, otherwise the CT iself loaded the startup oscillator too much, and it needed to be turned up very high to trigger the inverter gate, and at low suply voltage the thing wouldnt oscillate. (not a strong enough feedback signal) The configuration I showed you was arrived at mostly by experimentation, and it works down to very low supply voltages on the DC bus, you can see in the youtube link, I start the variac from 0, and corona first appears at about 5 volts input.

At the end of the day, why should this scheme:

be better for non DR coils than this

barring cascaded CTs, and over current detectors etc. Just from a signal detection point of view? I would have gone for the second one anyway, because as Richie said, CTs need to be loaded, to appear like a current source, other wise its like a 1:60 step-up transformer, pumping kilovlts into your driver!

Also does any one have any sort of data, of aproximately what the base current of a 1-4 Kw SSTC would be? what sort of magnitude? I only chose 1:60 for the CT because thats what I've seen on every schematic on the net.

Cheers,
Jesse

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