On Thu, 14 Aug 2003, Dave Tweed wrote:

> Vasile Surducan <vasile@S3.ITIM-CJ.RO> wrote:
> > On Wed, 13 Aug 2003, Dave Tweed wrote:
> > > Also, your <1nF load draws over 300mA rms of reactive current at 25 kHz,
> > > which is going to need relatively large wire to keep losses low.
> >
> > I think you have mistaken with 10^3. Is just about 250 microamperes...
>
> No, 1/(2*pi*f*C), given f=25000 and C=1e-9, definitely gives 6366 ohms
> impedance. With 2KVrms applied, you're definitely going to have 314 mA
> rms flowing.
>
> Note that I'm talking about reactive current, not just the in-phase current
> that represents power delivered to the load. The coil has to carry both
> types of current with low loss.


Yes, you have right, I've messed up the 10^3. However this reactive load
is ok for my transformer. It works continously from about one week.


>
> > > Yeah, you're going to be better off getting a broadband audio power
> > > amplifier and using a beefy audio transformer to step the voltage up.
> >
> >   This is not working. One of my coleague has project and built such an
> > amplifier 5 years ago. Just in 0...10KHz but up to 10KV. The system is
> > totaly instable with capacitive loads. It works only with resistive loads.
>
> Have you tried adding feedback with an appropriate compensating network?
>
  Sure. The compensation works ok only on some frequencies. There are
situations when the output transformer is resonating. Which should be ok
if it does not change the amplitude.



> > > An alternative approach would be to create a DC supply of 3KV (900 watts)
> > > and then build some sort of H-bridge driver out of high-voltage tubes.
> > > But controlling distortion with this setup might be a problem.
> >
> > This is not feasible at all. And you know well why :)
>
> Do I? Hey, I was just throwing out ideas, trying to be helpful, with the
> sketchy requirements you gave us. If you already know how to do this, why
> were you asking here?

  Is not feasible for me. I didn't say is not a good ideea !
  It needs first to project a DC/DC up to 3KV at least.
  Then, with a switching methode to get a sinusoidal shape using a pwm
or something like this. The load is still high for such a methode.
Then it's a real pain to get transistors which may work safely at this
voltage. If I'm just thinking I don't know any... :( 1500V is too low even
for a bridge supplied at 3KV.


>
> Seriously, if you already have coils that work over the entire frequency
> range, the obvious answer is to electromechanically switch among them,
> using relays or a motor-driven switch. How frequency-agile does the
> solution need to be?

  I was thinking to the motor driven switch. There is a problem with the
movement resolution. Because the proper resonance is at a very small air
gap distance. I know it could be changed using a highest AL ferrite but
I haven't any with AL greater than 5000. So the whole frequency range will
be in 1...2mm movement. The time required for changing frequency is not
essential now.

THX,
Vasile

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