Re: [TCML] understanding DRSSTC

["Udo Lenz" <udo_lenz@xxxxxxxxxxxxxx>]

   Hi Herwig,

> I'm about building a small DRSSTC. After having tuned the primary circuit to the natural 
> frequency of the secondary coil with topload = 304 kHz, I connected it through 10 kOhms 
> to a signal generator. Picking up the voltage of the secondary coil with an antenna, I 
> found two "humps" at 281 kHz and 328 kHz.  Calculated the coupling factor to about 0.16 
> which IMHO should be a good starting point - ok?

Looks ok to me.

> The amplitude at the lower hump is greater than that at the upper hump. Is the relation 
> between the amplitudes dependent on the quality factors of the coils or does it depend on 
> the L/C-ratio of the primary circuit? 

Possibly this is because the current in the secondary needed to get a certain voltage is lower
for lower frequencies. A lower current means a lower power loss due to copper resistance.

> In my understanding the lower the L/C-ratio the harder the half bridge can drive the 
> primary circuit - ok?

Yes, a lower  L/C-ratio will mean more current for a given bridge voltage. Optimally
that ratio should be chosen so that your bridges voltage and current capabilities will
be met at the same time.

> I *assume* that the system will work at 281 kHz, but do not know why. The OCD will not 
> provide a signal, if the system runs at low power. The feedback-CT and its logic will 
> tell the half bridge when to switch ON and OFF the primary circuit...

Usually DRSSTCs are run at the lower pole and they are tuned so that initially the secondary
frequency is somewhat (10% - 20%) higher than that of the primary tank. After arc formation, the
secondary frequency will drop due to the capacitance of the arc. Ideally that should shift
the secondary frequencie back to the primary frequency.

HTH, Udo


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