VTTC Startup Circuit

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Adam Horden" <electronicmadman-at-blueyonder.co.uk> 

Hi Guys,

I have been playing with the circuit 
<http://www.e-insite-dot-net/ednmag/archives/1994/101394/graph/21di7fg1.htm>http://www.e-insite-dot-net/ednmag/archives/1994/101394/graph/21di7fg1.htm 
here to slowley apply the voltage to the filimant transformers on my VTTC. 
I need a little help with it.

Varying the voltage on pin5 should allow the output period to be brought up 
slowley instead of instantley.I cant see how this is done in the circuit as 
there is nothing to charge up the resistor capacitor network. I think the 
resistor should go to supply and the end of the resistor to pin 5 and a 
capacitor from pin 5 to create a resistor capacitor time constant.

I did just that added a resistor capacitor to pin 5 and watched the voltage 
from switch on gradulay rise. I had a 1000uF and 2K resistor to give me 2 
seconds. The voltage rose form 0V to 12V over 2 seconds. The problem is 
this dident seem to make the sligest diffrent on the output pulse.

I take it this circuit uses phased angle control to fire the triac? This 
then should give me a varying voltage on the filimant transformer.So the 
longer the pulses the lower the voltage and the faster the pulse higher the 
voltage?I just get a fast going square wave on pin 3 to the opto triac 
regardless of the voltage on pin 5.

I got a 6.8k resistor and 1uF capacitor on pin 7 and 6 for the timeing 
components.

With a load on the triac from startup I just get a 240V on the filimant 
transformers.

Any ideas why this circuit dosent work?

I have removed the interter in the design as the pulse from the zero 
crossing is already low going and will trigger the 555 timer.

Am I wasting my time with this circuit or shall I find another way to ramp 
the voltage on the filimant transformers?

Any ideas would be very gratefull,

Thanks,
Adam