First drsstc project, looking for recommendations

[costas_p]

First test is completed , it wanted around 70V ac from variac to start oscillating, there is a buzzing sound coming from the coffeepot, after 1 minute of run on 70V ac everything was cool to the touch,
actual measurements not done cause my tools are on work place at the moment. I am verry happy nothing exploded yet

https://youtube.com/shorts/fxLiGhJB-ZI?feature=share

[AstRii]

(Anyone had issues with UD2.7 outputs being stressed by the low frequency signal of non-oscillation resulting in GDT saturation?)

While we're at it, how does one recognize if GDT is saturating?
I usually check primary current of a transformer to see if it doesn't peak way above nominal current, but current flowing to transistor's gates is by definition a high current peak, rendering my method useless in this case.
How do output waveforms of a GDT look like if it's saturating?

[davekni]

While we're at it, how does one recognize if GDT is saturating?
I usually check primary current of a transformer to see if it doesn't peak way above nominal current, but current flowing to transistor's gates is by definition a high current peak, rendering my method useless in this case.
How do output waveforms of a GDT look like if it's saturating?

Normal peak GDT input current is during voltage switching.  Saturation current is highest just before voltage reversal, and typically lasts longer, from the time of saturation until voltage reversal.  So, time of peak current is the key.  Saturation current will show up as relatively-rapid charging of coupling capacitor in UD2.7 output circuit and in higher output FET voltage drop.

[costas_p]

I finally managed to assemble the coil, and make some tests, coil achieved first light.

https://youtube.com/shorts/_saYnn-gCiA?feature=share

Phase lead is not yet tuned.
I managed to supply full ac mains power throught a variac without blowing anything, however the spark length did not had big difference from eg 70v (start oscillating) to full 220v.
What is the recommendation: make coupling factor higher, or start adjusting primary coil tap point ?

[davekni]

What is the recommendation: make coupling factor higher, or start adjusting primary coil tap point ?

If coupling is still at your original 0.12, I recommend increasing it to at least 0.14.  Even higher might be good, especially for initial testing.

Tuning would be easier with a scope.  Adjusting tuning will likely help too.  Higher coupling will allow success with a bit more detuning.  Thus I'd increase coupling first.

[costas_p]

By the way, when we talk about coupling factor, what is the correct way?

In my case I am using 1/4" copper tubing for primary coil, so to measure coupling I measure distance from top/bottom or middle of primary and the start of secondary winding ?

[davekni]

In my case I am using 1/4" copper tubing for primary coil, so to measure coupling I measure distance from top/bottom or middle of primary and the start of secondary winding ?

I'd guess that JavaTC is based on middle of primary winding.  However, someone else will need to answer this more definitively.

[costas_p]

I did all calculation from the beginning, my coupling factor as it is right now is 0.126, due to construction  I can remove another 5mm of secondary coil height for a maximum 0.138 coupling.

While playing midi (6 turn tap + 0.126 coupling) the coil happily output around 20cm sparks at 70-80v input , increasing the voltage from variac the OCD led starts flashing like crazy so I decrease voltage to lower.

I will experiment further in order to be able to use full main voltage.


Mean while doom theme song:

[davekni]

While playing midi (6 turn tap + 0.126 coupling) the coil happily output around 20cm sparks at 70-80v input , increasing the voltage from variac the OCD led starts flashing like crazy so I decrease voltage to lower.

Increased coupling will likely reduce OCD events, allowing higher voltage.