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Registered Member #9039
Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117
If so how do you apply RC snubbers to this schematic: I tried puting one in parallel with the GDT primary (1k + various caps) but it seemed to have littee effect on the ringing. 220R for R4 & R3 seemed the flatten out the ringing but then I am significantly reducing current to the FET gates.
GDT:
4 strands of AWG 30 twisted together to form 3 'wires'. 3 of these are then twisted together to form a trifilar 'wire' which was then wrapped around the torroid. Torroid material: N30
Ringing on GDT primary
Ringing on both GDT secondaries with peak voltage one lower than the other. Why?
Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
first put a 10K resistor across each gate to source second put RC snubbers across D1 and D2 as you have not given details of your oscilloscope settings I cannot help further
Registered Member #9039
Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117
johnf wrote ...
first put a 10K resistor across each gate to source second put RC snubbers across D1 and D2 as you have not given details of your oscilloscope settings I cannot help further
The oscilloscope is on the 1us scale, although there doesn't seem to be any way of telling how much the fine scale adjustment alters the time scale.
Resistor from gate to source? Oh I see what you mean - a discharge path that does not have to go through the GDT secondaries (inductors)
I was assuming the ringing was coming from the GDT but you have made me realize that it could be coming from the Tesla primary.......which would explain why trying various combinations of resistor + cap across the GDT secondaries was having little effect.
Is there any reason why it would interfere with the correct operation of this particular circuit? And I assume I could get away with 1N1819 as D2 since the current involved with the voltage spikes would be under 1A.
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
The snubbers on the daycounter website are design to prevent flyback on the output of the 'FETs. You seem to have a problem with ringing on the gates. This is due to nasty parasitic capacitiances and inductances between the output of the gate drivers and the inputs of the MOSFETs. One thing you can try is placing a small value resistor in series with the primary of the GDT. I would suggest trying 2 to 5 ohms.
On another note, your duty cycle seems to be more like 60/40 than 50/50. This is causing the capacitor to shift voltages to maintain GDT flux balance. To provide an even voltage, provide an even duty cycle.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
with no ac power to the output bridge, connect C12 in parallel with C9 to see more like operational gate waveforms. use a 10R 10W resistor in place of L1 if L1/L2 not ready.
Registered Member #9039
Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117
hen918 wrote ...
The snubbers on the daycounter website are design to prevent flyback on the output of the 'FETs. You seem to have a problem with ringing on the gates. This is due to nasty parasitic capacitiances and inductances between the output of the gate drivers and the inputs of the MOSFETs. One thing you can try is placing a small value resistor in series with the primary of the GDT. I would suggest trying 2 to 5 ohms.
On another note, your duty cycle seems to be more like 60/40 than 50/50. This is causing the capacitor to shift voltages to maintain GDT flux balance. To provide an even voltage, provide an even duty cycle.
I used this 555 astable circuit so that I could experiment with the duty cycle with a double gang pot for R1 and R2 plus 2 x 1.2k in series with both sides of the pot to fix the minimum and maximum duty cycles at about 10% and 90%.
So I could experiment with the effect of duty cycle.
I also have a 300pF tuning cap in parallel with the timing cap for fine adjustments to the frequency.
I have marked the position on my enclosure of 50% duty cycle.
But I have noticed that it really messes up the wave form if I go below 50% duty cycle.
I have my control box connected to the coil and driver circuit via a 3m long lead so that is probably a source of some of the parasitic capacitance and inductances.
But I didn't really want to be standing right next the the discharge while playing with the controls.
I will try your small value resistor trick and see if that improves the waveform.
Can you also suggest a website that fully explains and details strategies for dealing with ringing in this type of circuit?
Side note - if done correctly, using magnet wire can yield a good GDT with sufficient isolation between primary and secondary windings. However it's very easy to nick or stress the insulation, which can lead to a short especially with high bus voltages. As a result, I typically prefer using good (but as thin insulation as possible) insulated wire.
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Ringing on GDT. Can it be stopped with RC snubber? (no choice but to dupilcate)
