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Registered Member #55076
Joined: Sat May 23 2015, 08:26AM
Location:
Posts: 20
I have a MMC with 6 strings of 11 with each cap being 1.5Kv and .047uf. (25.6uf 16.5Kv) My NST is a 10Kva 50ma I used TeslaMap to work it out but heard someone say TeslaMap works out the MMC incorrectly, is this correct?
I've always used DeepFriedNeon's site (google it) and multiplied the capacitance by Phi (1.618). Has worked beautifully every time. No idea how TeslaMap performs.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Capacitor in series and parallel follow the same but inverse equations for that of resistors in series/parallel circuits.
So for each string: For total capacitance 0.047uF / 11 = 4.27nF For total voltage 1.5Kv * 11 = 16.5Kv
For the whole bank: Capacitance 4.27nF * 6 = 25.62nF (not uF)
For voltage we follow Kirchoff's good old law, voltage on parallel elements must be the same on all elements in the network. So your bank voltage is the same as the string voltage.
For more in depth info on caps in series and parallel:
@Sigurthr I don't know what you are getting at with multiplying by phi you don't need to do that. DeepFriedNeon's MMC page uses the same math as I have shown above.
As an additional note, this is a very small capacitance for a coil <200KHz You may end up with a very large primary and very high tank impedance if you keep your current MMC configuration.
The reasoning behind multiplying by Phi is it guarantees a Larger Than Resonant value that should prevent potentially-destructive resonant rise during a runaway condition (spark gap failure for example).
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Sigurthr wrote ...
The reasoning behind multiplying by Phi is it guarantees a Larger Than Resonant value that should prevent potentially-destructive resonant rise during a runaway condition (spark gap failure for example).
That doesn't make sense. Making the capacitor larger won't prevent resonant rise, it simply lowers the frequency at which it occurs in the primary tank.
I also can't fathom a spark gap failure. Are you expecting the gap to fail short? If this happened you would not have a runaway current in the primary, because there is simply no way for the NST to add additional energy (it's short with a gap failure). In my opinion this is one of the disadvantages of a SGTC vs a DR. Once the gap fires the only energy you have to resonate with is what is already stored in the tank, from that point on it's a decaying oscillation.
So in consultation by making the capacitor larger like that you are not doing yourself any favors nor are you preventing resonant rise.
Registered Member #55076
Joined: Sat May 23 2015, 08:26AM
Location:
Posts: 20
Goodchild wrote ...
Capacitance 4.27nF * 6 = 25.62nF (not uF)
As an additional note, this is a very small capacitance for a coil <200KHz You may end up with a very large primary and very high tank impedance if you keep your current MMC configuration.
Sorry I meant nF
as for the small capacitance what would you suggest I'm just going by what TeslaMap says is the optimum
Registered Member #39190
Joined: Sat Oct 26 2013, 09:15AM
Location: Boise National Forest
Posts: 65
Goodchild wrote ... That doesn't make sense. Making the capacitor larger won't prevent resonant rise, it simply lowers the frequency at which it occurs in the primary tank.
I wonder if Sigurthr meant to say that he uses an MMC with a voltage rating phi times the required voltage rating.
Nope, I'd been using Phi * capacitance. Voltage rating was always at least 400% expected working peak voltage.
It's been too many years since I've done spark gap coils, but I did have static gaps fail open circuit. They just stop firing (usually a result of electrode ablation). Rotary gaps can of course fail open quite easily.
I can't remember the specifics of it any more, but a lot of the literature I had read when I was learning about and still doing SG coils talked about how when you perfectly impedance match the tank capacitance to the NST secondary impedance there's potential for resonant rise if the gap fails to fire, where the voltage across the secondary of the NST can rise beyond the winding insulation ratings, causing NST failure. The idea was to use a larger than resonant value so that the cap doesn't have time to fully charge (because it is a lower impedance than the nst, and thus loads the output of the nst to a lower voltage initially as well as taking longer to reach a certain voltage) within the expected break rate. This way skipped beats don't stress the NST secondary insulation.
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
In spark gap coils, even if you maintain the rated terminal voltage, say 15kv, you are STILL exceeding the transformer's insulation rating. Why? Because when loaded to a Neon tube (a diode) it drops the terminal potential of the transformer to its rated insulation.
Don't believe me? Call up Franceformer and find out for yourself. I was surprised after I called with a battery of questions.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
Sigurthr wrote ...
Nope, I'd been using Phi * capacitance. Voltage rating was always at least 400% expected working peak voltage.
It's been too many years since I've done spark gap coils, but I did have static gaps fail open circuit. They just stop firing (usually a result of electrode ablation). Rotary gaps can of course fail open quite easily.
I can't remember the specifics of it any more, but a lot of the literature I had read when I was learning about and still doing SG coils talked about how when you perfectly impedance match the tank capacitance to the NST secondary impedance there's potential for resonant rise if the gap fails to fire, where the voltage across the secondary of the NST can rise beyond the winding insulation ratings, causing NST failure. The idea was to use a larger than resonant value so that the cap doesn't have time to fully charge (because it is a lower impedance than the nst, and thus loads the output of the nst to a lower voltage initially as well as taking longer to reach a certain voltage) within the expected break rate. This way skipped beats don't stress the NST secondary insulation.
I'm sorry but what you are saying really doesn't make sense to me. If the spark gap just stops firing the NST is more or less going to run open circuit, and because the NST is putting out 60Hz the tank capacitor is only going to pass a small amount of current (limited by Xc @ 60Hz in series configuration) Xc will be so high at 60Hz that for all intensive purposes the NST will be open circuit as Hazmatt suggest.
Regardless, even if you did mange to somehow match the NST impedance to the tank impedance at Fo (unlikely) you would generate the resonate high voltage at the L and C node of the tank, not across the NST as you suggest. A DR is a perfect example of this, when we create resonate rise in a DR tank the voltage on the H-Bridge output never rises above that of the drive voltage, however the L/C node will rise to many thousands of volts during normal operation. Hence why you have to rate the MMC to Ipk * Z but the bridge is only rated to 600V/1200V.
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MMC confusion
