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Registered Member #54484
Joined: Sun Feb 15 2015, 06:18PM
Location:
Posts: 46
I'm adding another 15kv 60mA NST in parallel with my existing NST same make and specs for 120mA. TeslaMap suggests an LTR of 34.3nF. The MMC calculator suggests 2 strings of 9 to = 33.3nF at 18kv peak. The peak voltage of my 15kv NST is 17.12kv. Is that a safe enough range or should I go with 3 strings of 13 to = 34.6nF at 26kv peak? And what do you believe the best capacitance value would be? If I go with 3 strings of 13 I'll have to purchase more caps to provide tap points of less value. Right now I have 40, I'll need 48 total to provide a total of 4 taps. 34.6, 32.1, 30 and 28.1. According to the calculator. Also, TeslaMap suggests a PFC cap of 322uF. Can I use these from ebay?
Coil specs:
NST X 2 (both are Franceformer with GFCI circuit removed) * Input 120v * Freq 60Hz * Output 15KV * Current 60mA
NST Protection: Terry filter
Richard Quick static gap. 6" PVC with 11 pieces of 3/4" copper pipe all spaced at 1/32"
Registered Member #6038
Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248
Been a while since I built an NST coil, but my recollection is that you want the MMC to match the transformer impedance at 60Hz. this will give max power transfer to the tank capacitor. If my calculations are correct you need about 21nF for MMC. Depending on what types of capacitors you are using, you can figure out the series/parallel config to give the right capacitance. Hope this helps.
Registered Member #54484
Joined: Sun Feb 15 2015, 06:18PM
Location:
Posts: 46
I believe your calculating the resonate capacitance. From what I've learned NST's prefer a larger than resonate capacitor (LTR). The coil will perform better and it will help preserve the life of the NST. I was just asking if an MMC with a max rating of 18kv is safe enough to handle the peak voltage of my NST which is 17.12kv.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
crashstudio wrote ...
I believe your calculating the resonate capacitance. From what I've learned NST's prefer a larger than resonate capacitor (LTR). The coil will perform better and it will help preserve the life of the NST. I was just asking if an MMC with a max rating of 18kv is safe enough to handle the peak voltage of my NST which is 17.12kv.
All MMC design is a trade off between life time and price. For a very long time the DC rating of a pulse capacitor have been used, despite we really should find the proper RMS voltage vs frequency in the datasheet and derate our capacitors accordingly.
We rely a great deal on the very short on-times, intermittent use and that our coils might sit for years without running and over a life time it might not see more than a couple of days of combined use.
Is your MMC good enough for hobby use, yes(until it explodes). Will it survive running 24/7, no.
Registered Member #54278
Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367
crashstudio wrote ...
I was just asking if an MMC with a max rating of 18kv is safe enough to handle the peak voltage of my NST which is 17.12kv.
Isn't the peak voltage of a 15kV NST Vrms*sqrt(2)=21,215V ? Additionally with the variac (o-140VAC input) multiply this product by 140/120 yielding just under 25kV.
What calculation gives you 17.12kV, I may have something off since I never used an MMC.
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
I believe you are correct, Signification. However, the spacing of the spark gap *should* limit the maximum voltage the MMC sees.
I've somehow managed to kill a cheap 24kV MMC with a 8.4kV transformer, even with a very conservative gap spacing (to be fair, they weren't suitable for high pulse currents). Your mileage may vary...
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Adding another NST. How should I reconfigure my MMC?
