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Registered Member #488
Joined: Tue Oct 24 2006, 02:19AM
Location: Maine
Posts: 8
Hello, I built this cap bank for my new SGTC. Each capacitor is rated at 50kv 910pF, 11 caps in parallel giving me an LTR capacitance of close to 0.01uF for my 12kv/30mA NST. The ceramic caps are made from Strontium Titanate dielectric: Below are the exact specifications:
I have been told that these caps will /work/ but will slowly loose capacitance. My question is: Is this cap bank going to hold up, without blowing up, or am I wasting the caps and my money? I know people usually use CD-942c polypropylene metal foil caps..... but the above bank is all I have right now.....is this going to be /OK/?
EDIT: How many ohms should I use for the bleeder resistors on this completely parallel bank? I see people with mmcs usualy use 10M per capacitor. So if I were to follow, 10x11 = 110M Ohms of resistance on my bank....
I have 2 high quality 100M tiger resistors I could use..... now bleed time ~ R*C*5 so if I have 100M Ohms = 100,000,000 * 0.00000001 * 5 = 5 seconds to be near 0 if on a full charge (very unlikely).....if I have 200M, then bleed time would be around 10 seconds........If I were to use both in parellel then the resistance (I believe) would drop to 50M = 2.5 sec bleed.....With the spark gap firing at a perfect ~120BPS, (charge for every 1/2 60hz cycle) then a 2.5 sec bleed may be to fast, and 100M is probably the right choice..... and 17000^2 / 100,000,000 = 2.89 Watts which I believe this one tiger resister can handle.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The fact that their voltage rating is specified as DC makes me feel bad straight away. Proper RF power capacitors are specified with an AC rating of some kind too, such as RMS current or kVAr.
Registered Member #488
Joined: Tue Oct 24 2006, 02:19AM
Location: Maine
Posts: 8
They have a printed stamp on them that says 20kv AC, I specifically asked the guy for the AC stamped caps before purchase, I should have mentioned that....they still have a 50kvdc rating......
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
Well, your peak charging voltage could easily hit around 17kV, that means the ring down will be 34kvAC peak to peak, which is within the 50kVDC rating (a good thing). But, I personally would save the caps for marx generator use (if that interests you), and buy the popular film caps for the TC.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I can't imagine that capacitor bank having too much stress with a 12kV/30mA NST unless you operate (into an extremely small inductance) / (at several MHz)
These are (I think) class II capacitors which are mainly for dc filtering and ac coupling.
The temperature coeficient N4700 means -4700 ppm/degree = -0.47% per degree so as the capacitors warm up by say 20 C the capacitance will change by -9.4% which will de-tune your TC, not so good. The capacitors will degrade with use but by hobby standards it should be ok.
So I think that capacitor bank will work ok for your NST powered SGTC with occasional re-tuning. (keep Fres below 1 MHz) Certainly they should give reasonable performance (better than typical bottle-capacitors) So I say GO FOR IT
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My cap Bank...will this work ok? (new question about bleed resistors)
