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Registered Member #1083
Joined: Mon Oct 29 2007, 06:16PM
Location: Upland, California
Posts: 256
I am considering buying one of these for my medium size tesla coil: How many volts AC would a capacitor rated for 35kV DC stand? Also, has anyone had any experience with these in an mmc? They're digikey part BC2000-ND. Do they look like they would work? They seem similar to these but are about 1/2 as expensive. What are the advantages/disadvantages of using a single heavy duty cap instead of an mmc? I will be using a 12kV 30ma nst as the power.
Registered Member #1103
Joined: Mon Nov 05 2007, 06:02PM
Location: Houston
Posts: 80
I can't answer your other questions, but let me say this.
wrote ... What are the advantages/disadvantages of using a single heavy duty cap instead of an mmc?
If you loose a cap or two in an MMC one can easily replace the blown cap and be back in service, however with a single capacitor one cannot easily replace it. I see this as a major limitation.
For 95 dollars you should be able to build a decent MMC.
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
Another way of looking at the single capacitor is that film foil caps (depending on construction) have a limit of how many shots they will handle over their lifetime, say 10,000 for instance. For a SGTC 10,000 shots goes by real fast even at 120 BPS.
I have a bit of a hard time explaining this, so bare with me.
12kv NST X1.414 = 17kv peak
The ringdown of the tank will oscillate between +17kv and -17kv, this is 100% reversal rating and you will add the two magnitudes together for 34kv. This is probably way too close to 35kv rating.
I would go for a 32kv rating maximum, which gives you peaks of 16kv for each ringdown half cycle, and 16kv * .707 = 11KVAC on your NST for safety margin.
Registered Member #690
Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616
I'm wondering how I ever managed to run an SGTC based on a 15kV/30mA NST with an MMC only rated for 22.4kVDC...
Its made of these:
It still works to this day, and the arrangement is 7 parallel strings of 14 in series for a total of 98 capacitors. If you order 2 more you reach the 100 piece price break and get all of them for $40.
The spark length with air blown static gap was about 30 inches before the secondary burned.
I guess these caps are made with a good amount of safety margin in their voltage rating...
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
ShawnLG wrote ...
Those Maxwell caps are only rated for 15% voltage reversal. They will eventually die in TC duty.
Not necessarily...
The name of the game is derating.
35kV and 15% reversal coresponds to 35+(35*0.15) = 40kV
A tesla system has, say, 90% reversal, so with 40kV to do good with, the peak charging voltage cold be as high as 40/1.9 = 21kV. You can call it a 21kV cap, rated at 90% revesal.
A transformer rated at 21/1.41 = 14,9kV would then allow you to utilize the full potential of the cap, and as such, it is the perfect match for the odd neon transformer. But you will only have the safety margin that`s designed into the cap itself.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Finn Hammer wrote ...
ShawnLG wrote ...
Those Maxwell caps are only rated for 15% voltage reversal. They will eventually die in TC duty.
Not necessarily...
The name of the game is derating.
35kV and 15% reversal coresponds to 35+(35*0.15) = 40kV
A tesla system has, say, 90% reversal, so with 40kV to do good with, the peak charging voltage cold be as high as 40/1.9 = 21kV. You can call it a 21kV cap, rated at 90% revesal.
A transformer rated at 21/1.41 = 14,9kV would then allow you to utilize the full potential of the cap, and as such, it is the perfect match for the odd neon transformer. But you will only have the safety margin that`s designed into the cap itself.
Cheers, Finn Hammer
Can someone explain me rightly what voltage reversal really is?
I thought is was the maximum percentage the voltage can ring under zero after cap was charged to it's rated voltage?
I still don't understand what significance this has, how in a world can this cause the cap to fail.
Only explanation I got is that high voltage reversal would cause high mechanical stresses to dielectric. (how can that be a problem?)
What does this have to do with the AC voltage rating of the cap? Why are AC ratings always so much lower than DC ratings?
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
It's all to do with charge trapped inside the dielectric of the capacitor I think Marko. This trapped charge causes additional stress on the dielectric when the plate potential reverses.
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MMC versus a single Maxwell
