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Re: What caused capacitor failure?
Original poster: "Terry Fritz" <twftesla-at-qwest-dot-net>
Hi Brett,
At 04:34 AM 10/22/2001 +0000, you wrote:
>Thanks to all on this list. You've been tremendously helpful, and this
>evening I finally got some sparkage as the fruits of my labor. Worked
>great, and even my wife was awed. She finally understood why all the
>effort.
>
>Unfortunately, I've already had a capacitor failure. Seemingly relevant
>specs of the coil:
>
>12kV 60mA NST
>MMC Built almost identically to the one Terry describes:
>http://users.better-dot-org/tfritz/site/MMCinfo/MMC.html
>
>I used the identical capacitors described ordered from DigiKey. However, I
>only used eight in series as I only needed to support 12kV. I built three
>strings, but only used two of them in parallel for a total capacitance of
>.014 microFarads (the transformer matches to .013 microFarads).
Those caps can take substantial over voltage with no problems. If MMC caps
blow "instantly", it is surely from over voltage. If they have a while to
"cook", it is from over heating due to too much current.
>
>I have a RQ-style cylinder spark gap with 10 .28 inch gaps in series.
>
>She ran beautifully for probably 10 minutes of total run time during the
>course of several hours (15-second or so at a time). However, shortly after
>lighting her up the last time one of the capacitors split and caught fire.
>I immediately shut down the coil. When I examined the capacitor bank more
>closely I found that ALL the capacitors have suffered stress. They have all
>at least cracked their outer coverings, many of them have completely broken
>away chunks exposing the foil inside. Unfortunately, I didn't check the
>caps as I went to see if they were gradually failing, or if this was an all
>at once, catastrophic thing. But based on how they look I would guess they
>were gradually failing.
If the caps arc internally, they often crack the case especially if they
are hot.
>
>Right after failure I checked the other caps and they were not hot... I
>don't think the failure was from heat break down. Instead, from the look of
>the other capacitors, it looks like they actually sort of blew themselves
>up.
>
>Any ideas what would cause this? My first guess would be too much voltage
>and they really did blow themselves up. However, the caps are rated at 1600
>VDC each, so with 8 in series I should be able to handle 12.8 kV.
The peak voltage is 12000 x 1.414 = 17000 volts. 8 caps per sting is
pushing it but still within the area where they should work well. If you
system resonated the voltage too high, you may have reached their limit.
They will start to fail at about 3000 volts peak each. Maybe your NST
voltage got up to around 25kV peak. Since your cap size is right near the
resonant size, this is very possible if you are not using safety gaps.
However, it would indicate that your NST is a good strong one and maybe
the MMCs saved it from destruction (the hard way ;-)). When you use MMC
caps at their limit, they do tend to act like safety gaps...
Bust apart one of the blown caps and unravel the foil roll. If the layers
are filled with little blow holes, it is too much voltage and you system is
resonating the voltage way too high. If they are all melted together in
sort of a solid lump in the middle, then too much current overheated them.
Running the numbers....
MMC Calculator Ver. 2.2 9/12/2000 Terry Fritz
Transformer voltage = 12000
Transformer current = .06
Firing voltage = 16970.52
Fo = 200000
Break rate = 200
Thermal dissipation constant = 40
Individual cap value = 5.6E-08
Strings Caps/Str Capacitance Voltage Temp C Cost I Arms
1 4 14.00 6400 51.42 7.20 :-(( :-(( 7.05
2 8 14.00 12800 12.86 28.80 :-(( :-| 7.05
3 12 14.00 19200 5.71 64.80 :-)) :-) 7.05
--------------------------------------------------------------------
4 16 14.00 25600 3.21 115.20 :-)) :-)) 7.05
5 20 14.00 32000 2.06 180.00 :-)) :-)) 7.05
So you are seeing 7 amps in the primary (rms). That will not fail the caps
as you describe although it is over the recommended limit. Since there is
room in the voltage too, it "should" have worked.
So I am pretty sure your primary circuit went well above 12kV due to
resonant rise. The caps plates are probably filled with little holes
inside and maybe melted as well...
>
>How can I prevent this problem in the future? Needless to say, that was an
>awfully expensive 10 minutes (although immensely satisfying). I'd like to
>figure out what I can do to prevent this failure before trying it again.
The Geek groups caps are about 1/3 the cost (or less if you bought direct
from DK) of the Panasonic caps. The Geek caps are a far better buy these days.
You should get the program to design MMCs or one of it's equivalents:
http://hot-streamer-dot-com/TeslaCoils/programs/Mmccalc2.zip
It will insure you voltage and current is ok. However, in this case, it
looks like serious resonant rise was the problem. "I" would go to a static
gap LTR cap size of 18nF. The larger cap will not ring up to high
voltages. I would also get a safety gap going to add further protection.
http://hot-streamer-dot-com/TeslaCoils/Misc/NSTStudy/NSTStudy.htm
This filter is nice too :-)
http://hot-streamer-dot-com/TeslaCoils/Misc/NSTFilt.jpg
So if you use the Panasonic caps you get:
MMC Calculator Ver. 2.2 9/12/2000 Terry Fritz
Transformer voltage = 12000
Transformer current = .06
Firing voltage = 16970.52
Fo = 200000
Break rate = 120
Thermal dissipation constant = 40
Individual cap value = 5.6E-08
Strings Caps/Str Capacitance Voltage Temp C Cost I Arms
1 3 18.67 4800 54.85 5.40 :-(( :-(( 7.28
2 6 18.67 9600 13.71 21.60 :-(( :-| 7.28
3 9 18.67 14400 6.09 48.60 :-| :-) 7.28
4 12 18.67 19200 3.43 86.40 :-)) :-)) 7.28
--------------------------------------------------------------------
4 13 17.23 20800 2.92 93.60 :-)) :-)) 6.72
5 15 18.67 24000 2.19 135.00 :-)) :-)) 7.28
5 16 17.50 25600 1.93 144.00 :-)) :-)) 6.82
6 18 18.67 28800 1.52 194.40 :-)) :-)) 7.28
6 19 17.68 30400 1.37 205.20 :-)) :-)) 6.9
So four strings of 12 would be nice but three strings of nine would work.
With the Geek caps:
MMC Calculator Ver. 2.2 9/12/2000 Terry Fritz
Transformer voltage = 12000
Transformer current = .06
Firing voltage = 16970.52
Fo = 200000
Break rate = 120
Thermal dissipation constant = 22
Individual cap value = 1.5E-07
Strings Caps/Str Capacitance Voltage Temp C Cost I Arms
1 8 18.75 16000 4.28 14.40 :-| :-)) 4.48
2 16 18.75 32000 1.07 57.60 :-)) :-)) 4.48
--------------------------------------------------------------------
2 17 17.65 34000 0.95 61.20 :-)) :-)) 4.22
So a 2 x 8 or even a 1 x 8 array would work there.
Or, just use this real nice chart the Geeks have:
http://hot-streamer-dot-com/temp/MMCcapSales.gif
They say only 8 caps are needed which is only $20.
http://www.thegeekgroup-dot-org/Sales/MMCsales.html
You may want to double check my numbers here since I am going real fast but
hopefully this is of help...
Sorry your cap blew up, but if Tesla coils were easy, everyone would make
them. It's just part of the challenge ;-))
Cheers,
Terry
>
>Thanks in advance for your help.
>
>Brett
>