Hi David,It ultimately depends on the dielectric system and degree of dielectric stress that Maxwell applied in the design of the specific cap. Unfortunately, Maxwell does not normally provide details about the dielectric material or construction on their spec sheets. Remember that Maxwell designed and developed most of their capacitors to meet specific customer requirements. Most of their higher-voltage capacitors in the 31xxx and 35xxx series were not originally designed for high power RF use, but for Pulse Forming Networks (PFN's) or Marx/impulse generators.
Some of their caps were designed for moderate to high rep-rate PFN's in pulsed laser systems. Caps for these applications used a PP-oil dielectric system to reduce internal dielectric losses and heating. The static dielectric constant of PP is only about 2/3 that of Mylar, so PP caps are physically larger for the same capacity and voltage rating - Mylar caps have a higher energy density than PP caps. In those cases where space was not a primary customer consideration, Maxwell could use a low-loss polypropylene dielectric system even for 1 PPS applications. For tighter space/higher energy density needs, Mylar-oil was used. Some 1 PPS caps in either series use PP, and these make excellent TC caps. Those caps using Mylar... not so much. In a high-power TC, internal heating may destroy a Mylar cap before the case barely gets warm.
Although which caps use PP can sometimes be gleaned from Maxwell's spec sheet (i.e., low % dissipation and 10e6-10e9 shot expected lifetime), only Maxwell knows for sure. Lower shot life caps use a more highly-stressed dielectric system, hence the shorter expected lifetime. I'd recommend staying away from any caps spec'ed with less than 10e5 shot lifetime. Also, stay away from any surplus cap with a bulged case, since corona has begun to damage the dielectric system, causing a buildup of gas pressure.
For Maxwell caps that may appear on eBay or the surplus market, YMMV. A call to GAEP capacitor engineers is advisable before making a large investment in what may actually be a lossy Mylar cap.
Unfortunately, I don't have the spec sheet for the 31885. However, based upon the lack of capacitor heating during extended runs in the Green Monster, these must use a PP-oil dielectric system. A Mylar cap would have overheated and failed in short order... :)
Best wishes, Bert David Rieben wrote:
Bert, Thanks for the info. I reviewed what you and David Speck said in answer to Tim's inquiry about the 31xxx series GA/Maxwells that are only rated at 1 PPS and it seems that you are suggesting that most of these are not very suitable for Tesla use, and I would tend to mostly agree. However, I have acquired (4) each used surplus 31885 cat # .1 uFd, 75 kV rated Maxwells (4"x 6"x 19" single ended plastic case) several years ago, via eBay (one is currently being used as the main tank cap for my Green Monster coil - the other 3 are "spares"), and this cap seems darn-near indestructable for running my coil at 300 to 400 PPS with 14,400 volts from an overdriven 10 kVA rated pig, that's typically processing about 75 amps with about 200 to 220 volts input from the control variacs. I have never been able to detect any significant warmth after even a several minute run under these conditions. And I have come back a few minutes later to feel and see if some of the internal "warmth" finally reached the surface of the outer casing. Have you been able to gather any info about these 31885 cat # Maxwells? Thanks,David On Wednesday, October 21, 2015 6:07 PM, Bert Hickman <bert@xxxxxxxxxxxxxxxxxxxxx> wrote: Hi Dave, It's a mixed bag with the 35xxx series as well, and many caps in this series are excellent for TC use. The Maxwell spec sheets for many in this series have NO stated PPS rating. For these, the Max. Dissipation Factor (MDF) or Design Life (# of charge discharge cycles, with 90% survival) can provide some useful hints as to their suitability for TC use. Mylar caps will have a higher factor (0.5% or higher) while polypropylene typically run 0.1 - 0.3% at 120 Hz. Design life for caps rated for high PPS will also have a long Design Life (millions or even billions of shots). For example, the popular 35174 (0.06 uF at 40 kV) is rated for 100 PPS, has a an MDF of 0.3%, and a design life of 100 million shots - very suitable. A similarly packaged 35282 (0.06 uF at 50 kV) is rated for 1 PPS, has an MDF of 0.5%, and a shot life of only 100,000. Some caps in this series have shot lifetimes of only 30,000, and you never know how many are left on YOUR surplus cap. Over the years I've accumulated a number of spec sheets, so I may be able to provide you with more information if you can provide me with series numbers. However, in all cases, a call to the GAEP capacitor engineers is really the best bet. Best wishes, Bert
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