[TCML] Capacitor Questions

[Bert Hickman]

Bob Deuel wrote:
> Hello:
> 
> I have questions regarding a couple capacitors that I have:
> 
> 1) The first capacitor is a Sprague type 45P5 "Vitamin Q" rated at
> 0.012 mfd. @ 25,000 VDC. It physically measures approximately
> 2.5-inches in diameter and 6.5 inches long with end caps threaded for
> 1/4-20 thread. It is an oil filled capacitor that has a glass
> cylinder with metal end caps. I understand that some capacitor
> dielectrics do not take kindly the rigors of Tesla Coil operation and
> overheat. I wondering if anyone has used a capacitor of this type?
> This one would use for a future coil if it is acceptable.
> 
> 2) I plan on building a table top coil using a 5kv @ 20 ma. NST soon.
> I keep looking at a capacitor that is on my bench that I believe was
> from an older IBM computer monitor. It is made by ERO and is
> 9000pF+80-20%/20KV. It actually measures 10,300 pF which is great.
> Physically, it is potted unit in a 1.5x1.75x2.75 plastic case with a
> insulated HV lead and a "ground" lead. Are capacitors like these
> acceptable for a small coil or should I forget it and go with a MMC?
> 
> I would appreciate your input on the feasibility of these two
> capacitors.
> 
> Thanks, Bob Deuel, K2GLO 
> _______________________________________________ Tesla mailing list 
> Tesla at www.pupman.com http://www.pupman.com/mailman/listinfo/tesla
> 

Hi Bob,

The first capacitor uses a paper-mineral oil dielectric system - all of 
Sprague's vintage "Vitamin Q" capacitors used this older dielectric 
system. Vitamin Q caps were designed for use within DC filtering and 
audio frequency coupling in vacuum tube circuits. Although paper-oil 
capacitors can be used for short runs in Tesla Coils, the lossy 
dielectric does heat up from the RF. The unique glass package style for 
your cap sounds quite similar to "Glassmike" caps with larger diameter 
and bigger end studs. The problem with cylindrical oil-filled capacitors 
in general is that they are very unforgiving when subjected to internal 
gas pressure. Internal gases can build as corona begins attacks the 
dielectric fluid along the capacitor plate edges during high voltage/RF 
stress. This pressure may cause the case to rupture, sometimes 
violently, and usually with no warning. If you do try this capacitor in 
a Tesla Coil tank circuit, you may want to enclose it within a suitable 
container to capture glass fragments and mineral oil in case it does 
decide to catastrophically fail.

Based on the small size and wide tolerance, the second cap is almost 
certainly a DC-rated ceramic capacitor. There are various types of 
ceramic high voltage capacitors, and some are much better suited for 
Tesla Coil/RF use than yours. High k barium titanate blends (Z5U or Y5U) 
are used for DC/filtering applications. However, these heat up and 
change capacitance value when subjected to RF. Also, high tank circuit 
currents have been known to gradually remove small pieces of the thin 
evaporated metal from the ceramic, reducing the capacitance value and, 
eventually, destroying the caps. A somewhat better choice would be 
strontium titanate ceramic HV caps that are "pulse rated". These use a 
lower loss dielectric blend with beefier termination and metalization to 
withstand high discharge currents. Examples of the latter are N4700 
series HV ceramic caps, such as those from Murata, TDK, AVX, etc. 
However, these tend to be rather pricey, even on eBay.

In short, I'd recommend making an MMC...  :^)

Bert
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