Re: Barium Titanite caps

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-uswest-dot-net>" <bert.hickman-at-aquila-dot-net>

Jeff,

There's simply no single type of capacitor that's superior to others...
capacitors must be matched to their target application. In a Tesla Coil,
the high voltages, high RF current, and large voltage reversals place very
heavy demands on the tank capacitors - Tesla Coils can be real "cap
killers"! Although ceramic (Barium Titanate) caps will indeed work in this
application, they are not necessarily superior to true low loss pulse
capacitors
using polypropylene or polyethylene dielectric systems. 

Simply stated, the main advantages of ceramic caps is simply that they pack
a large amount of capacitance into a small volume, and they are somewhat
more available than true pulse caps. They are not necessarily any cheaper.
When
properly made (i.e., with thick metallization), they can pass significant
RF current. While these caps can sometimes be found at reasonable prices in
the surplus market, at Ham Fests, or on Ebay, new HV ceramic caps are high
priced, and exorbitant if they are pulse rated. 

A typical ceramic's capacitance tends to vary as a function of both voltage
and temperature - these properties come with the high-k ferroelectric
dielectric
material. This voltage sensitivity tends to "smear" the sharpness of the
frequency response of the primary circuit. And, the low voltage "instrument
tuning" point may be significantly different than the tuning point at high
voltages - at least one coiler has recently mentioned this effect without
identifying the underlying cause. And, unless the caps are rated for high
current pulse duty, they will have relatively thin evaporated silver
plates. During Tesla Coil use, many coilers have found that this thin metal
layer slowly disappears (evaporates?), reducing their effective capacitance
over time. BTW, you can often see this with ceramic capacitors that were
pulled from pulsed excimer laser systems - their stamped values are often
10-25% higher than their actual values. These problems do not occur with
polypropylene pulse caps or MMC's.

Now, if you can find a batch of doorknob or hockey puck caps at a
reasonable price, you can indeed make them work in Tesla Coil service. As
an example, a bank of 36 2200 pF at 40 kV ceramic caps previously used in a
Tesla Coil just sold 2 days ago on Ebay for $255.00 (or about $7/cap)- see
http://cgi.ebay-dot-com/aw-cgi/eBayISAPI.dll?ViewItem&item=1227513715

Now this was not a bad price for these type caps - however, if you look
carefully, you can often find surplus HV polypropylene pulse caps for
significantly less, and you'll tend to get better long term coil
performance. For example, surplus 0.03 uF 35 kV Maxwell caps are available
for $85 or four for $300 at:
http://www.teslasystems-dot-com/products/index.html
An identical Maxwell cap just recently sold on Ebay last week for $114, and
All Electronics used to sell these on Ebay as well. 

Finally, you can often build an MMC that performs just as well (in a Tesla
Coil) as a Maxwell cap for significantly less, especially for a small to
medium sized coil. And it's adjustable and repairable. It would be my first
choice for a beginning coiler.

Hope this helped remove some mystery...

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com

Tesla list wrote:
> 
> Original poster: "cougercat by way of Terry Fritz <twftesla-at-uswest-dot-net>"
<felix1063-at-home-dot-com>
> 
> What are the characteristics of Barium Titanate capacitors that make them
> superior to other capacitors that can be found out there today?
> 
> --jeff