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Re: [TCML] Capacitor dielectric examples and Tesla coil equations

Paul,
Capacitors can be lossy in various ways. The dielectric itself can heat up when subjected to RF voltages, such as with Mylar. However the capacitor may also have excessive equivalent series resistance (ESR) for other reasons. High ESR can occur when the metalization layers are very thin (and resistive), the capacitor leads are thin and flimsy (as in many of the eBay Chinese HV caps!), or the interconnection between the flame-sprayed ends to the capacitor plates are poorly made. All of these other problems will contribute to excessive heating, especially when these caps are subjected to the high currents seen in a TC primary circuit.
Polystyrene is a very good, low loss dielectric. In fact, it is one of the best dielectric systems for precision analog circuitry. However, if the internal design and construction of the caps is not robust, polystyrene caps will still overheat in RF or pulsed power applications. Although these caps may work fine for filter and voltage multiplier circuits, I have little doubt that the eBay Chinese HV caps will quickly fail when used in a SGTC. 

Bert
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Paul Brodie wrote:

A recent comment on this list said that the polystyrene caps made in China that are sold on eBay almost all of the time should be avoided for MMC use because they are lossy and will heat up and fail. IIRC, this comment was made by someone who I believe is a very highly regarded TC builder and long time contributor to the TCML.

I'm not trying to start an argument but I would like to know which is correct. Is polystyrene lossy or not?
Paul
Think Positive
----- Original Message ----- From: futuret@xxxxxxx To: tesla@xxxxxxxxxx Sent: Tuesday, December 01, 2009 20:02 
  Subject: Re: [TCML] Capacitor dielectric examples and Tesla coil equations



   Those who may not be familiar with capacitor dielectric materials should be
  aware that many of the materials listed are lossy, and not well suited for
  Tesla coil use.  The best common dielectrics for low loss in TC use are:

     Polypropylene, Polyethylene, Polystyrene, Ruby Mica, some ceramics,
and perhaps a few other materials. 
  Cheers,
  John
------------------- 
  -----Original Message-----
  From: Brian Hall <brianh4242@xxxxxxxxxxx>
  To: tesla@xxxxxxxxxx
  Sent: Tue, Dec 1, 2009 7:37 pm
  Subject: [TCML] Capacitor dielectric examples and Tesla coil equations
  <snip>


      =======================================================================
      INFORMATION FOR BUILDING CAPACITORS:                           Comments
                                                                       |
                                                     Puncture Voltage  |
      Material                   Dielectric strength   per Mil         |
                                        "K"           (0.001 inch)     |
                                         |              |              |
      -----------------------------------------------------------------------
      Miscellaneous
      -------------
      Vacuum                             1.0

      Paper, bond                        3.0            200
      Paper, Royal Grey                  3.0            200
      Paper, telephone, treated          2.5 - 4        200 - 250      *
      Paper, Parafin Coated              2 -3.5
      Paper, Kraft                       2.2

      Oil, Castor                        4.67
      Oil, Mineral, Squibb               2.7            200
      Oil, Mineral                       2.2
      Oil, Transformer                   2.1 - 2.5      75             *

      Rubber                             3.0
      Rubber, Hard                       3.0            160 - 500      *
      Rubber, Vulcanized                 3.2 - 3.9                     1

      Fibre                              5.0 - 7.5      150 - 180
      Fibre, Red                         5.0

      Mica                               4.5 - 8.0      3800 - 5600
      Mica, Ruby                         5.4            3800 - 5600

      Quartz                             3.8 - 5.0      1000
      Quartz (Fused)                     4.2            150 - 200      *

      Shellac                            2.5 - 4.0      200 - 400
      Spar Varnish                       4.8 - 5.5

      Steatite, low loss                 5.8            150 - 315
      Steatites (Magnesium silicate,etc) 5.5 - 7.5      200 - 300

      Cambric (Varnished)                4.0                           2
      Alsimag 196                        5.7
      Gutta Percha                       4.0                           3
      Amber                              3.0 - 7.0                     4
      Resin                              2.48 - 2.57
      Enamel                             5.1            450
      Mycalex                            7.4            250
      Silicone RTV                       3.6            550
      Wood                               2.0 - 5.2
      Wax (Parafin)                      2.1 - 2.5      250 - 450      *
      Beeswax                            2.9 - 3.0
      Slate                              7.0                           5
      Barium titanate(25 C)              1200
      Bariam titanate                    6000
      Titanium dioxide                   125
      Cellulose acetate                  3.3 - 3.9      250 - 600
      Casein, Moulded                    6.4                           6
      Polytetraflourethylene             2.0                           A
      Aluminum oxide                     8.7
      Tantalum pentoxide                 22
Glass 
      -----
      Glass                              4.8 - 10       300
      Plate Glass                        6.8 - 8.4
      Pyrex Glass                        4.8 - 10       335
      Window Glass                       7.6 - 7.8      200 - 250


      Ceramics
      --------
      Cordierite ceramics                5.0 - 5.5      100
      Magnesium titanate ceramic         12 - 18        150
      Porcelain                          5.1 - 7.5      40 - 280
      Titanium dioxide ceramic           70 - 90        100
      Titanium-zirconium dioxide ceramic 40 - 60        150
Plastics 
      --------
      Bakelite                           4.4 - 5.8      300
      Bakelite, Mica filled              4.7            325 - 375
      Epoxy Circuit Board                5.2            700
      Formica                            4.6 - 4.9      450            7
      Nylon (lowest values of 3 types)   3.2            407
      PVC (rigid type)                   2.95           725
      Plexiglass                         2.8            450 - 990
      Polyethylene                       2.2 - 2.3      450 - 1200
      Polycarbonate (Lexan)              2.96           400
      Polyethylene Terphthalate (Mylar)  3.0 - 3.1      7500
      Polystyrene                        2.5 - 2.6      500 - 700
      Teflon                             2.1            1000 - 2000
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