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Rolled cap margins?



To take into account that:

1. It is better to use 2 caps in series, rating each of them e.g. 10 kV DC,
instead of trying to built a single one for 20 kV DC. Thus, it is not worth
using 90 mils of dielectric if you are not going to stress the cap with more
than 10 kV DC (e.g. you can use an external safety gap on the cap).

2. Two dielectric layers are better than only one.

I have (again) modified my tentative capacitor design as follows:

- paper    INNER LAYER
- Cu
- paper
- 0.2 mm LDPE
- paper
- 0.8 mm HDPE
- paper    
- Cu
- paper
- 0.2 mm LDPE
- paper
- 0.8 mm HDPE   OUTER LAYER

Paper is 0.05 mm (40g/m2) kraft paper not waxed.
HDPE is 0.8 mm (31 mil) HDPE 
LDPE is 0.2 mm (7.9 mil) LDPE 
Cu is 0.1 mm copper sheet
capacitor is filled with oil

If I rate this capacitor as a 10 kV DC device, how much wide is the margin
I have to leave on the electrode sheet sides? 1", 1.5" or how much?

Please, help me with this decision, because I wouldn't like to leave too much
space if it is not needed: that will reduce the capacitance very quickly :(



________________________________________________________________________

 Marco Denicolai                   Vista Communication Instruments, Inc.
 Hardware Development Manager      www.vistacom.fi   

 marco-at-vistacom.fi                 Kaisaniemenkatu 13 A
 fax:    +358-9-622-5610           SF-00100 HELSINKI
 phone:  +358-9-622-623-15         Finland

   Remember, Murphy was an optimist! I am not...
________________________________________________________________________