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Plastic Capacitors
From: Mark Finnis <mefinnis-at-medicine.adelaide.edu.au>
Have built my first two caps with rolled polyethylene/alum flashing and
have been quite happy.
Wandering through wire shop the other day buying some 1mm (19SWG) for my
new former, when I noticed some rolled sheets on the shelf....
This stuff was "slot insulation", various sizes & components, eg.
5/5/5 mil each of nomex/mylar/nomex
nomex is actually polythene saturated & bonded each side
effectively 7 layers bonded together, stable to 250 degrees C !!!
rated at 17 kVAC !!!
This would match stated figure for mylar from list at 7,500 kV / mil
puncture VDC.
Thought ...... this would make a *great* cap ....... alas, $30 / m (very
dear).
Looking at the plastics chart for puncture V versus dielectric K, mylar
looks very attractive:
Polyethylene Terphthalate (Mylar) K = 3.0 - 3.1 Vp = 7500 V
Available in rolls at about 2x cost of LDPE. But remember we get 50%
greater K and only have to use a couple of layers (5 mil), so gain even
more in the total C stakes. Actually doing the sums, this is barely more
expensive than LDPE, and is much higher grade / purity.
OK, so why don't we use this stuff ????
Anyone tried it ??
I am contemplating when my current caps die (I am a realist, really) to
re-roll them with mylar.
Basic assumptions:
1. For LDPE we use K = 2.0, not 2.2 to account for losses via "home-made"
OK, lets take mylar K = 2.7 (less 10%)
2. Reported puncture V for LDPE varies from 400-1200 V/mil. We use lowest
figure as "experience" shows TC use hard on dielectric.
OK, lets take Vp for mylar = 5000V
Any comments welcomed ;-)
___________________________________________________________
Mark Finnis Hm: 61 8 82895205
Staff Specialist Wk: 61 8 82224000
Intensive Care Unit Fax: 61 8 82236340
Royal Adelaide Hospital Mbl: 041 23242268
www.health.adelaide.edu.au/icu
___________________________________________________________