Cables for LightSpeed Plasma ignition.

A while back someone recommended that user's of Plasma ignitions not
use the recommended RG 400 but instead use shielded wire. Klaus
contacted me, the designer, for my comments about this substitution.
Coaxial cable is used for several reasons: it is rugged, it has a
high voltage rating, it has coax connectors designed for its use, and
it has well-defined electrical characteristics, foremost among these is
the capacitance per foot. RG58, no longer recommended, is 28pf/ft and
RG400 is 29.3pf/ft. Shielded wire, such as the high-rel,
aerospace-rated piece I have in my hand, has 121pf/ft, over 4 times as
much! This capacitive loading on the coil drivers has several effects:
1. it slows down the rate-of-rise of the voltage to the coil, lowering
the spark voltage; 2. It imposes a much higher loading on the driver
transistors, which could lead to output transistor failures; 3. It
increases the unit's overall current drain; 4. the higher current drain
raises the power dissipation of the unit, making it run hotter.
In addition, the voltage rating of the shielded wire was never
specified. These output lines to the coils have over 800V P-P!
250V-rated shielded wire could eventually break-down, causing ignition
failures in flight!
Klaus has had several people who have reported a rough-running
engine. When questioned by Klaus it was found that they used the
shielded wires in making the connections to the coils. After changing
to the RG400, the problem went away. I would seriously recommend that
anyone who has used this shielded wire change back to the recommended
cable!

I have a dual plasma II on my experimental. It has RG58 that came with
the ignition kit. Are you saying I should change to RG400?
My engine (Franklin) runs rough if I turn off one of the ignition
sides.
Would that help? It seems OK if I have both ignitions on?
Franklins are known to drop by 250 rpm with one side off.

-----------------------------------------
SQ2000 canard: http://www.abri.com/sq2000

ELIPPSE wrote:
A while back someone recommended that user's of Plasma ignitions not
use the recommended RG 400 but instead use shielded wire. Klaus
contacted me, the designer, for my comments about this substitution.
Coaxial cable is used for several reasons: it is rugged, it has a
high voltage rating, it has coax connectors designed for its use, and
it has well-defined electrical characteristics, foremost among these
is
the capacitance per foot. RG58, no longer recommended, is 28pf/ft and
RG400 is 29.3pf/ft. Shielded wire, such as the high-rel,
....................
Klaus has had several people who have reported a rough-running
engine. When questioned by Klaus it was found that they used the
shielded wires in making the connections to the coils. After changing
to the RG400, the problem went away. I would seriously recommend that
anyone who has used this shielded wire change back to the recommended
cable!

Hi, Ignord!
I use RG58U on the dual Plasma I's on my Lancair. Some of the RG58,
under heat with a short radius bend, had the center conductor
occasionally shorting to the braid. This was more of a problem with
foam dielectric rather than polyethelene. The RG400 has a much higher
temperature range. If you have the foam dielectric cable and have some
tight bends you might consider either straightening out the cable more
or, if that's not possible, replacing with the RG400. Of course, your
problem may have another source, so you might want to do a little
substitution/swapping trouble-shooting . Paul