ELIPPSE
January 11th 05, 02:17 AM
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!
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!