APPENDIX A
Curtis 1232E/34E/36E/38E & 1232SE/34SE/36SE/38SE Manual, os 31 – May 2017 Return to TOC
pg. 142
Conducted paths are created by the wires connected to the controller. ese wires act as antennas and
the amount of RF energy coupled into them is generally proportional to their length. e RF voltages
and currents induced in each wire are applied to the controller pin to which the wire is connected.
Curtis controllers include bypass capacitors on the printed circuit board’s throttle wires to reduce the
impact of this RF energy on the internal circuitry. In some applications, additional ltering in the
form of ferrite beads may also be required on various wires to achieve desired performance levels.
Radiated paths are created when the controller circuitry is immersed in an external eld. is
coupling can be reduced by placing the controller as far as possible from the noise source or by
enclosing the controller in a metal box. Some Curtis controllers are enclosed by a heatsink that also
provides shielding around the controller circuitry, while others are partially shielded or unshielded.
In some applications, the vehicle designer will need to mount the controller within a shielded box on
the end product. e box can be constructed of just about any metal, although steel and aluminum
are most commonly used.
Most coated plastics do not provide good shielding because the coatings are not true metals, but
rather a mixture of small metal particles in a non-conductive binder. ese relatively isolated particles
may appear to be good based on a dc resistance measurement but do not provide adequate electron
mobility to yield good shielding eectiveness. Electroless plating of plastic will yield a true metal and
can thus be eective as an RF shield, but it is usually more expensive than the coatings.
A contiguous metal enclosure without any holes or seams, known as a Faraday cage, provides the
best shielding for the given material and frequency. When a hole or holes are added, RF currents
owing on the outside surface of the shield must take a longer path to get around the hole than if
the surface was contiguous. As more “bending” is required of these currents, more energy is coupled
to the inside surface, and thus the shielding eectiveness is reduced. e reduction in shielding is a
function of the longest linear dimension of a hole rather than the area. is concept is oen applied
where ventilation is necessary, in which case many small holes are preferable to a few larger ones.
Applying this same concept to seams or joints between adjacent pieces or segments of a shielded
enclosure, it is important to minimize the open length of these seams. Seam length is the distance
between points where good ohmic contact is made. is contact can be provided by solder, welds, or
pressure contact. If pressure contact is used, attention must be paid to the corrosion characteristics
of the shield material and any corrosion-resistant processes applied to the base material. If the ohmic
contact itself is not continuous, the shielding eectiveness can be maximized by making the joints
between adjacent pieces overlapping rather than abutted.
e shielding eectiveness of an enclosure is further reduced when a wire passes through a hole in
the enclosure; RF energy on the wire from an external eld is re-radiated into the interior of the
enclosure. is coupling mechanism can be reduced by ltering the wire where it passes through
the shield boundary.
Given the safety considerations involved in connecting electrical components to the chassis or
frame in battery powered vehicles, such ltering will usually consist of a series inductor (or ferrite
bead) rather than a shunt capacitor. If a capacitor is used, it must have a voltage rating and leakage
characteristics that will allow the end product to meet applicable safety regulations.
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