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2.Power
Dissipation
The power rating for a given element is based
on the surface area of the resistive film. The
Duncan 98E is rated at 3 watts per square inch
(4.65 milliwatts per square millimeter). This
is a conservative rating based on operation in
free air at 60°C, derating to zero watts at 125°C.
With some high temperature substrates the derating
curve can be extended to 150°C. If properly heat-sinked,
the rating can be considerably increased although
it is not recommended to use this type of element
for high power applications that would necessitate
operating at an elevated temperature.
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3. Linearity
Conformity of the voltage transfer function to
a given law (linear, square, trigonometric, etc.)
is dependent somewhat on film geometry and size.
Element lengths of one to ten inches normally
display a linearity of ±2% or better. Non-linear
law elements and extremely long or short elements
tend to be proportionately higher. The longest
active length that may be economically produced
is roughly 20 inches (500mm). Lower function conformity
tolerances can be achieved by means of a proprietary
film-trimming process. Duncan 98E elements have
been fabricated with linearity tolerances below
±.025% using this method, however, additional
processing means additional cost and the wise
system designer will specify a realistic linearity
requirement to avoid unnecessary expense.
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4.
Contacts
Proper contact design greatly affects a number
of performance characteristics in a conductive
plastic potentiometer. The quantity of contact
points directly determines the current carrying
capacity (ampacity) of the contact, the apparent
contact resistance, and noise or output smoothness
to a lesser degree. It is, therefore, common practice
in thick-film devices like the 98E to use "multi-finger"
contact configurations consisting of a group of
aligned contact springs each yielding a discrete
point of electrical continuity on the carbon film.
Spring force is an additional factor involved.
Inadequate contact force will result in poor contact
resistance and output smoothness characteristics,
while too much force will cause excessive contact
and film wear, resulting in a decreased useful
life. To achieve a happy medium, a contact force
of 3 to 5 grams per contact is generally recommended,
although some designers have specified as much
as 15 grams per contact in cases where contact
resistance was of primary concern. The material
used in fabricating a contact is also important.
For best results, a noble metal alloy is optimal,
such as Neyoro 28A or Paliney 6 (both proprietary
alloys of the J.M. Ney Corp.). Although somewhat
costly, a precious metal contact alloy will definitely
yield the best electrical and life performance.
As a viable alternative, there are several base
metal alloys which are commercially available
at a lower cost, if the designer can compromise
life and other characteristics for the sake of
economy. Many nickel-chromium and nickel-tin alloys
are suitable for this use in short-lived commercial
applications. Matched contact assemblies are available
through Duncan to complement the 98E design you
require.
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5.Terminations
The screen printed terminations normally used
on the Duncan 98E element are not solderable.
The common method of termination using solder
techniques is to fabricate the element with terminal
lugs, eyelets, or solderable lead-wires. In many
applications, a spring-pressure termination is
used in lieu of soldering. If a true "solder
pad" configuration is a necessity, it can
be provided by using a plated and etched copper-clad
substrate similar to a printed circuit board.
It should be noted, however, that the additional
etching and plating operations are an extra expense
in producing an element. The termination for the
moving contact can also be provided on the element
by means of an additional strip of low resistance
conductive plastic film acting as a slip ring
or adjacent "pick-off track," in conjunction
with a dual contact arrangement.
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6. Substrates
The Duncan 98E can be provided on a glass-laminate
board, thermoset plastic base, and some high temperature
thermo-plastic substrates. Substrate configurations
may be either rigid or flexible, and mounted by
means of adhesives or mechanical fasteners. It
is not recommended that the element be cemented
or laminated to a secondary rigid baseplate as
this adds costly assembly steps to the fabrication
process. A single glass-laminate sheet is the
most economical approach to substrate selection,
especially in small production quantities. If
the element is to be mass produced, then tooling
is more easily justified, and the substrate may
be a molded plastic configuration of any size,
shape and thickness desired. A typical element
would be screen printed on .01 to .06 inch thick
glass-laminate board. The board may be glass-epoxy,
although glass-polymide is very popular due to
its stable temperature characteristics and is
recommended by Duncan.
The Duncan 98E element has been used in airborne,
medical instrument, automotive and commercial
appliance applications. It is a unique device
by virtue of its cost effectiveness in both small
quantity usage and mass production. Call you local
Duncan representative for quotations and technical
assistance on your special potentiometer requirements.
You may be surprised to find out that custom doesn't
necessarily mean expensive!
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is the
active film rotation angle expressed in radians,
and the denominator is the natural logarithm of
the ratio of the film outer radius
and the inner radius
expressed
in constant units. The economic range of resistivity
for the Duncan 98E conductive plastic is approximately
from 250 to 10,000 ohms-per-square. This range
may be extended somewhat as necessity demands,
however, special process techniques are required
to achieve abnormal resistivity levels, with a
corresponding impact on cost. 