Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to thermal print bars for use,
~or example, in facsimile systems.
In a known type of facsimile system a thermal print bar
- is moved relative to a receptor of heat sensitive paper. A surface
of the print bar is caused to undergo localized heating in response
to an incoming facsimile signal so an array of pels, or darkened areas,
appearing on the heat sensitive paper accurately resemble pictorial
or other information contained in the facsimile signal.
A previously proposed form of thermal print bar
has a strip of thick film resistive material printed on a substrate.
Thick film conductors connect contact areas along both edges of
the trip to drive circuitry which, when energi~ed, passes current
across the strip between preselected pairs of contact areas to
produce localized heating effects at each current path. A problem
with known thermal print bars stems from the need to site
conductors and their associated contact areas very close together
to achieve high resolution. Typically the centre-to-centre spacing
of contact areas is 0.01". Depending on the pattern of contact
areas in the resistive material, many parallel current paths
may be set up between the contact areas of a selected pair with
adjacent contact areas which have not been so selected~
This has two undesirable effects: firstly, more
power is required to heat the resistive material in the select
current path to compensate for power dissipated in adjacent parts
of the resistive strip. This makes necessary higher power handling
drive elec~ronics with a commensurate cost increase. Secondly,
peripheral heating adjacent the selected current path may lead
to undesirable spreading of the pels; the resulting facsimile
on the heat sensitive paper may then become correspondingly indistinct~
Laser machining of the thick film resistiYe material înto
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individual elements with a density of about 200 elements per inch
or connecting external diodes are two methods used to reduce this
leakage current. ~lowever both methods are expensive because of
the extra manufacturing process steps required.
According to the invention there is provided a
thermal print bar for use with a thermally sensitive receptor
sheet, said bar comprising a strip of resistive material
deposited on a substrate material, a plurality of conductors
deposited on the substrate material and connecting contacts along
both edges of the strip to terminals, characterized in that the
resistive material has a negative temperature coefficient of
resistance greater than 1000 parts per million per degree centrigrade.
In use, a current path ls established between a
selected pair of contacts which produces a heat;ng effect which,
in turn, reduces the resistance of the current path. Parallel
current paths existing between said sellected pair of contacts via
non-selected contacts are, by virtue ol the enhanced conductivity
of the current path between the selected pair of contacts,
rendered o~ insufficient magnitude to produce a heating effect
materially altering conductivity.
In a preferred arrangement contact areas are
regularly spaced along both edges of the strip, the contact areas
along one edge being staggered with respect to the contact areas
along the other edge. In such a staggered arrangement, a selected
pair Gf contacts might be any one contact along one side and either
one of the two closest contacts on the other side of the strip.
In a practical arrangement of the in~ention,
drive circuitry initiates current between the contact areas of
selected pairs of contact areas1 selection and timing circuitry
being controlled by an incoming signal.
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An embodiment of the invention will now be
described by way of example with reference to the accompanying
drawing which shows a part of a thermal print bar with schematically
indicated current paths.
Referring to the drawing in detail, there is
shown a part of an electrically resistive strip or bar 10 positioned
on a substrate 11 of electrically insulating material. A first
series or pattern 12 of conductors have contact areas 13 along
one side of the strip and a second series or pattern 14 of
conductors has contact areas 15 along the other side of the strip 10.
; The contact areas along each strip side are regularly spaced9 the
areas 13 being staggered with respect to the areas 15. On applying
a potential difference between a pair of contact areas, for example,
the contact areas 13a and 15a, one each side of the strip, current
flowing across the uniformly resistive strip follows a direct
path and produces localized heating of the strip at the current path.
Only certain contact area pairs are selected to produce a heating
effect~ Thus the contact area 15a can make a pair with either of
the two contact areas ~3a and ~3b which are located on the other
side of the strip and displaced longitudinally ~ith respect to it.
The resistive strip 10 comprises a thick film
material having a marked negative temperature coefficient of
resistance of 5000 parts per million per degree centigrade.
Typically a temperature increase of about 300C is required in
order to activate the heat sensitive paper. Thus when subjected
to such a temperature increase, the resistance of elements
fabricated in the thick film strip 10 decreases by:
5000 x 300 x 100
1,000,000
= 150% reduction in resistance.
., -.
If a constant voltage source is utilized, the
power P = V /R dissipated in an element increases as the
temperature of the element increases until an equilibrium position
is attained.
One example of a suitable thick film material
useful in this application is FIRON made by E.M.C.A. under the
specification TM-103.
- The substrate is of suitable electrically
insulated material, for example glass, quartz or ceramic. The
condustors comprise a three-part layer of conducting material,
for example, titanium, palladium, gold. The techniques for
manufacturing the thermal printing bar described are well-known
in electronics production art, and will not be described in
detail here.
It will be understood that drive circuitry is
required in order to apply potential differences to selected
pairs of the contact areas and timing and addressing circuitry
is required to ensure that an input facsimile signal is accurately
reproduced on m~vement of the thermal print bar relative to a
receptor sheet of heat sensitive paper.
As shown in the figure, in addition to the direct -
current path linking a selected pair of contact areas, for example,
the path of resistance Rl linking contact areas 13a and 15a,
additional parallel currents are initiated by applying a potential
difference between contact areas 13a and 15a, the path of one
such current being represented by the resistances R2 and R3 which
respectively link contact areas 13a and 13b and the contact
areas 13a and 15b.
The benefit of using a thick film material having a high
negative temperature coefficient to resistance is that the ratio
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R2 + R3
Rl increases rapidly consequent upon the reduction in the
resistance of Rl by virtue of the temperature increase in Rl.
The power dissipated in resistance of R2 and R3 is therefore
made relatively insignificant. The heating at the current path
is therefore more localized which ensures a greater resolution
between printed elements. In addition, the power(VR)dissipated
in all of the parallel current paths is markedly reduced
thereby permitting use of drive circuitry with lower power
handling components.
It may be determined that if the thick film
resistive material used has an insignificant temperature coefficient
of resistancel for example ~ 100 parts per million per degree
centigrade which has hitherto been usecl, then a 300C increase
in temperature produces only a 3% change in resistance and the
R2 ~ R3
ratio Rl stays practicably stable as the temperature of R
increases.
Although having an immecliate applicability to
facsimile systems, the invention could find a use on many -
structures where, in operation, a heat pattern is required,
especially where the geometry of conductors to the resistive
strip is complex and laser scribing o~ the strip unacceptable.
Although a thick film material is pre~erred in the invention thin
film materials are available with the property of a high negative
temperature coefficient of resistance and could be used in the
manner described.
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