CA1098581A - Thermal print bar - Google Patents
Thermal print barInfo
- Publication number
- CA1098581A CA1098581A CA310,594A CA310594A CA1098581A CA 1098581 A CA1098581 A CA 1098581A CA 310594 A CA310594 A CA 310594A CA 1098581 A CA1098581 A CA 1098581A
- Authority
- CA
- Canada
- Prior art keywords
- contact areas
- resistive material
- thermal print
- print bar
- strip
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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- Electronic Switches (AREA)
Abstract
Abstract of the Disclosure In a thermal print bar for use in facsimile systems, an array of individually selectable electrical current paths are defined in a strip of resistive materials having a high negative temperature coefficient of resistance . In operation, the resistance of a selected current path is markedly reduced in contrast with the resistance of unselected current paths parallel to the selected path so that the parallel current paths do not experience joule heating. Electrical power saving and visual contrast result.
-i-
-i-
Description
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 .
..
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.
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 .
..
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.
- 2 -, . .
~i8~
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 4 _ . . .
8~
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.
.
~i8~
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 4 _ . . .
8~
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.
.
Claims (4)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A thermal print bar comprising an elongate strip of resistive material deposited on a substrate material, said strip having a pair of edges, a plurality of conductors deposited on the substrate material and connecting contact areas along both edges of the resistive material to terminals, wherein the resistive material has, in an operating regime, a negative temperature coefficient of resistance greater than 1000 parts per million per degree centigrade.
2. A thermal print bar as claimed in claim 1, wherein the contact areas along each edge of the strip are regularly spaced from one another, the regularly spaced contact areas along one of the edges being staggered with respect to the regularly spaced contact areas along the other edge.
3. A thermal print bar as claimed in claim 1, wherein the resistive material is screen printed as a thick film.
4. A thermal print bar comprising:
a region of resistive material deposited on a substrate;
a plurality of conductors deposited on the substrate, each of the conductors having a contact area with the resistive material, and means for applying a potential difference across the resistive material between contact areas of selected pairs thereof to cause Joule effect heating of the resistive material between the selected contact areas;
wherein the resistive material has a negative temperature coefficient of resistance sufficiently high that on application of said potential difference, Joule effect heating of the resistive material and consequent increased electrical current between the contact areas of each selected pair of conductors reinforce one another and become progressively localized in a zone extending directly between such contact areas.
a region of resistive material deposited on a substrate;
a plurality of conductors deposited on the substrate, each of the conductors having a contact area with the resistive material, and means for applying a potential difference across the resistive material between contact areas of selected pairs thereof to cause Joule effect heating of the resistive material between the selected contact areas;
wherein the resistive material has a negative temperature coefficient of resistance sufficiently high that on application of said potential difference, Joule effect heating of the resistive material and consequent increased electrical current between the contact areas of each selected pair of conductors reinforce one another and become progressively localized in a zone extending directly between such contact areas.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA310,594A CA1098581A (en) | 1978-09-05 | 1978-09-05 | Thermal print bar |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA310,594A CA1098581A (en) | 1978-09-05 | 1978-09-05 | Thermal print bar |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1098581A true CA1098581A (en) | 1981-03-31 |
Family
ID=4112285
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA310,594A Expired CA1098581A (en) | 1978-09-05 | 1978-09-05 | Thermal print bar |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1098581A (en) |
-
1978
- 1978-09-05 CA CA310,594A patent/CA1098581A/en not_active Expired
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| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |