EP0044756B1 - Heating resistor and thermal printing head using such a heating resistor - Google Patents

Description

The invention relates to a structure of heating elements, of the type of resistors mounted in series in the form of rectilinear plates, more particularly intended for the production of thermal printer heads. It also relates to the circuit for controlling thermal printer heads, which is simplified by the adoption of resistors according to the invention.

Such a printer head is described in patent FR-A-2 220 139.

Thermal printers are peripheral devices of computer or telecommunications systems, in which the printing of a line of text is obtained by means of a strip of heating resistors: the heat given off by an elementary resistance chemically modifies the paper on which registration is done. Printing a line of characters using a thermal printer head is obtained by repeating several lines of dots at the rate of 8 points per millimeter. A thermal printer head, for a standard paper size 21 cm wide, has 1728 resistors, deposited on a glass or ceramic plate. Each resistor has dimensions of the order of 250 microns in width and the resistors are spaced from each other by 250 microns.

Thermal printer heads pose two problems: that of controlling a specific resistance and that of heat dissipation.

Each programmed unit resistor is controlled by a circuit comprising, among other things, two transistors and a diode. The diodes connected in series with the non-programmed resistors limit the potential across its terminals and prevent them from heating. A thermal printer head therefore requires a circuit comprising as many diodes as there are heating resistors or at best, according to the scheme adopted, a number of diodes equal to half the number of heating resistors, the number of diodes remaining significant since 'so it is at least 863 diodes, for 1728 points.

The characteristics of the resistors according to the invention which comprise at least one layer operating as a non-linear resistance with a negative temperature coefficient, with a tilting point, make it possible to eliminate the diodes in the supply circuit of the heating resistors.

Furthermore, the heating resistors, which are small, have a low calorific capacity, and the heat released is partly absorbed by the substrate whose calorific capacity is much greater.

Indeed, an in-line heating resistance strip is produced on a substrate which is a glass or ceramic plate, the length of which is equal to the width of the printing paper, and the thickness of which is of the order of a few millimeters so as to ensure the rigidity and non-brittleness of the thermal printer module. It is therefore an improvement to the thermal heads provided by the invention according to which the heating resistors comprise a warmer outer layer which dissipates the heat preferentially towards the paper rather than towards the substrate wafer.

More specifically, the invention consists of an electrical heating resistor, deposited on an insulating substrate of glass or ceramic, whose heat capacity is much higher than that of the heating resistor, characterized in that it comprises at least a first layer of a material of relatively constant resistivity as a function of temperature, deposited on the substrate and at least a second surface layer of a material of resistivity varying non-linearly with temperature, with a negative temperature coefficient, this second layer being deposited on the first layer.

• - fig. 1 schéma électrique de l'alimentation des résistances d'une tête d'imprimante thermique.
• - fig. 2 vue en coupe d'une résistance chauffante, montrant la dissipation thermique.
• - fig. 3 vue en coupe d'une résistance chauffante selon l'invention.
• - fig. 4 courbe des caractéristiques de résistance en fonction de la température d'une résistance à coefficient de température négatif.
• - fig. 5 vue en coupe d'une résistance selon l'invention, selon un perfectionnement.

The invention will be better understood by the explanations which follow, which describe an example of application of a thermal printer head, and are based on the following figures, which represent:

• - fig. 1 electrical diagram of the power supply to the resistors of a thermal printer head.
• - fig. 2 sectional view of a heating resistor, showing the heat dissipation.
• - fig. 3 sectional view of a heating resistor according to the invention.
• - fig. 4 curve of the resistance characteristics as a function of the temperature of a resistance with a negative temperature coefficient.
• - fig. 5 sectional view of a resistor according to the invention, according to an improvement.

Fig. 1 shows the electrical diagram of the current supply to the heating resistors in a thermal head. The explanation of its operation will better show the advantages of the invention.

Given the large number of heating resistors in a thermal printer head, 863 or 1728 resistors according to the scheme adopted for a standard paper size, only part of the thermal head is shown diagrammatically. 1.

The heating resistors, numbered from 1 to 5 are connected in series and are supplied by group, from several power transistors, two of which have been represented in 6 and 7. The power transistor 6 supplies the resistors 1, 4 and 5, while the power transistor 7 supplies the resistors 2 and 3. The groups are interdigitated and the choice or the programming of a resistor which must heat is determined by a transistor such as 8, 9 or 10, the base of which is controlled by a shift register. Resistor 1 is controlled by transistor 6 and transistor 8, resistor 2 is controlled by transistor 7 and transistor 8, the resistor 3 is controlled by the transistor 7 and the transistor 9 ... and so on.

In order to prevent the current sent into a resistor from passing through other resistors, it is necessary to mount diodes in series with power transistors 6 and 7: these are diodes 12, 13 and 14 in fig . 1. In fact, the heating resistors according to the known art have relatively low values and the simple leakage current through an unprogrammed transistor is sufficient to heat an unprogrammed resistor: the presence of the diodes limits the potential at the terminals of the resistors not programmed.

Although the diagram in fig. 1 is only very partial compared to the complete diagram of a thermal printer head, it appears that the electrical mounting requires the mounting, inter alia, of a large number of diodes which are implanted on relatively complex circuits by the large number of conductors they require as well as the large number of welds, which represents a disadvantage for the industrial assembly of a thermal printer head.

The replacement of conventional heating resistors by heating resistors according to the invention has the advantage of eliminating the diodes, because of the high value that the heating resistors have when cold, a value which decreases very quickly as soon as the surface layer which is consisting of a resistance with a negative temperature coefficient has reached and exceeded its tipping point.

Fig. 2 shows the sectional view of a heating resistor according to the known art. This figure provides a better understanding of the problems of heat dissipation.

On a substrate 15, glass or ceramic, is deposited a heating resistor and the sheet of paper 17 scrolls in contact with the heating resistor. The relative scales of fig. 2 have been adopted so as to allow a better view of the figure, and, in fact, for a thickness of the substrate 15 of the order of approximately 1 to 5 mm, each heating resistor 16 deposited by screen printing, by vacuum evaporation or by any other similar process has a thickness which is best counted in tenths of a millimeter. In addition, the resistors according to the known art are deposited by means of one or more passages by accumulation of layers which are all made from the same base material, and therefore all the layers have the same characteristics of resistivity and temperature coefficient.

When a heating resistor such as 16 is programmed, the heat it gives off is symbolized in FIG. 2 by means of arrows which represent an amount »Q« of heat which dissipates towards the substrate and other arrows which represent an amount of heat »q« which dissipates towards the sheet of paper. The heat capacities in the presence of the thick substrate 15, of the relatively thin resistor 16, and of the sheet of paper 17 which, moreover, passes past the resistor, mean that a good part of the energy involved is in fact dissipated towards the substrate, which is not the goal.

The replacement of such a conventional resistance with a resistance according to the invention, in addition to the previous advantage described on the occasion of FIG. 1 makes it possible to dissipate the heat preferentially towards the sheet of paper, that is to say that by adopting resistors according to the invention in fact makes it possible to simplify the whole electronic control part since the required energy is less and that the components such as transistors must dissipate less energy.

Fig. 3 shows a sectional view of a resistor according to the invention.

On a wafer of substrate 15 are deposited, by any process in accordance with what a person skilled in the art can do, at least a first resistance layer 18 which will be conventionally called fixed as opposed to a second layer 19 of non-linearly variable resistance with a negative temperature coefficient. On the side of fig. 3 is symbolized by the electrical diagram of the two resistors 18 and 19, the first layer 18 being a fixed resistor and the second layer 19 being a variable NTC resistor mounted in parallel with the fixed resistor R. At ordinary temperature, all of the two resistors 18 and 19 which constitute the resistance according to the invention has a high value. When a current is addressed via the control transistors through this compound heating resistor, the resistor R heats the variable resistor CTN until it reaches its tipping point: from this temperature the CTN considerably decreases in resistance, it becomes conductive and the heat released is largely released by the external surface of the heating resistance, that is to say in contact with the sheet of paper.

The first resistance layer 18 can be of the linear type, that is to say that its value practically does not vary with temperature, in comparison with the variation of the CTN. But it is an improvement to the invention to use as a first layer a resistance with positive temperature coefficient PTC: this behaves like a linear resistance up to its tilting temperature, temperature at which its resistance increases considerably and abruptly. This solution, however, requires a fairly good choice of materials from the CTP and the CTN, so that the two changeover temperatures substantially overlap, that is to say that the CTN becomes "conductive" when the CTP ceases to 'be.

To simplify the figure and the description, only the limiting case of the invention has been shown when a single layer of fixed resistance 18 is deposited on the substrate. A more general case which will be explained later plans to file suc several layers of fixed resistances.

In order to prevent part of the heat given off by the CTN 19, when the latter has passed its tipping point, from being communicated to the substrate 16, the invention provides that the layer 19 is deposited on the layer 18 without exceed it in such a way that the layer 19 does not have contact with the substrate 15.

Fig. 4 represents the resistance characteristics as a function of the temperature of the resistors with a non-linear negative temperature coefficient.

The temperatures being given on the abscissa, the resistances are represented on the ordinate. When a CTN resistor heats up, its resistance gradually decreases until a so-called tilting temperature T _b is reached. In a low temperature zone surrounding the tilting temperature T _b , that is to say more or less 3 to 5 ° on average, the resistance of the CTN decreases considerably and it is currently known to produce CTNs whose resistance is affected on either side of the changeover temperature by a coefficient exceeding 10 ³ and reaching 10 ⁴ to 10 ⁵ . This is how we know how to make thermistors which for an initial value of 2.10 ⁴ to 10 ⁵ ohms at ordinary temperature do no more than 2 to 5 ohms at 80 ° C.

The heating resistors according to the invention therefore have the advantage of having a high value at ordinary temperature and then, when they have been programmed and the underlying resistance layer has heated the variable resistance layer, they do not have more than a low resistance value, which allows on the one hand to remove diodes in the supply circuit since the other resistors, not programmed, have high values, and on the other hand to dissipate the heat mainly towards paper.

An improvement to the invention consists in permanently programming a low current through all of the heating resistors so as to maintain them at a constant temperature apart from any programming which is slightly lower than the switching temperature. Thus, when a resistor must be programmed to write a point on the sheet of paper, it only takes a small current through the circuit of the control transistors to switch the surface layer with negative temperature coefficient, which increases the speed. response of the thermal printer head and reduces the necessary power dissipated through the control transistors.

Fig. 5 shows an improvement made to the structure of the heating resistors according to the invention. The description of the invention, in fig. 3, was based on the simplest case where a variable resistivity layer 19 is deposited on a single layer with constant resistivity 18. However, it is an improvement to produce a variable resistivity layer 19 on a plurality of layers with constant resistivity, two of which have been represented at 18 and at 20 in FIG. 5.

Thus according to this improvement to the invention, it is advantageous to deposit the variable resistivity layer 19 on a support composed of a plurality of layers from 1 to n, the resistivity of each layer decreasing from layer 1 to layer n. In addition, each layer is deposited so that only the first layer touches the substrate 15 of the thermal printer head, so as to focus the heat given off in each layer, heat which is greater in one layer than in the previous one, towards the external surface of the thermal resistance, which means that the external surface is the warmest and which modifies the paper used in the thermal printer.

Claims (7)

1. Electrical heating resistance, deposited on an insulating substrate (15) made of glass or ceramic, the thermal capacity of which is much higher than that of the heating resistance, characterized in that it comprises at least a fist layer (15) of material with a resistivity relatively constant in function of the temperature, deposited on the substrate (15) and at least a second superficial layer (19) of a material with a resistivity variable in a non-linear manner with temperature, and showing a negative temperature coefficient (NTC), this second layer (19) being deposited on the first layer.

2. Electrical heating resistance according to claim 1, characterized in that its resistance, raised to ordinary temperature (20°C), becomes weak once the constant resistivity layer (18), passed through by a current, bring the variable resistivity layer (19) to the temperature of sudden variation of its resistivity, called tipping temperature of the NTC.

3. Electrical heating resistance according to claim 2, characterized in that, once the variable resistivity layer (19) has reached its tipping temperature, the calorific emission is localized in the superficial layer (19).

4. Electrical heating resistance according to claim 1, characterized in that the superficial layer (19), of variable resistivity, is maintained at a temperature close to and slightly below its tipping temperature, by application of a permanent current passing through the subjacent resistive layer.

5. Electrical heating resistance according to claim 1, characterized in that the superificial layer (19), with a variable resistivity, is deposited on a plurality of layers (18, 20) with a constant resistivity, the resistivity of these layers ranging in decreasing order from, that (15) which is deposited on the substrate (15) to that (20) which is underlying the superificial layer (19).

6. Thermal printing head, comprising at least one strip of heating resistances in line, characterized in that it comprises resistances according to any one of claims 1 to 5.

7. Thermal printing head according to claim 6, comprising heating resistances mounted in series (.1 to 5) supplied by power transistors (6, 7) and controlled by transistors (8, 9, 10) each of which authorizes the passage of the current through a determined resistance, characterized in that the heating resistances are connected by direct connections to the power transistors and to the control transistors.

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