JPH066927Y2 - Thermal head - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to a thermal head, and more particularly to improvement of a drive circuit for a resistance heating element.

[Conventional technology]

An example of a conventional thermal head is Nikkei Electronics,
No. 311, pages 90 to 94 (published February 28, 1983).

The basic circuit configuration is as shown in FIG. 2. One terminal 1a of each heating element 1 is connected to the common electrode power supply terminal 2 through the common connection line 2a, and the other terminal 1b is the driving IC 3 respectively. Is directly connected to the individual output terminal 3a. 25
Are various input / output terminals and power supply terminals for driving the thermal head.

FIG. 3 shows a circuit configuration inside a general driving IC 3, which includes a shift register circuit 21 and a data latch circuit 2.
2, a gate circuit 23, and a driving transistor 24.

Of the input / output signals and the power supply terminal 25, 25-1 to 25-
5 is an enable input, a latch input, a data input, a clock input, and a data output, respectively.
25-8 is a power supply terminal. Of these, 25-8 is a ground terminal, and a predetermined voltage is applied between the ground terminal 25-8 and the power supply terminal 2. In the thermal head having such a configuration, the heating element is selectively energized by the combination of the data input signal and the enable signal.

[Problems to be solved by the invention]

However, in the thermal head having the above-mentioned configuration, the lengths of the current paths from the power supply terminals 2 to the respective heating elements, that is, the wiring lengths, are different, so that even if the voltage applied to the power supply terminals 2 is constant, both ends of each heating element are Such voltage values are different due to the voltage drop due to the common connection line 2a, and therefore the current values flowing through the heating elements are different, and therefore the heat generation amounts are also different. That is, the heat generation amount becomes smaller as the heating element is farthest from the power supply terminal. Therefore, density unevenness occurs in printing on a thermal recording paper or the like, and print quality deteriorates. This density unevenness is most noticeable in so-called solid black printing in which all heating elements generate heat.

This invention eliminates the above-mentioned disadvantage that the voltage applied to both ends of the heating element becomes smaller as the heating element is farther from the common electrode power supply terminal of the thermal head, resulting in uneven density. It is intended to provide a good thermal head.

[Means for solving problems]

The thermal head of the present invention comprises a plurality of resistance heating elements, a first power supply terminal connected to one terminal of the plurality of resistance heating elements through a common connection line, and a plurality of the resistance heating elements. And a driving transistor interposed between the other terminal of the corresponding resistance heating element and the second power supply terminal, which are provided corresponding to each of the above-mentioned driving transistors, and each of the driving transistors corresponds to the corresponding resistance heating element. From the one terminal of the
The longer the wiring distance to the power supply terminal, the smaller the saturation voltage is.

[Action]

Each heating element, through the drive transistor, common connection line,
It is connected to a constant voltage source. Therefore, the current I flowing through the heating element when the transistor is turned on is given by I = (V _op −V _on ) / (R _dot + R _c ...... (1) where V _op is the power supply voltage and V _on Is the saturation voltage of the transistor, R _dot is the resistance of the heating element, and R _c is the wiring resistance of the common connection line.

Since the heat generation amount of the heating element is proportional to I ² R _dot , it is necessary to keep I constant in order to keep the heat generation amount constant. That is, (1)
V _on = −R _{c If} V _on = V _op −IR _dot −IR _c (2) (2) is fixed and V _op , I, and R _dot are constant, V _on is −R _c.
Proportional to. That is, it can be seen that decreasing V _on with an increase in R _c is a condition for making I constant and thus making the heat generation amount constant. Since R _c is proportional to the wiring length from the heating element to the common electrode terminal, the longer the wiring length,
By making the saturation voltage of the corresponding drive transistor smaller, the heat generation amount of each heating element can be made constant.

〔Example〕

The overall circuit configuration of the thermal head of one embodiment of the present invention is the same as that shown in FIGS. 2 and 3. The difference is that each of the drive transistors has a different saturation voltage value depending on the wiring length from the corresponding heating element to the common electrode 2. That is, the longer the wiring length, the lower the saturation voltage.

FIG. 1 is a partial circuit diagram for explaining the relationship between the heat generation amount of each heating element and the saturation voltage of the driving transistor.
Only one and one drive transistor are shown. In the figure, 1 is a heating element, 24 is an output transistor in the driving IC, that is, a driving transistor, 2, 25-8 are power supply terminals for energizing the heating element, and 2 is a positive side. Reference numeral 15 is a base current input terminal of the drive transistor 24. When turning on the transistor 24,
A base current that saturates is input through the terminal 15. Reference numeral 16 is a wiring resistance of the common connection line, which schematically represents the electric resistance from one terminal of the heating element 1 to the terminal 2, and depends on the length of the current path from the terminal 2 to the heating element 1.

The heat generation amount of the heating element 1 is proportional to the power consumption of the heating element. This power consumption is given by the product of the square of the current I flowing through the heating element and the resistance R _{dot of the} heating element. Therefore, it is necessary to keep the current I constant in order to keep the power consumption constant. The current I is given by the following equation.

I = (V _op −V _on ) / (R _dot + R _c ) ... (3) where V _op is the power supply voltage, V _on is the saturation voltage of the drive transistor 24, and R _c is the wiring resistance of the common connection line. . V _op ,
Since R _dot is constant, the condition for making I constant is to modify equation (3) to obtain V _on = V _op −IR _dot −IR _c (4) From this equation, increase of R _c It can be seen that V _on should be decreased in accordance with the above. That is, it is understood that the heating element located farther from the terminal 2 should correspond to one having a smaller saturation voltage V _{on of the} drive transistor.

The saturation voltage V _on also depends _on the flowing current value, that is, the common resistance R _c , but the current dissimilarity of V _on when the saturation voltage is sufficiently saturated is usually small, and is almost constant in the practical use range of the thermal head. You can think of it.

The size of V _on of the drive transistor can be designed intentionally to some extent in terms of design, and the variation in V _{on in} the manufacturing process can be easily ranked and used. That is, when the average value of V _on is different for each driving IC, the larger the average value of V _on , the closer to the common electrode power supply terminal 2 the heating element may be connected to.

[Effect of device]

As described above, according to the present invention, since a driving transistor having a small saturation voltage is used as a driving transistor corresponding to a heating element having a long wiring from the common electrode power supply terminal, the heating value of each heating element is substantially the same. As a result, a thermal head with uniform printing can be realized. This feature is more effective than the so-called solid black printing, and high quality can be maintained even when such printing is performed. Further, it is particularly effective when the amount of heat generated by each heating element is delicately controlled as in gradation printing.

[Brief description of drawings]

FIG. 1 is a wiring diagram showing one heating element and a driving transistor connected thereto and wiring resistance in one embodiment of the present invention, FIG. 2 is a diagram showing an overall circuit configuration of a thermal head, and FIG. 3 is a driving circuit. It is a figure which shows the outline in IC. 1 ... Resistance heating element, 2 ... Common electrode power supply terminal, 16 ...
Wiring resistance of common connection line, 24 ... Driving transistor, 2
5-8 ... Ground terminal.

Claims (1)

[Scope of utility model registration request] 1. A plurality of resistance heating elements, a first power supply terminal connected to one terminal of the plurality of resistance heating elements via a common connection line, and each of the plurality of resistance heating elements. Correspondingly provided,
A driving transistor interposed between the other terminal of the corresponding resistance heating element and the second power supply terminal, and a constant voltage is applied between the first power supply terminal and the second power supply terminal; A thermal head characterized in that the longer the wiring distance from the first power supply terminal to the resistance heating element, the smaller the saturation voltage of the corresponding drive transistor, and the more constant the heating value of the resistance heating element.