JPH0531938A - Driver circuit for thermal head - Google Patents

Abstract

PURPOSE:To reduce printing density unevenness largely by a low-cost constitution. CONSTITUTION:When strobe pulses STB are at a high level, the high level is output from an AND circuit 3 when a data DATA is at the high level, thus conducting electricity through a Tr 4 and conducting electricity through a heating resistance element 1. When the curren amplification factor hfe of the Tr 4 is increased sufficiently, VZ=VBE+I0XRS holds, but load currents I0 are kept constant because VZ, VBE and RS are kept constant and constant currents are made to flow through said heating resistance element 1 quite whether other heating resistance elements shows a conductive state or a non-conductive state or the change of loss in a common resistor, thus preventing the generation of density unevenness.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a drive circuit for a thermal head.

[0002]

2. Description of the Related Art Generally, in a printer provided with a thermal head and capable of printing multi-gradation images, Joule heat generated by the heating resistance element is supplied when electric power is supplied to the heating resistance element constituting the thermal head. Printing is performed by developing a color on the thermal recording paper or transferring the ink or dye of the transfer paper onto the image receiving paper. Note that here, the printing includes printing.

In a printer using such a thermal head, a constant voltage power source is used as a power source for supplying electric power to the heating resistance element, and the fluctuation of the voltage is suppressed to make the print density uniform. However, even if it is a constant voltage power supply, when the load current changes, a transient fluctuation of the power supply voltage occurs, and such a fluctuation of the power supply voltage appears as a fluctuation of the amount of power supplied to the heating resistance element. Even when the number of the resistance elements is large and the number of the resistance elements is small, even if the pixels should originally have the same density, the printed density becomes different, resulting in uneven density. Further, there is a resistance (hereinafter referred to as a common resistance) in the power supply line that supplies power to the heating resistance element or in the printed wiring inside the thermal head, and the power supply voltage applied to the heating resistance element due to the loss due to the common resistance. There was also a problem that the density was reduced, resulting in uneven density.

Therefore, conventionally, various proposals have been made in order to prevent the uneven density due to the load fluctuation of the power source as described above. For example, Japanese Patent Laid-Open No. 60-23062 discloses printing density unevenness by controlling the energization time to the heating resistance element according to the ratio of black pixels or white pixels in the image and the ambient temperature of the thermal head. It has been proposed to reduce.

[0005]

SUMMARY OF THE INVENTION
In the one disclosed in Japanese Patent Laid-Open No. 60-23062, a unit for counting the number of black pixels or the number of white pixels, a unit for detecting the ambient temperature of the thermal head, and a heating resistor element according to the output of these units. Therefore, it is necessary to provide a means for deciding the energization time to the device, which complicates the configuration, and is therefore expensive. SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems and to provide a thermal head drive circuit capable of greatly reducing print density unevenness with an inexpensive structure.

[0006]

In order to achieve the above object, a thermal head drive circuit of the present invention comprises a heating resistor element and a plurality of switch circuits for energizing / de-energizing the heating resistor element. In the thermal head drive circuit configured as described above, the switch circuit has a constant current characteristic.

[0007]

Since the switch circuit for energizing / de-energizing the heating resistance element has a constant current characteristic, when the heating resistance element is energized, the current flowing through the heating resistance element is different from that of the other heating resistance element. Is constant regardless of whether or not the power is on. Therefore, even if the loss due to the common resistance changes, a constant current flows through the heating resistor element, so that it is possible to prevent uneven density.

[0008]

Embodiments will be described below with reference to the drawings.
FIG. 1 is a diagram showing a configuration of an embodiment of a thermal head drive circuit according to the present invention. In the figure, 1 is a heating resistor element, 2 is a switch circuit, 3 is an AND circuit, 4 is a switching transistor (hereinafter, referred to as Tr. 5) is a constant voltage diode, and V _B is a power supply voltage.

In the configuration of FIG. 1, the strobe pulse S
When TB is at a high level, if the data DATA is at a high level, the AND circuit 3 outputs a high level, whereby Tr4 becomes conductive and the heating resistor element 1 is energized. At this time, assuming that the current amplification factor h _{fe of} Tr4 is sufficiently large, the following equation (1) is established. V _Z = V _BE + I ₀ × R _S (1) where V _Z is the voltage between the terminals of the constant voltage diode 5, V _BE is the voltage between the base and emitter of Tr4, and I ₀ is the load current flowing through the heating resistor element 1. , R _S represents the emitter resistance. (1)
In the formula, since V _Z , V _BE , and R _S are constant, the load current I ₀ is constant, and the other heating resistor elements are in the energized state or the non-energized state, or the loss in the common resistance. However, since a constant current flows through the heating resistor element 1 irrespective of whether or not the density changes, density unevenness does not occur.

In the configuration shown in FIG. 1, when the potential at the position indicated by A in the figure is constant, it is apparent that the constant voltage diode 5 of FIG. 1 can be replaced by the resistor 6 as shown in FIG. Is. That is, when the potential at the point _A is V _A , the following expression (2) is established similarly to the above expression (1). V _A = V _BE + I ₀ × R _S (2) Here, since V _A , V _BE , and R _S are constant, I ₀ is constant, and a constant current flows through the heating resistor element 1. ..

FIG. 3 is a diagram showing the configuration of another embodiment of the present invention, in which the switch circuit 2 is a field effect transistor (FE).
T) 7 is an example. The same parts as those in FIG. 1 are designated by the same reference numerals. The same applies hereinafter. FET7
Has a constant current characteristic, it is possible to make the current flowing through the heating resistance element 1 constant during energization, thereby preventing uneven density.

FIG. 4 is a diagram showing a configuration of still another embodiment of the present invention, which is an example in which the switch circuit 2 is configured by a series circuit of Tr4 and FET8. By forming a constant current circuit with the FET 8, it is possible to make the current flowing through the heating resistance element 1 constant during energization.

FIG. 5 is a diagram showing the configuration of still another embodiment of the present invention, which is an example in which the switch circuit 2 is constituted by a current mirror circuit, and the current flowing through the heating resistor element 1 is constant when energized. Therefore, it is possible to prevent uneven density.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and various modifications can be made. In short, a switch circuit having a constant current characteristic may be provided. It is a thing.

[0015]

As is apparent from the above description, according to the present invention, it is possible to prevent density unevenness with a simple circuit without performing complicated processing as in the prior art.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of an exemplary embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of another embodiment of the present invention.

FIG. 3 is a diagram showing the configuration of another embodiment of the present invention.

FIG. 4 is a diagram showing a configuration of still another embodiment of the present invention.

FIG. 5 is a diagram showing the configuration of still another embodiment of the present invention.

[Explanation of symbols]

1 ... Heating resistance element, 2 ... Switch circuit, V _B ... Power supply voltage.

Claims (1)

Claim: What is claimed is: 1. A thermal head drive circuit comprising a heating resistor element and a plurality of switch circuits for energizing / de-energizing the heating resistor element, wherein the switch circuit has a constant current characteristic. A thermal head drive circuit characterized by comprising.