JPS60262670A - Thermal head driving circuit - Google Patents

Abstract

PURPOSE:To maintain a uniform printed density even when printing at high speed, by a construction wherein one end of each heating element is connected to a switching element, while the other end is connected to a common terminal, and the common terminal is connected to a power source through a special resistance element. CONSTITUTION:One end of each of a plurality of heating elements 2 arranged in a thermal head 3 is connected to a current passage controlling circuit 8 and the power source 4 through the switching element 1, while the other end is connected to the common terminal, which is connected to the power source 4 through a positive temperature coefficient resistance element 10 the temperature of which is raised through self-heating.

Description

DETAILED DESCRIPTION OF THE INVENTION (1) Field of Application in Industry The present invention relates to a thermal printer, and in particular, the common end of a plurality of heating elements corresponding to printed dots is connected to a power source, and the other end of each of the elements is connected to a switch element. The present invention relates to a thermal head drive circuit that alleviates density unevenness when selectively driven by being connected to a thermal head drive circuit.

(2) Prior Art Conventionally, a driving circuit for a thermal head is used which includes a plurality of heating elements corresponding to a plurality of vertical dots in a print line. One example is as shown in FIG.
A thermal head 3 is provided in which the heating element 2 is connected in parallel.
The common terminals of the elements 1 to 2n are connected to the power source 4, and the other ends of the elements are connected to the switching elements 11 to 1% for selective driving. The heating element is a resistive layer formed on a glass body corresponding to a dot and covered with a protective layer. This thermal head is pressed onto a platen via a ribbon and paper, and the resistive layer is heated. Heating melts the ink on the ribbon and prints on the paper. In this type of printer, the heating element 21
The print density is determined by the temperature of ~2%.

(3) Problems to be solved by the invention In the case of low-speed printing, as shown in FIG. does not occur. However, in the case of high-speed printing, as shown in (6) in the same figure, for the head energization in ■,
The balance between heating and heat radiation is disrupted and heat is accumulated in the thermal head, causing the surface temperature of the heating element (2) to gradually rise and uneven shading to occur. As described above, the pattern with a higher printing duty has a more significant difference in shading, whereas the pattern with a lower printing duty does not have a difference in shading. That is, when high-speed printing is performed, there is a problem in that unevenness in density is likely to occur, and the printing quality is greatly influenced by the printing duty of the printing pattern.

In addition to cases where the same dot out of multiple vertical dots accumulates heat over time and is caused by an increase in the surface temperature of the heat generating element, especially in high-speed printers, unevenness in density in the thermal head is caused by positional differences among the multiple dots. This may be due to differences in power consumption when the number of dots that are driven simultaneously in close proximity to each other is large and when the number of dots is small, that is, when they are heated simultaneously.

When the thermal head is driven at high speed in this manner, unevenness in density occurs in printing due to heat accumulation in the head. Various heat accumulation prevention measures have been proposed to prevent this, but none of them are perfect.

Therefore, a method has been proposed in which the uneven density is improved by detecting heat accumulation in the head and changing the driving conditions of the head. FIG. 4 shows an example of this type of countermeasure. In the figure, a switch element 11 a heating element 2. Thermal head 3. Power supply 4 is the second
This corresponds to the configuration with the same number in the figure. In FIG. 4, a heat sink (radiation plate) 5 is provided on the thermal head 5, and a sensor 6 such as a thermistor is attached thereon. As mentioned above, in the case of high-speed printing, etc., when heat is accumulated in the thermal head and the temperature of the heat sink 5 rises, the output of the sensor is detected by the temperature detection and temperature compensation section 7, and the corresponding compensation signal is output as shown by the solid line. It is returned to the power supply 4 to lower the power supply voltage, or it is returned to the energization control section 8 as shown by the dotted line, and feedback control is performed to shorten the energization time of the switch element 1. This improves the unevenness of print density caused by heat accumulation in the thermal head.

In general, uneven shading caused by heat accumulation in the head can be roughly divided into those that appear on a per-line basis per character and on a per-page basis, but the method shown in Figure 4 is mainly effective only for uneven shading on a page-by-page basis. The drawback was that it was not effective for uneven shading on a line-by-line basis. This is because the sensor is attached to a heat sink that includes the head, and since the heat sink has a large heat capacity, it takes time for the temperature to rise to a controllable level, and the control system does not work during that time.

Further, as is clear from the figure, the configuration shown in FIG. 4 also has the problem that the control system including the sensor 6, temperature detection and temperature compensation circuit 71, energization control section 8, etc. is complicated and expensive.

(4) Means and action for solving the problems In the present invention,
Among the above-mentioned uneven shading caused by heat storage in the head, an effective and simple solution to the uneven shading caused by one line per character is to create a system between the common end of the heating element of the thermal head and the power supply due to self-heating. This is provided with a resistance element that has positive temperature characteristics that increase the temperature.

As mentioned above, one of the causes of uneven density that occurs in thermal heads is the difference in power consumption of the head when there are many dots that are positioned close to each other and are simultaneously driven, and when there are few dots that are driven at the same time. becomes a problem. Therefore, in order to reduce this difference in power consumption, the applicant proposed a drive circuit shown in FIG. 5. That is, a resistor (R) 10 is inserted between the common end of the thermal head 3 and the power source 4, and when a large number of dots are driven simultaneously, a large voltage drop from the power source voltage occurs and a low voltage is applied to each heating element. so that
This is designed to compensate for the effects of heat storage. The present invention utilizes this proposed circuit and replaces the resistor 10 with a resistive element having the required positive temperature characteristics. For this,
In addition to reducing the difference in power consumption when the number of dots driven simultaneously by the thermal head is large and small, it also makes the positive temperature characteristics of the resistor element equivalent to the temperature rise characteristics such as the required two-line revolution per character. By doing so, it is possible to reduce unevenness in density due to heat accumulation in the thermal head.

(5) Implementation example FIG. 1 is an explanatory diagram of the configuration of an embodiment of the present invention.

In the same figure, the difference from the conventional example in FIG. 4 is that the sensor 6
The feedback control system consisting of the temperature detection and temperature compensation section 7 is omitted, and a resistance element 11 is provided between the common terminal of the thermal head 3 and the power source 4. The resistance value R' of the resistance element 11 has a positive temperature coefficient like a thermistor, and changes its value due to self-heating. This temperature characteristic changes depending on the heat accumulation in the head of about 2 batting averages per character. In order to match this temperature characteristic,
A heat sink (heat sink) 12 may be attached if necessary.

As explained in the proposed example of FIG. 5, the insertion resistor 11 increases or decreases the voltage drop from the power supply voltage according to the number of dots driven simultaneously to adjust the amount of heat stored in the thermal head 5, and also increases or decreases the amount of heat stored in the thermal head 5. Since the resistance value R' increases in accordance with the characteristics set isometrically for age, etc., it is possible to improve the unevenness of shading even more finely than in the case of FIG. 5.

(6) As described in detail, according to the present invention, a resistance element having a positive temperature characteristic that heats up due to self-heating is provided between the common end of the heating element of the thermal head and the power source. It is something. This reduces the amount of heat stored in the thermal head, which has a large voltage drop from the power supply voltage when a large number of dots are driven at the same time, and allows the insertion resistance value itself to change depending on the drive current value. It is possible to directly control density unevenness such as a 2nd batting average for fine character units by direct open control. Since the time delay seen in conventional feedback control via a heat sink is completely eliminated, it is no longer limited to page units.

Another great advantage is that the circuit configuration can be significantly simplified.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram of the configuration of the embodiment of the present invention, FIG. 2 is an explanatory diagram of the configuration of the conventional example, FIG. 3 (α) and (b) are explanatory diagrams of the operation of the conventional example, and FIG. Figure 5 is an explanatory diagram of a conventionally proposed example. In the figure, 1 is a switch element, 2 is a heating element, 5 is a thermal head, 4 is a power supply, 5.12 is a heat sink, and 8 is a current supply. In the control section, 11 indicates a resistance element. Patent applicant Fujitsu Ltd. Sub-agent Patent attorney 1) Yoshishige Saka Figure 1 Figure 2 Figure 3 (aJ Figure 4)

Claims (1)

[Claims] In a thermal head drive circuit for a dot printer in which a common end of a plurality of heat generating elements corresponding to printed dots is connected to a power source and the other end of each of the elements is connected to a switch element to selectively drive the heat generating elements, the common end of the heat generating elements of the thermal head is A thermal head drive circuit characterized in that a resistor element having a positive temperature characteristic that heats up due to self-heating is provided between an end and a power source.