JPS62204970A - Thermal printer - Google Patents

Abstract

PURPOSE:To reduce variations in a raised temperature of each heat generating element due to variations in the number of times of driving, secure favorable printing quality and maintain a long useful life of the heat generating elements, by providing a common voltage-dividing resistor for each heating resistor group, and varying a driving voltage according the the number of times the heat generating elements in the same group are driven. CONSTITUTION:Only for a period for which a strobe pulse 33 is outputted, selection of a head-driving circuit is performed according to printing data, and heat generating resistors H1-H24 in a heat generating resistor array 20 which correspond to the printing data are energized. Of the heat generating elements in the array 20, those belonging to odd-numbered dot lines and those belonging to even-numbered dot lines are connected to a common DC power source respectively through voltage-dividing resistors 41, 42. As the number of black dots in a group of the heat generating resistors for odd-numbered or even-numbered dot lines is increased, a current passed to each of the heat generating elements is decreased, and power consumption is decreased. Accordingly, by appropriately setting the resistances of the resistors 41, 42 according to the resistances of the heat generating elements, a power source voltage and heat-releasing characteristics of a printing head, it is possible to cancel variations in a raised temperature of each of the heat generating elements due to variations in the number of print dots.

Description

DETAILED DESCRIPTION OF THE INVENTION 1 Ball 11 The present invention relates to a thermal printer, and particularly to an improvement of a head drive circuit in a serial thermal printer using a thermal head.

Prior Art In such a thermal printer, a print head having a large number of heating elements is moved back and forth in front of printing paper held on a platen, and each heating element prints dots while contacting the thermal paper directly or through a thermal transfer film. Printing is performed by repeating heating and cooling according to the pattern.

Conventionally, thermal heads used in this type of printer have one or two rows of heating elements arranged perpendicular to the row direction, but the head drive power supply depends on the driving state of each heating element. It was configured so that a constant voltage was applied regardless.

For this reason, each heating element is placed close to each other and thermally interferes with each other, so that there are cases where many heating elements are operated at the same time and cases where only a small number of heating elements are driven.
The upper limits of temperature of the heating elements are different, and therefore, a pattern with a large number of black dots tends to have fine details easily destroyed by overheating of the head, and a pattern with a small number of black dots has the disadvantage that the print density is insufficient. Furthermore, there is a problem in that the life of the heating element tends to deteriorate due to heating of the head in a pattern with a large number of black dots.

OBJECTS OF THE INVENTION The present invention has been made in order to eliminate the drawbacks of the above-mentioned conventional printers, and its purpose is to provide a thermal printer that maintains substantially constant print density and good print quality regardless of the print pattern. It's about doing.

An object of the present invention is to provide a thermal printer that can extend the life of the heating element.

1. A thermal printer according to the present invention includes a plurality of heating elements for printing arranged orthogonally to the printing direction, and a first heating element among which the heating elements are divided into first and second heating element groups. a common first resistor connected in series to all of the heating elements of the heating element group; and a common second resistor connected in series to all of the heating elements of the second heating element group. and these are the first
The heating element is characterized in that power is supplied to each of the heating elements through the second resistor.

Example Next, the present invention will be explained with reference to the drawings.

FIG. 1 is a circuit diagram showing an actual example of the present invention, and FIG. 2 is a diagram showing the operation of this embodiment.

Referring to FIG. 1, serial data 31 transferred from the head control circuit 30 is stored in the shift register 11 by a shift pulse 3'2 and converted into parallel data.

The print data output from the shift register 11 is supplied to the head drive circuit 13 via the AND gate 12, and the head drive circuit is selected in accordance with the print data only during the period when the strobe pulse 33 is output. Therefore,
Heating elements 11 to 1H corresponding to the printing data of the heating element row 20
24 is energized. Each of the heating elements belonging to the odd-numbered dot rows of the heating element row 20 is connected to a common DC power source through a voltage dividing resistor 41, and each heating element belonging to an even-numbered dot row is connected to a common DC power source through a voltage dividing resistor 42. There is.

Here, the number of black dots in the odd-numbered dot rows of the dot pattern is nO, and the number of black dots in the even-numbered dot rows is n[.

The power supply voltage is set to V1, and the resistance value of the voltage dividing resistors 41 and 42 is set to R1.
If the resistance value of each heating element is r, the current flowing through the heating element group in odd-numbered dot rows is IO=V (r/no+R), and the current flowing through the heating element group in even-numbered dot rows is I[=V (r /
nE +R).

The drive current flowing through each heating element in odd and even dot rows is
10-V/(r+n0R), 1E=V/(r+nF
R). At this time, the power converted into heat by each heating element is PO = rV2/ (r + nOR), PE = rV
2/(r+nE R)”.

As described above, as the numbers no and nE of black dots included in the heating element groups in the odd-numbered dot rows and even-numbered dot rows increase, the current i0 flowing through each heating element increases. i E decreases, and the power consumption PO and PE of each heating element also decreases, and this situation is reflected in the second
It is shown in figures (A) to (E).

(A) is the dot pattern, (B) is the number of black dots, (C)
is the total current I=IO+Ij, (D) is the heating element current, (
E) is its power. Here, if the resistance values of the voltage dividing resistors 41 and 42 are appropriately set according to the heating element resistance r, the power supply voltage 2, and the heat dissipation characteristics of the thermal head, the temperature rise of each heating element due to a change in the number of printed dots can be reduced. This means that fluctuations can be offset.

As explained above, according to the present invention, a common voltage dividing resistor is provided for each heating element group, and the driving voltage is adjusted according to the number of driving circuits for the heating elements included in the same heating element group. By configuring the heating element to change, it is possible to reduce fluctuations in the temperature rise of the heating element due to changes in the number of drive circuits, thereby ensuring good printing quality and maintaining a long life of the heating element.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention, and FIG. 2 is a diagram for explaining the operation of the embodiment circuit of the present invention. Explanation of symbols of main parts 20...Heating element group 41.42...Resistor element

Claims (1)

[Claims] For all of the plural heating elements for printing arranged perpendicularly to the printing direction and each heating element of the first heating element group out of which the heating elements are divided into the first and second heating element groups. A common first resistor connected in series, and a common second resistor connected in series to all of the heating elements of the second heating element group, these first and second resistors A thermal printer characterized in that power is supplied to each of the heating elements through the body.