JPH0542700A - Thermal printing head improved in printing density difference - Google Patents

Abstract

PURPOSE:To provide a direct drive type thermal printing head improved in printing density difference. CONSTITUTION:A plurality of drive elements 5 driving heating resistors 1 are divided into a plurality of blocks 7-10 and a current is supplied to the respective blocks by ON-signals of strobe signals S7-S10. The resistance value of the heating resistors R' the of block 10 reduced in the number of the drive elements operated at the same time (the number of dots applied at the same time) is set so as to be larger than that of the heating resistors R of the other blocks 7-9 so as to obtain the same supply power in all of the blocks.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a direct drive system of a thermal print head which has an improved print density difference.

[0002]

2. Description of the Related Art FIG. 2 shows a schematic diagram of a thermal print head of a direct drive system. 2A, a heating resistor 1, a common electrode 2, a lead electrode 3, and a ground terminal 4 are formed on a ceramic substrate (not shown) by a thick film technique, and a driving integrated circuit device 5 (hereinafter, The drive element 5) is mounted. 2B is an equivalent circuit showing the electric resistance of the thermal print head shown in FIG. In the figure, R is the resistance value of the heating resistor 1, Rcom is the conductor resistance value of the common electrode 2, and Rcom is
If lead is the conductor resistance value of the lead electrode 3 including the drive element 5 and the ground, Ric is the internal resistance value of the drive element 5, and the supply voltage is Vset, the supply power Po for one dot is given by the following equation. Become. Po = [(Vset) ² / (N × Rcom + R + Ric + Rlead) ² ] × R In the above formula, N is the number of simultaneously applied dots, but if there is a difference in the number of simultaneously applied dots, one dot is supplied. A difference also occurs in the electric power Po.

In the driving of the thermal print head by the direct drive method, when all the heating resistors are driven at the same time, the circuit current becomes excessive. Therefore, a method of operating the driving element by dividing into several blocks is adopted. ing. In the direct drive method as shown in FIG. 1, the recording data sent to the latch circuit 6 is divided into four blocks by the driving units 7 to 10 and the strobe signal S.
It operates in response to the ON signal from 7 to S10, and the heating resistors 1 belonging to each block are energized to generate heat to record data.

In this case, the number of driving elements 5 divided into four blocks and operated may vary depending on the blocks. For example, when the total number of drive elements 5 is 27 and the block is divided into four blocks and operated, three blocks 7 to 9 each have seven drive elements N1 to N7 and the remaining one block 1
Six drive elements N1 to N6 are assigned to 0. Thus, when different numbers of drive elements 5 are allocated to the block, (the number of drive elements to be operated simultaneously) × (the number of output bits of the drive elements) = (the maximum number of simultaneously applied dots Nmax), and the heating resistor Since all the resistance values of 1 are set to the same target value, the difference in the number N of simultaneously applied dots affects the supplied power Po, as is apparent from the above equation. For this reason, the print density of the block 10 to which the six drive elements having a small number of simultaneously applied dots are allocated has a small loss, and therefore the other three print densities cannot be obtained.
It has the disadvantage that it is darker than one block 7-9.

[0005]

SUMMARY OF THE INVENTION The present invention provides a thermal print head capable of reducing the difference in print density even if the number of drive elements in the divided blocks is different.

[0006]

According to the present invention, the resistance values of the heating resistors are not all set to the same resistance value as in the prior art, but the target resistance value is set to other values for a block having a small number of simultaneously applied dots. It is characterized in that the difference in print density between the blocks is reduced by setting the value so that the same supply power as that of the blocks can be obtained.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In the present invention, in a block 10 having a small number of simultaneously applied dots, the target resistance value is set in other blocks 7 to 10.
It is characterized in that the difference in print density between blocks is eliminated by setting the value so that the same supply power as that of No. 9 can be obtained. In the case of a thermal print head for a facsimile application, the average resistance value of the heating resistor is generally 1000Ω, but the resistance value R ′ of the heating resistor of the block 10 having a small number of simultaneously applied dots is 1060Ω and trimming is performed. , And is set to be slightly larger than the resistance value R of the heating resistors of the other blocks 7 to 9.

The resistance value is corrected by a trimming method as shown in FIG. As described above, the target value for trimming the heating resistor is the block 10 with a small number of simultaneously applied dots.
Only, the target value higher than other blocks will be set. First, a probe pin is applied between the lead electrode b and the common electrode a of the heating resistor, and the resistance value between a and b is measured. Next, since the resistance value between a and b is higher than the target resistance value, pulses are applied until reaching the target resistance value. Next, after the trimming of ab is completed, a probe pin is applied between ac and trimming is performed in the same manner. By performing the trimming as described above, it is possible to set the resistance value R ′ of the heating resistor of the block having a small number of simultaneously applied dots to be slightly larger than the resistance value R of the heating resistor of the other blocks. it can.

[0009]

INDUSTRIAL APPLICABILITY The present invention can reduce the difference in print density between divided blocks in a direct drive type thermal print head, and can be used regardless of the effective print width.

[Brief description of drawings]

FIG. 1 is a drive circuit diagram of a thermal print head of the present invention.

FIG. 2 is an explanatory diagram of a thermal print head.

FIG. 3 is an explanatory diagram of resistance value trimming.

[Explanation of symbols]

1 ... Heating resistor 5 ... Drive element 6 ... Latch circuit 7-9 ... Drive block with many applied dots 10 ...
・ Drive block with few applied dots

Claims (1)

[Claims] 1. In a thermal print head of a direct drive system, a plurality of driving elements for driving a heating resistor are divided into a plurality of blocks and driven, and
The resistance value of the heating resistor driven by the driving element of the block in which the number of driving elements belonging to the block is small is made larger than the resistance value of the heating resistor driven by the driving element belonging to another block. The thermal print head with improved print density difference.