JPH0281647A - Thermal head driving apparatus - Google Patents

Abstract

PURPOSE:To enhance printing quality by uniforming the heat distribution on a thermal head by providing a constant current control circuit for keeping the current proportional to a control level constant at every printing control unit and driving the resistor of the thermal head by the output current of the constant current control circuit. CONSTITUTION:An operational amplifier 7 constitutes an adder circuit along with input resistors 8-10 and a feedback resistor 11. The out current IO of a constant current is controlled on the basis of the output VOP of the operational amplifier 7 and the base potential of a transistor 19 is controlled by a resistor 17 and the collector current flowing to the transistor 19, that is, the supply current of a resistor 20 is made variable. A transistor 1 and a resistance ladder circuit consisting of resistors RHn2, RLn3, RRn4, RR(n+1)5 are arranged according to thermal head arrangement. The resistance ladder circuit has such a characteristic that potential is gradually lowered from the center of a continuous current supply element to the end part thereof and considerably lowered with respect to an isolated current supply element and, therefore, an extremely good characteristic is developed when the distribution state of temp. is corrected.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a printing device using a thermal head, such as a typewriter or a printer.

BACKGROUND OF THE INVENTION In recent years, thermal printing devices used in portable typewriters and the like are aiming at high printing quality, and the miniaturization of each printing element is progressing. The heat generating part of the thermal head is basically a resistor, and the resistor generates heat by applying a regulated constant voltage to the boat and energizing it. 7th
The figure shows an example of a drive circuit using this constant voltage control method.

In FIG. 7, a transistor 21 is a main switching transistor that controls the application of voltage to a resistive load 22, which is a heating element of the thermal head.

The transistor 23 is a switching transistor that controls the main transistor 21. When the transistor 23 is turned on, the transistor 21 is turned on, and when the transistor 23 is turned off, the transistor 21 is turned off. 24.25 is the resistance.

VELM is a control signal for selecting and energizing this printing element.* Vaa is a power supply for this circuit.
It is generally regulated to a constant voltage.

Since this drive method is characterized by applying a constant voltage, there is a large variation in heat distribution between printing elements due to variations in resistance value between the printing elements of the resistive load 22 and fluctuations in resistance value due to temperature rise after heat generation. It has the following drawbacks. In particular, this has been a bottleneck when it comes to miniaturizing printing elements and achieving high printing quality. Therefore, constant current drive systems such as US Pat. No. 452,346 have been devised, in which each printing element is driven with a constant current and the value of the current applied is controlled according to the number of printing elements to be energized. According to this method, it is possible to realize a circuit that supplies a constant current to each printing element, and it is possible to reduce fluctuations in the amount of heat generated due to variations or fluctuations in the resistance value of the heating element. It also includes a secondary current value correction circuit that increases or decreases the current value according to the number of energized printing elements among all printing elements.

Problems to be Solved by the Invention A general thermal print head has the following problems as shown in FIG.
Various printed characters and figures are constructed by arranging printing elements that form printed characters in the vertical direction and scanning the required units in the horizontal direction. The printing element array 26 has elements from the 1st element to the 2nd element, and printing is performed by selecting an arbitrary nth printing element 27 among these elements and generating heat. 0 Selected energization printing The elements are hereinafter abbreviated as energized elements, and the non-energized printing elements that are not selected are hereinafter abbreviated as non-energized elements. For example, assuming that the printing elements from the n-th element to the n+i-th printing element generate heat as energized elements to form characters, a chain of heating elements from the n-th energizing element to the n+i-th heating element is generated. This results in variations in the temperature distribution of the heating element of the thermal head, as shown in FIG.

In the above example, there is no variation in resistance value of the heating element, and
This is a graph assuming that the energizing currents of the n+i-th energizing elements are constant. This difference in temperature distribution causes deterioration in printing quality, such as voids and blanks, around the shaper pattern. In the above conventional configuration, in order to solve the above problem, a slightly thicker current is applied to the n-th and n+i-th printing elements at both ends of the series of current-carrying elements than to the other current-carrying elements. The temperature distribution is corrected by increasing the amount of heat generated. FIG. 10 shows the current pattern of the current-carrying elements, and FIG. 11 shows the temperature distribution corrected as a result. However, the conventional configuration is insufficient in correcting the temperature distribution for printing elements that are becoming smaller and finer, and a more optimal temperature distribution correction is desired. Further, the conventional configuration requires a large number of control voltages and requires a special regulator circuit to be configured, which has the disadvantage of poor cost performance.

The present invention is intended to solve the above-mentioned problems, and an object of the present invention is to provide a thermal head drive device that can uniformly distribute heat on a thermal head and improve printing quality.

Means for Solving the Problems In order to solve the above-mentioned problem, the present invention provides a resistance ladder circuit spanning between printing control units of a thermal head that forms a printed figure, and generates a voltage according to a division ratio of the resistance ladder circuit. An adder circuit for superimposing the temperature distribution correction level on the reference current level is provided, and a constant current control circuit for maintaining a constant current proportional to the control level generated by the adder circuit is provided for each print control unit, and the constant current By driving the resistor of the thermal head with the output current of the control circuit, the current value applied to the resistor of the thermal head is varied according to the voltage division ratio of the resistor ladder circuit.

Operation: With the above configuration, changes in the voltage level of the resistor ladder circuit due to the energized printing element are monitored.

Obtain the voltage for correcting the temperature distribution of the thermal head.

The thermal head is caused to generate a current pattern proportional to this correction voltage pattern. In this way, by varying the value of the current flowing through the resistor according to the voltage division ratio of the resistor ladder circuit, a heat amount that contrasts the temperature distribution curve is generated, thereby making the temperature distribution of the connection of current-carrying elements uniform. be able to. As a result, a thermal head drive device with good print quality and high cost performance can be obtained.

Example An embodiment of the present invention will be described below based on the drawings.

FIG. 1 shows a circuit diagram of an arbitrary n-th drive circuit in an array of printing elements forming a thermal head in a thermal head drive device according to an embodiment of the present invention. is the main control signal for energizing this printing element, and at the same time operates as a power supply line within the circuit. The main control signal V@LMfi is applied only when the printing element is energized, and the main control signal V ELMn is not applied if the printing element is not energized. The applied signal VELMn is regulated to a constant voltage.

VID connects resistor Rfit12 and resistor Rt, ゎ3 and resistor Ra, which constitute a resistor ladder circuit, through transistor 1.
It is a reference voltage applied to n4, resistor RRfi, and 5.

6 is the base resistance of transistor 1. Reference voltage VR
D is the resistance ladder circuit that determines the temperature distribution correction potential Va of the operational amplifier 7. This is a control voltage, and the minimum conduction current is determined by this control voltage Vctll.

The operational amplifier 7 includes input resistors 8, 9.degree. 10 and a feedback resistor 11 to form an adder circuit. operational amplifier 7
Since the power is supplied by the main control signal VgLMn, it is a single power supply, and a high slew rate is used to realize high-speed energization start and stop. The output VOp of the operational amplifier 7 is the reference voltage V IIEF
is set as a reference potential, and is varied by a value added by a resistor voltage division ratio of resistance ladder circuit voltage vc3 and energizing current control voltage Vcu*, and outputted. 13 inside the broken line is a constant current control circuit.

Although the present embodiment is configured with a constant current control circuit using a current mirror circuit, it may be replaced with a constant current line using an operational amplifier shown in FIG. At that time VI
+LMn may be non-regulated. Since this constant current control circuit is a general circuit, its explanation will be omitted. Transistors 14, 15, and 16 constitute a typical current mirror circuit, and current 1.
An output current Io proportional to can be obtained through transistor 15,16. The proportionality coefficient is realized by making the transistor 16 a multi-emitter. The base potential of the transistor 19 is controlled by the constant output current IO controlled by the operational amplifier output VOp and the resistor 17, and the collector current flowing through the transistor 19, that is, the current flowing through the resistor 20 is varied.

FIG. 3 is a diagram in which the resistor ladder circuit consisting of transistor 1, resistor RH, resistor 2, resistor RL, resistor 3, resistor R1t, 4, and resistor RIIn+15 of FIG. 1 is arranged in accordance with the arrangement of the thermal head. Figure 4 shows the nth printing element in this array.
It is illustrated that five printing elements from the -2nd printing element to the n+2nd printing element are energized. The potential of the resistance ladder circuit output vc, which is a temperature distribution correction potential, varies depending on the arrangement of adjacent current-carrying elements, and a potential pattern having a curve as shown in FIG. 5 is generated. Vcn-x to Va
Potentials are also generated for vcs other than □Y, but these printing elements can be ignored because they are non-energized elements.

- For boat fishing In the resistor ladder circuit shown in Figure 4, the potential gradually decreases from the center to the ends of the continuous current-carrying elements.

Furthermore, since the potential of an isolated current-carrying element is significantly lowered, it has extremely good characteristics when correcting the temperature distribution state. The temperature distribution correction potential generated at the resistance ladder circuit output vct1 is the reference voltage V 11! Since the operational amplifier output VOP is generated in proportion to the difference from F, the amount of heat generated is controlled and the temperature distribution is corrected by passing a current corresponding to this voltage level.

FIG. 6 shows the pattern of the applied current, and the broken line 28 shows the distribution of the amount of heat generated as a result of the above-mentioned applied current pattern. The shaded area is Vc! This is the minimum reference current according to Ill. This makes it possible to achieve a uniform temperature distribution in the arrangement of the current-carrying elements.

Note that the transistor 1 in FIG. 1 is a basic switching element, and can also be implemented as a PNP transistor or a field effect transistor. Further, the adder circuit of the open amplifier 7 can also be realized using a transistor.

Effects of the Invention As described above, according to the thermal head drive device of the present invention, a resistance ladder circuit is provided between adjacent printing elements, and a level potential is generated to correct the temperature distribution generated in the arrangement of the current-carrying elements of the thermal head. The amount of heat generated can be well controlled by varying the current flowing through the thermal head. As a result, it is possible to obtain a uniform temperature distribution,
An excellent thermal head drive device that can ensure extremely good printing quality and has high cost performance can be obtained.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of a thermal head drive device according to an embodiment of the present invention, Fig. 2 is a circuit diagram in which the constant current control section of Fig. 1 is realized using an operational amplifier, and Fig. 3 is an arrangement of printing elements. A diagram showing a resistance ladder circuit. FIG. 4 is a simplified diagram of FIG. 3 showing the arrangement of current-carrying elements. Fig. 5 is a diagram showing the potential pattern of the current-carrying elements by the resistance ladder circuit of the present invention, and Fig. 6 shows the pattern of the conducting current generated based on the pattern of Fig. 5 and the distribution of the amount of heat generated by this. Figure 7 is a circuit diagram of a thermal head drive device using a conventional constant voltage control method, Figure 8 is a diagram showing the arrangement of printing elements of the thermal head and the scanning direction of the thermal head, and Figure 9 is a diagram showing the energization. A diagram of the temperature distribution that occurs when no correction is made based on the arrangement of elements, Figure 10 is a diagram showing the pattern of current applied by the conventional control method, and Figure 11 is a diagram of the temperature distribution that occurs when the current pattern of Figure 1θ is applied. It is. 2.3, 4.5...Resistance ladder circuit, 7, 8, 9°t
o, it... Addition circuit, 13... Constant current control circuit, 20... Resistor, V ELM... Main control signal, V
RD...base voltage, Va...resistance ladder circuit output (temperature distribution correction potential), VcU*...conducting current control voltage, V REF...reference voltage. Agent Yoshi Morimoto Buddhist painting 2 r --------1"'1 3rd fig. 4 den-3 t1 L12 Q 7 1 to 5 fig. l-2 fIψ2 2nd to χ-2 9 -l 9π4t Futsu 2

Claims (1)

[Claims] 1. A thermal head drive device for a printing device that performs printing using heat generation, in which a resistor ladder circuit is provided between print control units of a thermal head that forms a print figure, and a resistor ladder circuit is generated according to a voltage division ratio between the resistor ladder circuit and the resistor ladder circuit. an adding circuit that superimposes the temperature distribution correction level generated by the adding circuit on the reference current level; and a resistor of the thermal head, which is provided for each printing control unit to maintain a constant current proportional to the control level generated by the adding circuit; 1. A thermal head drive device comprising: a constant current control circuit for driving a thermal head; and configured to vary a current value applied to a resistor of a thermal head according to a voltage division ratio of a resistor ladder circuit.