JPS6221560A - Thermal printer - Google Patents

Abstract

PURPOSE:To obtain a thermal printer capable of continually maintaining a constant density independently of the kind of data to be printed, by a method wherein the temperature of a thermal head is indirectly detected by a condenser connected to feed transistors for heating elements. CONSTITUTION:Feed transistors 3 are turned ON to pass an electric current to heating elements 2, and simultaneously a capacitor 7 starts and electric discharge through diodes 5 and resistors 6. After a printing is started, the calorific value of a thermal head 1 is varied according to the number of conduction dot in conduction timings t1,...tn and simultaneously the discharge value from the capacitor 7 is varied, which implies that the charging level Vc is indicative of the temperature of the head 1 during printing. If the charging level Vc is inputted to the one end of a voltage comparison circuit 10 as the reference, and the power source Vh to be supplied to the thermal head is partially inputted to the input terminal of the compared voltage, the voltage always proportional to Vc can be obtained. In this construction, the applied voltage is so controlled as to ensure a suitable printing density on the basis of the temperature of the thermal head 1.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a thermal printer having a driving means that suppresses heat accumulation in a thermal head and realizes high-speed driving.

[Prior art]

In recent years, there has been an increasing demand for higher speeds, especially for serial type thermal printers, and various thermal head control methods have been used. As an example, the special public service 55-4Q63
History control, represented by No. 1, is most commonly used.

However, in order to store data that has already been printed using history control, there are limits to memory capacity and processing speed, and it has been impossible to process all the data printed on the - line. , Solid printing that occurs when printing image data exceeds the limits of history control, so especially in serial printers, it can cause dot irregularities such as trailing, fill-in of white characters, and even head temperature fluctuations. This has led to various problems such as a decrease in the life of the head due to an increase in .

Another method used is to bring a thermistor into contact with the thermal head and determine the energy to the heat generating element based on the detection result, but this method has problems such as uneven printing due to the slow detection response time. Was. Furthermore, it is difficult to mount the thermistor in close contact with the head, which increases the cost and causes inaccuracy in accuracy.

[Problem to be solved by the invention]

The present invention has been made for the purpose of eliminating the above-described drawbacks of conventional thermal printers and providing a driving device for a thermal printer that can always maintain a constant density regardless of the data to be printed. .

[Means for solving problems]

The driving means of the thermal printer according to the present invention includes a power supply transistor that supplies power to a heat generating element, a diode connected to an output terminal to the heat generating element for all or at least two or more of the power supply transistors. and a resistor, the other ends of the series circuit are commonly connected, and a capacitor has one end connected to this, and the voltage applied to the heating element is determined according to the charge level of the capacitor. It is characterized by

〔Example〕

The thermal printer of the present invention will be described in detail below using examples.

FIG. 1 is a schematic diagram of a drive circuit for a thermal printer according to the invention. 1 is a thermal head equipped with a heat generating element 2;
Reference numeral 3 corresponds to the heat generating element 2 and controls power supply to the power supply transistor, and 4 is a series circuit of a diode 5 and a resistor 6 connected to the output terminal of this power supply transistor, which is usually installed in all heat generating elements. Although it is preferable to do so, it is possible to omit it and reduce its number. The other ends of the plurality of series circuits 4 are commonly connected and connected to a capacitor 7. 8 is a power supply, 9 is a charging circuit for capacitor 7;
1.93 and 94.95 are resistors, and 92 is a transistor. The transistor 92 and the resistor 93.94 form a constant voltage means, and the potential thereof is VZ. The charge level of capacitor 7 is represented by Va. 10 is a voltage comparator circuit that obtains a voltage vh proportional to VC,
Vh divided into the reference source VC and the input terminal of the compared voltage
have. Resistor 12 and a type of resistor volume 1
3 forms a voltage dividing circuit that divides the output voltage vh. 14 is a stabilizing smoothing capacitor. Reference numeral 11 denotes a power transistor that is turned on and off according to the output of the voltage comparison circuit. Voltage comparison circuit 10. Resistor 12.13. Power transistor 11. The capacitor 14 constitutes a power supply circuit 15. 16 is an I10 port that outputs energization data to the power supply transistor 3. [Function] The present invention will be explained in detail using the second @(α)(b).

21 in Figure 2 (α) is a printed character.

represents a symbol. 22 indicates the timing of each dot row. FIG. 2 Ch) is a graph showing the change in the charge level Vc of the capacitor 7 when printing the above-mentioned characters and symbols 21, and 23 is its characteristic curve.

Generally, heat generation and cooling are approximated by charging and discharging a capacitor. The present invention uses this principle to indirectly and accurately detect the temperature of the thermal head.

At the same time that the power supply transistor 3 is turned on and the heat generating element 2 is energized, discharging is started from the capacitor 7 via the diode 5 and the resistor 6. This means an increase in temperature in the thermal head. When printing is started, at the energization timing t1...t%, the amount of heat generated by the thermal head changes depending on the number of energized dots, and at the same time, the amount of discharge from the capacitor 7 changes. For example, t.

The number of dots used at this point is 2, and the time constant for only discharging at this time is T1, the value of the capacitor 7 is C1, and the value of the resistor 6 is R, as shown below.

T, =OX-H However, in a region where the printing duty is high, such as in the period from t6 to t1o, the time constant T6 becomes as follows.

T,=OXa In other words, the higher the print duty, the smaller the time constant. This means that at 6tl or t, which directly represents the rate of temperature rise of the thermal head, the temperature of the thermal head rises slowly and the discharge time constant is large, but in the period from t6 to tto, The temperature of the thermal head increases rapidly, and the discharge time constant also decreases.

Furthermore, the temperature achieved is lower when only one dot is used, and higher when all dots are used. This is expressed on the circuit as follows.

The voltage Vz generated by the constant voltage means represents the room temperature, and the minimum potential v0 when two dots are on at the same time, such as at time t1, is the value of the resistor 95 R
6, R/ 2 V, = VzX 1. +,,.

becomes. However, in the period from t6 to t1o, if the minimum attainable potential is v2, then y, = Vz That is, the circuit of the present invention expresses exactly the same heat generation characteristics as the thermal head.

When the power supply to the heating element 2 is stopped, the charging circuit 9 starts charging the capacitor 7, and the final potential VZ
Return to. The thermal head also reaches ambient temperature at this time.

Considering the above, resistor 6,91,93゜94.95
．． By selecting the value of the capacitor 7, it is possible to approximate the time constants of charging for cooling and discharging for heating, respectively, and the charge level Vc represents the head temperature during printing.

This charge level, /I/Vc, is input as a reference to one end of the voltage comparison circuit 10, and the power supply vh to the thermal head is divided and inputted to the input terminal of the voltage to be compared, so that a voltage always proportional to VC is input. With this configuration, when Vc is low, that is, when the temperature of the thermal head is high, vh is small, and conversely, when Vc is high, vh is high, and the temperature of the thermal head 1 is high. The applied voltage is controlled so that the print density is always appropriate for the temperature.

The diode 5 is installed to prevent energization to neighboring heat generating elements when the power supply transistor 6 corresponding to the heat generating element 2 to be energized is turned on.

Further, due to the temperature characteristics of the diode, when the ambient temperature is high, the forward voltage decreases, so the discharge speed from the capacitor 7 becomes faster, which means that the influence of the ambient temperature on the thermal head has just been detected. Also, the transistor 92. The voltage of the constant voltage means formed by the resistors 95.94 also has some temperature characteristics and serves to compensate the print density of the thermal head with respect to the ambient temperature.

[Other Examples]

FIG. 3 is a schematic diagram of another embodiment of the drive device for a thermal printer according to the present invention, and the same parts as in FIG. 1 are designated by the same numbers. 31 is a diode array, and 32 is a resistor array. Half of the output terminals of the power supply transistor array 30 are randomly selected and a series circuit of a diode and a resistor is connected to each of them. 65 is A applicable to transformer
Used when using an O power supply. 34 is a rectifier diode, and a Zener diode 96 is used as a constant voltage means. If the input voltage Vi is obtained from the output of a stabilized power supply, the Zener diode may be a resistor. 18 is
The CPU outputs print data to the power supply transistor 35 and performs other functions of the thermal printer, such as motor control and communication with the host converter.

The power supply transistor array 50 is a head drive IC in which power supply transistors 35 including a front-stage amplification transistor 56 and a power transistor 37 are arranged in correspondence with the number of thermal heads.

In recent years, the number of heat generating elements in thermal heads has been increasing due to higher density and higher printing quality. Connecting all of the above series circuits of diodes and resistors to all of the 16-dot and 24-dot elements would increase costs and be difficult to implement, so a representative one of the elements was selected. A similar effect can also be achieved by doing so. At this time, if an odd-numbered or an even-numbered element is selected, the thermal head will receive erroneous information because of patterns that are relatively likely to exist in print information. For this reason, they are selected at random to minimize such errors. Furthermore, in the case of a kanji printer or the like, by using such a method, there is no difference in characteristics from the embodiment shown in FIG. 1 at all.

In this way, by indirectly detecting the temperature of the thermal head with the capacitor connected to the power supply transistor of the heating element, it is possible to control the energy applied to the thermal head.

Note that the power supply transistor used in the explanation of the present invention is N
Since it is a PN type transistor, discharging from the capacitor that seals the thermal head temperature means heat generation, but in the case of a PNP type transistor, charging means heat generation.

At this time, the direction of the diode is reversed.

〔Effect of the invention〕

As detailed above, according to the present invention, it is possible to indirectly but accurately detect the humidity of a thermal head in a thermal printer, and the humidity can be processed on a circuit board. Therefore, compared to conventional methods, it is possible to have an extremely simple configuration. In addition, when conventionally mounting a thermistor on a thermal head, there was a large variation in mounting depending on the accuracy, but with the present invention, it is extremely easy to select the accuracy of the parts, and the temperature of the thermal head can be simulated. It becomes possible to do so.

Furthermore, unlike methods using microcomputers, processing time can be saved, making it possible to realize high-speed printers, and countermeasures such as using an additional CPU for motor control were previously required. However, the ripple effects are significant, as thermal head control, print data input, motor control, etc. can all be processed by a single CPU.

Also, compared to the method of varying the energization time to the thermal head using similar means, the energization time is always the same, so
This has the advantage that the size of the dots does not change.

As described above, the present invention can be widely applied to thermal printers that thermally print on circular paper or plain paper via a thermal transfer ribbon.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of an embodiment of a drive device for a thermal printer according to the present invention. 1... Thermal head 3... Power supply transistor 5... Diode 6... Resistor 7... Capacitor Figure 2 (α) (b) is the main FIG. 3 is an explanatory diagram illustrating the operation of the thermal printer of the invention. FIG. 3 is a schematic diagram of another embodiment of the thermal printer of the present invention. Above Figure 1 7-Person-1-Co (a) Figure 2

Claims (1)

[Claims] having a plurality of heat generating elements, selectively energizing the heat generating elements;
In a thermal printer that prints on plain paper via thermal paper or a thermal film, a power supply transistor that corresponds to each of the heat generating elements and controls energization as a driving means of the thermal printer, and at least two of the power supply transistors are provided. A series circuit of a diode and a resistor each connected to one or more output terminals, and at least one capacitor commonly connected to the other end of the plurality of series circuits, and depending on the charge level of the capacitor. A thermal printer characterized in that a voltage applied to the heating element is determined.