JPS61286170A - Thermal head drive circuit - Google Patents

Abstract

PURPOSE:To obtain high-quality printing at high speeds by providing a preheating circuit using a thermistor and a serial resistor for a heating drive circuit for printing. CONSTITUTION:Switching circuits S1-S24 are turned ON by selection signals SL1-SL24, and electric currents I1-I24 flow to heat selected heating elements TH1-TH24 for printing. Currents Ir1-Ir24 for preheating flow through resistors R1-R24 connected to PTC thermistor RTH and thermal heads TH1-TH24 not selected. The currents Ir1-Ir24 for preheating are varied with change in the resistance values of PCT thermistor RTH having positive temperature coefficients based on the temperature change of thermal head 11 due to the heating conditions of the heating elements TH1-TH24 for printing. By providing the preheating circuit, high-quality printing can be obtained at high speeds, and a simplified flow-cost thermal head drive circuit can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a thermal head drive circuit for a thermal printer or a thermal transfer type thermal printer, and more particularly to a thermal head drive circuit for a copying type thermal printer having a preheating function.

(Prior art) As a conventional preheating method for thermal printers when the power is turned on or when the temperature is low, the print head is preheated by flowing current with a minute pulse width even when no printing is being performed on the printing heating element of the print head. What it does is known. However, with preheating of only the print head, for example, when printing on copy thermal paper or high-speed printing, sufficient heat transfer does not occur to the second sheet of paper, resulting in blurred characters and poor print quality. There was a drawback.

Thermal transfer printers also have similar drawbacks.

The coloring principle of a copying thermal printer is as shown in Fig. 2. When an applied pulse is applied to the heating element 2 of the print head 1, it generates heat, first coloring the first sheet of paper (thermal paper) 3, and then coloring the first sheet of paper (thermal paper). Thermal transfer ink 4 applied to the back side of the first sheet of paper is melted and transferred to plain paper 5. In a copying thermal printer, the relationship between the energy (E) applied to the first sheet of paper (thermal paper) and the second sheet (thermal transfer) and the ambient temperature (T) is as shown in FIG. Here, 6 indicates the energy E1 that causes the first sheet to develop color, and 7 indicates the energy E2 that causes the second sheet to develop color. 1st sheet of paper and 2nd sheet
In order to cause sufficient color development and transfer to the second sheet of paper, it is necessary to apply more than the energy required for the second sheet of paper (E2) to the print head, but in order to apply appropriate energy to the paper, Consideration must be given to detecting the ambient temperature using a thermistor or the like and feeding it back to the applied pulse width.

(Problem to be solved by the invention) However, when the printing speed is 30 characters/second, the period of the applied pulse is as short as about 2 ms, so especially at low temperatures and when printing the first character, sufficient heat is required. There was no transfer, and color development and transfer were insufficient.

The electrical energy (E) is E=wt as shown by the formula below.
= M2t (joules) E = electrical energy W = power t; application time ■ = applied voltage R; heating element resistance value By increasing the applied voltage (V), the electrical energy of the heating element (
Although E) can be obtained, increasing the applied voltage will significantly reduce the life of the print head.

In addition, in order to perform preheating using minute pulses,
This method requires a control circuit for instructing the pulse width and the number of pulses, and has the disadvantage of requiring a complex and expensive drive circuit including logic circuits and comparators. Furthermore, when a thermal head with a built-in thermistor or a heating element for preheating is used, there is a drawback that the thermal head itself is expensive.

The purpose of this invention is to improve the print quality and obtain high-speed printing by improving the preheating method of thermal printers (including thermal transfer methods). This is due to the addition of a preheating circuit.

A further object of the present invention is to provide a simple and inexpensive thermal head using a conventional thermal head and control circuit, without adding a thermal head with a special structure or a complicated control circuit.

(Means for Solving the Problems) In order to solve the above-mentioned problems, the present invention provides a thermal printer having a thermal print head in which a plurality of heating elements for printing are arranged, in which current is supplied to each heating element for printing. It is characterized by comprising a resistor for minute heating current connected in parallel with each switching circuit that connects and disconnects, and a thermistor commonly connected to each resistor.

(Function) According to the present invention having the above-described configuration, the resistance value of the thermistor changes in accordance with the temperature change of the printing heating element due to the connection/disconnection of the current supplied to the printing heating element by each switching circuit. The current supplied for preheating each heating element for printing is controlled by a resistor for a minute heating current and a thermistor connected in parallel with each switching circuit.

Therefore, the present invention can solve the above problems and provide a thermal head drive circuit that can achieve high-speed printing and high print quality.

(Example) Hereinafter, one embodiment of the present invention will be described based on the drawings.

FIG. 1 is a circuit diagram showing an embodiment of the present invention. In the figure, 11 is a thermal head, and a heating element T for printing.
HI to TH24 are arranged in one row. Reference numeral 12 denotes a switching unit which connects and disconnects the currents 1 to 124 flowing to each printing heating element TI (1 to TH24) of the thermal head 11, and connects and disconnects the printing heating element THI to selected by the selection signal SLI to 5L24. The switching circuits 81 to S24 corresponding to TH24 are in the 0N10FF state.13
is a preheating circuit, PTC (posiWe Tempe
one end of each of the resistors R1 to R24 is connected to the thermistor RTH, and the other end of each of the resistors R1 to R24 is connected to the switching unit 12.
The switching circuits 81 to S24 are connected in parallel. Further, the PTC thermistor RTH has a resistance value that changes depending on the heat generation temperature of the thermal head 11 or the ambient temperature, and in this embodiment, one with a positive temperature coefficient is used.

Next, the operation of this embodiment will be explained.

As a normal printing operation, switching circuits 81 to S24 are turned on by selection signals SL1 to SL24, currents 1 to I24 flow to printing heating elements THI to TH24, and the selected printing heating elements TRI to TH24 are turned on. generates a fever. Therefore, if one or more of the selection signals SLI to S and L24 are not supplied to the switching circuits 81 to S24, the unselected thermal heads THI to TH24 will have the PTC thermistor RTH and its unselected thermal heads THI to TH24. Thermal head THI~TH24
Preheating currents Irl to Ir24 flow through resistors R1 to R24 connected to the resistors R1 to R24, and preheating is performed. In addition, the preheating currents Irl to Ir24 are applied to the heating elements TH1 to TH for printing.
Based on the temperature change of the thermal head 11 due to the heat generation state of 24, the resistance value of the PCT thermistor RTH changes.

Furthermore, the difference between the electrical energy (Ep) required for printing and the electrical energy (EH) required for preheating is Ep >
The relationship EH is established, and as a method to satisfy this relationship, the preheating currents Irl to Ir24 flowing through the resistors R1 to R24 shown in FIG.
), it is possible to share the power supply voltage. In other words, in order to prevent color development (or transfer) due to heat generation during preheating, it is necessary to select the preheating series resistance value Rn so as not to cause color development (or transfer) according to the following formula.

Rn > Eta R7H(R(zN')Eta" Electrical energy required for color development per unit time ■ = Applied voltage Rn = Series resistance value for preheating RTH (uxN) = P Minimum resistance value of CT thermistor In this example, Although the voltage applied to the preheating circuit 13 shown in Figure 1 and the driving voltage during printing are the same, the same effect can be obtained even if the voltage is different, and the mounting position of the thermistor can be adjusted depending on the surrounding environment. It can be placed separate from the print head to detect the temperature or internal temperature of the printer, or it can be mounted inside the print head to detect the temperature of the print head itself. The same effect can be obtained by using a preheating circuit in which a thermistor is placed in parallel with each switching circuit and a resistor is connected in series.

Furthermore, if it is implemented as a print head preheating circuit for a thermal transfer printer that uses a thin film ribbon instead of the single sheet of paper (thermal paper) in a decoding thermal printer, it can achieve the same effect as that of the thermal printer, and generate less heat. Although the present invention is applied to a serial thermal print head having a vertically aligned body, it can be similarly used as a print head preheating circuit for a line thermal printer device with a fixed print head.

Note that the above preheating is a circuit that is constantly performed while power is supplied to the printer device, but if the circuit temporarily stops preheating when the printer is out of service for a certain period of time, the life of the printing heating element can be further extended. It has a prolonging effect.

(Effects of the Invention) As explained above, according to the present invention, the print head can be preheated by providing a preheating circuit using a thermistor and a series resistor in the printing heating element drive circuit. Therefore, high-speed printing and high printing quality can be obtained, and since there is no need to add a thermal head with a special structure or a complicated control circuit, a simple and inexpensive thermal head drive circuit can be provided.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the present invention, FIG. 2 is a diagram showing the printing principle of decoding type thermal printing, and FIG. 3 is a diagram showing the relationship between applied energy and ambient temperature. 1... Print head, 2... Heating element, 3... First sheet of paper (thermal paper), 4... Thermal transfer ink, 5... 2
Sheet paper (plain paper), 11... Thermal head, 12
...Switching section, 13...Preheating circuit.

Claims (3)

[Claims] (1) In a thermal printer having a thermal print head in which a plurality of heating elements for printing are arranged, a small heating current is connected in parallel with each switching circuit that connects and disconnects the current supplied to each heating element for printing. It is equipped with a resistor and a thermistor commonly connected to each resistor, and preheats each printing heating element by the resistance value of the thermistor that changes according to the temperature change of the printing heating element due to connection/disconnection of each switching circuit. A thermal head drive circuit characterized by: (2) The thermal head drive circuit according to claim 1, wherein the thermistor has a positive temperature coefficient. (3) The thermal head drive according to claim 1, wherein the resistance value of the resistor is set so that the heat generation temperature during preheating of the printing heating element becomes a temperature that allows printing. circuit.