JPH06246955A - Apparatus for controlling thermal head - Google Patents

Abstract

PURPOSE:To make the apparatus able to maintain voltage given to a group of heat registers, and to follow promptly a rapid change of photographic printing density, and simplify its constitution. CONSTITUTION:The control apparatus for a thermal head can control photographs and a density of the photographs by applying terminal voltage to a plurality of terminals of heat resistors in an alignment-connected manner, selecting a heat resistor to be conducted, and controlling a period of voltage application at the time, wherein the voltage control circuit 100 is adapted to obtain the total amount of resistor selecting pulses for applying voltage to each heat resistor Rn-R1 at a set time unit, and make the electric potential of terminal voltage variable in proportion to changes of the total.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a printer for printing with a thermal head, and more particularly to a thermal head controller.

[0002]

2. Description of the Related Art In a printer that prints using a thermal head, the printing density is controlled by the application time of a voltage applied to a plurality of heating resistors of the thermal head. The voltage application time is set by an application pulse that controls on / off of the transistors connected in series to the plurality of heating resistors. FIG. 3 shows a conventional thermal head and its peripheral circuits.

The heating resistors Rn, ... R2, R1 are arranged in parallel, one end of each of which is a transistor Qn,
... connected to the collectors of Q2 and Q1. The other ends of the heating resistors Rn, ... R2, R1 are commonly connected to the emitter line (positive power supply line 1) of the power supply transistor Tr via the resistor r1. Furthermore, the other ends of the heating resistors Rn, ... R2, R1 are commonly connected to the connection point of the resistors r11 and r12 that form the voltage detection circuit. Next, the common emitters of the transistors Qn, ... Q2, Q1 are grounded via the resistor r2 and connected to the negative power supply terminal 12 through the negative power supply line 2.

A series circuit of resistors r11, r12, r13 and r14 is connected between the positive power supply line 1 and the negative power supply line. The connection point between the resistors r12 and r13 is connected to the inverting input terminal of the operational amplifier 13, and the connection point between the resistors r13 and r14 is connected to the non-inverting input terminal of the operational amplifier 13 via the capacitor 14. The output terminal of the operational amplifier 13 is connected to the base of the transistor Tr, and the current of the transistor Tr is controlled in accordance with the change in the voltage VH applied to the heating resistor group.
Compensates to keep a constant value. Transistor Tr
The collector of is connected to the power supply terminal 11 on the positive side.
ON / OFF control of the transistors Qn, ... Q2, Q1 (control for determining the voltage application time to be applied to the heating resistor) is performed by a pulse applied to terminals Pn, ... P2, P1 led to the bases of the transistors Qn ,. Done by.

The heating resistors Rn, ... R2, R1, the transistors Qn, ... Q2, Q1, and the resistors r1, r2 are the equalizing circuits of the thermal head unit, and the resistors r11 to r14, the operational amplifier 13, the transistor Tr. The circuit formed by, for example, constitutes a constant voltage circuit.

Next, the function and operation will be described. In a printer using a thermal head, the print density is determined by the voltage application time applied to the heating resistor. When the print density is increased, the application period of the pulse applied to the base is lengthened, and when the print density is decreased, the pulse application period is shortened. In some pixels, there is no image, so the transistor corresponding to this portion is turned off.

Now, assuming that the heating resistors Rn, ... R2, R1 form pixels for one line of the print, the number of transistors turned on varies depending on the print content and varies depending on the line. Then, the voltage VH applied to the heating resistors Rn, ... R2, R1 in parallel changes. Then, even if pulses of the same period are given, the print densities cause different density unevenness between the lines. Therefore, in order to prevent such a problem, the constant voltage circuit operates, detects the fluctuation of the voltage VH, controls the voltage of the power supply line 1 through the operational amplifier 13 and the transistor Tr, and keeps the voltage VH constant. I am trying to become.

[0008]

According to the above-described conventional thermal head device, the unevenness of density is caused by the operation of the heating resistor group in which the voltage is selectively applied and the application period is controlled to be different. In order to prevent (keep the voltage VH constant), the lead wire 15 for remote sensing,
16 is provided and the fluctuation detecting circuit of the voltage VH is connected.
For this reason, there is a problem that the heating resistor peripheral circuit becomes complicated due to the connection of the lead wires 15 and 16, and the configuration of the head itself becomes large. Furthermore, since the constant voltage circuit is of a feedback type, a rapid change between lines,
For example, when there is a change from gray close to white to black, the operation cannot follow the change, and there is a problem that VH cannot be stably maintained at a desired value.

Therefore, the present invention provides a highly reliable thermal head control device which can stably maintain the voltage applied to the heating resistor group and can quickly follow a rapid change in printing density. The purpose is to provide.

[0010]

SUMMARY OF THE INVENTION According to the present invention, a terminal voltage is applied to the terminals of a plurality of heat generating resistors, a heat generating resistor that conducts is selected, and the voltage application time at that time is controlled. In the controller of the thermal head capable of controlling the printing and the printing density by, the sum of the resistor selection pulses for applying the voltage to each heating resistor is obtained in a predetermined time unit,
There is provided means for varying the potential of the terminal voltage in proportion to the change in the total sum.

[0011]

According to the above-mentioned means, the power supply voltage is controlled according to the number of heating resistors which are conductive and generate heat, and a constant voltage is always applied to the parallel circuit of the heating resistors.

[0012]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1A shows an embodiment of the present invention.
The heating resistors Rn, ... R2, R1 are arranged in parallel, and one end of each of them is a transistor Qn ,.
Connected to the collector. Further, the heating resistor Rn, ...
The other ends of R2 and R1 are commonly connected to the positive power supply terminal 21 via a resistor r1. Next, the transistor Qn,
The common emitters of Q2 and Q1 are grounded via the resistor r2 and are also connected to the negative power supply terminal 22.
Base terminals Pn of the transistors Qn, ... Q2, Q1
A pulse for turning on / off the transistor is supplied to P2 and P1. Depending on the pulse period supplied to this terminal, the current time of the corresponding heating resistor is different and the print density by the corresponding pixel can be controlled. The voltage control circuit 10 is connected to the positive power supply terminal 21 and the negative power supply terminal 22.
The control voltage from 0 is supplied.

First, the operating principle of this circuit will be described with reference to FIG. Now, the heating resistors Rn, ... R2, R1
Is for one line, and each resistor constitutes a pixel. Here, when printing the first line, for example, as shown in FIG.
It is assumed that the pixel density of the heating resistor R2 is high, the pixel density of the heating resistor R2 is low, and the pixel density of the heating resistor R1 is intermediate. Then, the pulse period given to each base terminal differs depending on the active state. Then, each pulse rises and falls at the same time through the first line, and the time varies.

When the number of pulses is measured every unit time, it can be expressed stepwise as shown by (P1 + P2 + ... Pn) in the figure. This means that the number of heat generating resistors connected in parallel, in which current flows, gradually decreases. At the same time, the applied voltage VH applied to the resistor group and any countermeasures are taken. It means that it increases gradually when not applied. So with this device,
The power supply voltage V applied from the voltage control circuit 100 is controlled so that the applied voltage VH does not change (becomes constant) with the change in the number of elements.

The voltage control circuit 100 controls the value of the voltage V by detecting the change in the number of pulses in units of a predetermined time. In the case of this example, the value of the voltage V is reduced in the first line, similarly to the change waveform of the pulse number.

FIG. 2 shows a configuration example of the voltage control circuit 100. In the figure, 101 is a level switching circuit, which can switch the level of the power supply voltage and supply it to the power supply terminal of the thermal head unit 200 with the waveform as shown in FIG. A control signal for level switching is given from the level setting logic circuit 102. The level setting logic circuit 102 measures the pulse (the number of pulses given to the base terminals Pn, ... P2, P1) generated by the pulse generation circuit 103 in a predetermined time unit, and creates a control signal corresponding to the number. is doing. The pulse generation circuit 103 is
The density of each part of the image for one line (density in each pixel) is determined according to the content of the image data, and a pulse corresponding to this is supplied to the corresponding transistor. Also,
The pulse from the pulse generation circuit 103 is supplied to the time adjustment circuit 1
04 (corresponding to the determination time of the level setting logic circuit 102)
Via the base terminals Pn, ... P2, P1 of the transistor
Is given as described in FIG.

When the power supply voltage is changed in accordance with the number of pulses applied to the base terminal of the transistor and the voltage commonly applied to the heating resistor groups connected in parallel is kept constant, the power supply voltage can be controlled by various methods. Examples are possible. In the example of FIG. 2, the configuration in which the control is performed in real time has been described. However, when the image content is known in advance, the control waveform for each line is obtained, stored in the memory, and set as the printing speed. The power supply voltage may be controlled in synchronization.
Further, an overall pulse length adjusting function may be provided according to the characteristics of the thermal head or the thermal paper and the transfer speed characteristics depending on the use environment.

[0019]

As described above, according to the present invention,
The voltage applied to the heating resistor group can be kept stable,
It is possible to quickly follow rapid changes in printing density,
Performance and reliability can be improved. Further, unlike the conventional case, it is not necessary to connect the constant voltage circuit to the thermal head, and the structure can be simplified without complication.

[Brief description of drawings]

FIG. 1 is a diagram shown for explaining an embodiment of the present invention.

FIG. 2 is a diagram showing an example of a power supply voltage control circuit in FIG.

FIG. 3 is a diagram showing a conventional thermal head device.

[Explanation of symbols]

Qn to Q1 ... Transistor, Rn to R1 ... Heating resistor, r
1, r2 ... Resistors, 21, 21 ... Power supply terminals, 100 ... Voltage control circuit.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location B41J 3/20 115 C

Claims (1)

[Claims] 1. A print and print density can be controlled by applying a terminal voltage to the terminals of a plurality of heating resistors, selecting a heating resistor that conducts, and controlling the voltage application time at that time. In the controller for the thermal head, means for determining the sum of resistor selection pulses for applying a voltage to each heating resistor in a predetermined time unit and varying the potential of the terminal voltage in proportion to the change in the sum is provided. A thermal head control device characterized in that