JPH05301370A - Thermal head - Google Patents

Abstract

PURPOSE:To attempt improvement of printing quality by making energy to be applied to a heating element constant by a method wherein printing density is prevented from being decreased as a result of a drop in printing voltage to be impressed on the heating element. CONSTITUTION:When printing voltage to be impressed to a heating element 21 is decreased by a resistance value (r) of a common electrode line 22, a control pulse CP of a pulse width corresponding thereto is generated from a pulse generating means 40. A NAND gate 50 increases or decreases a pulse width of a strobe signal SB from an outside according to the pulse width of the control pulse CP, and gives a strobe signal SBa of a pulse width corresponding to a drop in voltage of the common electrode line 22 to a gate circuit 33 via an inverter 34. Thereby, the gate circuit 33 is opened, and an output of a latch circuit 32 is given a switching element 23. Only the switching element 23 corresponding to the heating element 21 to be printed is turned on for specific time. Thus, always constant energy is applied to the heating element 21.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal head used for recording in a facsimile, a printer or the like, and more particularly to a pulse width setting method for strobe signals for driving the thermal head.

[0002]

2. Description of the Related Art FIG. 2 is a block diagram showing a configuration example of a conventional thermal head. With this thermal head,
A plurality of heating elements 1 are formed in a row on the substrate, one end of each heating element 1 is connected to the common electrode line 2, and the other end is connected to the switching element 3. The common electrode line 2 has a resistance value represented by a symbol r, and is connected to the + electrode of the driving power supply 4 through the resistance value r. The negative electrode of the driving power source 4 is commonly connected to the ground side of each switching element 3, and a passage for a current flowing through each heating element 1 is formed. Further, a shift register 11 that converts the serial data DA for printing into parallel data in synchronization with the clock signal CK is provided. A latch circuit 12 for latching parallel data output from the shift register 11 by a latch signal LH is connected to the output side of the shift register 11, and the latch circuit 12 is provided.
Gate circuit 1 consisting of a plurality of AND gates on the output side of
3 is connected. The gate circuit 13 is gate-controlled by the output of the inverter 14 which inverts the strobe signal SB (usually "L" active) supplied from the external circuit, and supplies the output of the latch circuit 12 to the switching element 3 to provide the switching element 3 therewith. Is a circuit that turns on and off.

Next, the operation will be described. Clock signal CK
And the serial data DA for printing is the shift register 11
Is supplied to the shift register 11, the shift register 11 sequentially shifts the serial data DA in synchronization with the clock signal CK. When the serial data DA is stored in the shift register 11, the shift register 11 is driven by the latch signal LH.
Parallel data is latched by the latch circuit 12 and sent to the gate circuit 13. In the gate circuit 13, the gate is opened by the signal obtained by inverting the strobe signal SB in the inverter 14, and the output of the latch circuit 12 is selectively supplied to the switching element 3 so that only the switching element 3 corresponding to the heating element 1 to be printed is provided. To turn on. This allows
An electric current flows through an arbitrary heating element 1, and characters, figures, etc. are recorded on a thermosensitive recording paper (not shown).

[0004]

However, the thermal head having the above structure has the following problems. In the conventional thermal head, the resistance value r of the common electrode line 2 is
As a result, a voltage drop occurs and the printing voltage applied to each heating element 1 becomes a value lower than the output voltage of the driving power supply 4. further,
The printing voltage applied to the heating elements 1 varies greatly depending on the number of heating elements 1 that are energized. Therefore, not only the print density on the recording paper becomes light, but also the print density for each line changes. The reason is that the print density of the thermal head depends on the energy applied to the heating element 1, that is, the applied power × strobe pulse width. Therefore, when the strobe pulse SB is constant, a decrease in the printing voltage applied to the heating element 1 results in a decrease in the power applied to the heating element 1 and a decrease in the energy applied to the heating element 1.

As described above, in the conventional thermal head,
Not only is the print density on the recording paper reduced, but the print density changes line by line. Moreover, the decrease in the print density greatly decreases as the number of heating elements 1 that are simultaneously energized increases. Such a problem deteriorates the print quality of the recording, and can be said to be a fatal defect particularly in applications such as gradation recording. The present invention, as a problem that the above-mentioned conventional technology has,
The present invention provides a thermal head which solves the problem that the print density is reduced due to the reduction of the print voltage applied to the heating element.

[0006]

In order to solve the above-mentioned problems, a first invention is to provide a plurality of printing heating elements connected in parallel to a common electrode line for supplying a power supply current, and a strobe signal provided from the outside. A gate circuit that is gate-controlled by the output circuit for outputting data for printing, and a plurality of gate circuits that are connected in series to each of the heating elements and that turn on / off by the output of the gate circuit to switch the power supply current flowing through the heating element. A thermal head having a switching element is provided with the following means. That is, in the present invention, a pulse that generates a control pulse that is synchronized with the strobe signal, has a pulse width smaller than the pulse width of the strobe signal, and has a pulse width that changes corresponding to the printing voltage applied to the heating element. Generating means and gate means for controlling the conduction state of the strobe signal by the control pulse to gate-control the gate circuit are provided.

In the second invention, the pulse generating means of the first invention is constituted by a monostable multivibrator and the gate means is constituted by a NAND gate. Here, the monostable multivibrator is a circuit that is triggered by an edge of a strobe signal supplied from the outside and generates a control pulse having a pulse width corresponding to the printing voltage of the heating element. N
The AND gate is a circuit that obtains the NAND of the control pulse and the strobe signal.

[0008]

According to the first aspect of the invention, since the thermal head is constructed as described above, when the printing voltage applied to the heating element changes, the change is detected by the pulse generating means, and the pulse width corresponding to the change is detected. The control pulse is output to the gate means. Based on the control pulse, the gate means changes the pulse width of the strobe signal from the outside, which is a timing signal for energizing the heating element, according to the voltage drop of the common electrode line. The gate circuit is opened by the strobe signal having the changed pulse width, and the on / off operation of the switching element is controlled by the printing data. As a result, a constant amount of energy is always applied to the heating element to prevent the print density from decreasing.

According to the second invention, when the strobe signal is supplied from the outside, the monostable multivibrator is triggered by the rising or falling of the strobe signal, and the control pulse having the pulse width corresponding to the printing voltage is generated. Generated and applied to the NAND gate. In the NAND gate,
The logical AND of the control pulse and the strobe signal from the outside is obtained, a strobe signal having a pulse width corresponding to the change of the printing voltage is generated, and the opening / closing operation of the gate circuit is controlled. Therefore, the above problem can be solved.

[0010]

1 is a schematic block diagram of a thermal head showing an embodiment of the present invention. In this thermal head, a plurality of heating elements 21 are formed in a row on a substrate made of alumina or the like, and one end of each heating element 21 is commonly connected to a common electrode line 22 and the other end is each Each is connected to the switching element 23. Each switching element 23 is composed of, for example, an NPN transistor 23a, and its collector is commonly connected to the heating element 21 and its emitter is commonly connected to the negative electrode of the driving power supply 24. The common electrode line 22 has a resistance value represented by a symbol r as in the conventional case, and the driving power source 24 is supplied through the resistance value r.
Is connected to the + electrode of.

Further, as in the conventional case, a shift register 31 for converting the printing serial data DA into parallel data by the clock signal CK is provided, and the latch circuit 32 is connected to the output side thereof. The latch circuit 32 is a circuit for latching the parallel data of the shift register 31 based on the latch signal LH, and the gate circuit 33 is connected to the output side thereof. The gate circuit 33 includes the heating element 2
It is composed of the same number of 2-input AND gates 33a as one, one input side of which is connected to the output side of the latch circuit 32 and the other input side is connected to the output side of the signal inverting inverter 34, and The output side of the AND gate 33a is connected to the base of each transistor 23a.

The thermal head of this embodiment differs from the conventional one in that the pulse generating means 40 for detecting the printing voltage applied to the heating element 21 and generating the control pulse CP having a pulse width corresponding to the printing voltage, and the control pulse. That is, the gate means (for example, a 2-input NAND gate) 50 that takes the logic between the CP and the strobe signal SB supplied from the external circuit is added. The pulse generating means 40 includes a heating element 21.
The resistors 41 and 42 for dividing the printing voltage applied to are connected to the time constant determining terminal of the monostable multivibrator 44 together with the capacitor 43. The monostable multivibrator 44 is a circuit that is triggered by, for example, the rise of the strobe signal SB supplied from an external circuit, and outputs a control pulse CP having a predetermined pulse width. This monostable multivibrator 44
For example, TYPE SN74221 from TI (Texas Instruments) is suitable. 2 inputs N connected to the output side of the monostable multivibrator 44
The AND gate 50 is a circuit that obtains the NAND of the control pulse CP and the strobe signal SB from the external circuit, and supplies the strobe signal SBa with the controlled pulse width to the inverter 34.

Next, the operation of FIG. 1 will be described with reference to FIG. FIG. 3 is a time chart showing the operation of the thermal head of FIG. In FIG. 3, Ts is the pulse width of the strobe signal SB, Tm is the pulse width of the control pulse CP, T
st is the pulse width of the strobe signal SBa output from the NAND gate 50. In FIG. 1, when the clock signal CK and the serial data DA for printing are supplied from the outside to the shift register 31, the shift register 31 sequentially shifts the serial data DA in synchronization with the clock signal CK. When the serial data DA is stored in the shift register 31, the latch signal LH causes the parallel data in the shift register 31 to be latched by the latch circuit 32 and sent to the gate circuit 33. In the gate circuit 33, each AND gate 33 is controlled by the output of the inverter 34.
a is opened, and the output of the latch circuit 32 is supplied to the base of the transistor 23a forming each switching element 23. As a result, only the transistor 23a corresponding to the heating element 21 to be printed is turned on, any heating element 21 is energized by the driving power source 24, and characters, figures, etc. are recorded on a recording sheet such as a thermosensitive recording sheet (not shown). .. Here, the input of the inverter 34 is a strobe signal having a constant pulse width supplied from an external circuit in the conventional case, but in the present embodiment, a strobe signal SB having a constant pulse width supplied from the external circuit and a control pulse. It is a NAND output with CP. This operation will be described with reference to FIG.

The voltage excluding the voltage drop due to the resistance value r of the common electrode line 22, that is, the true voltage (printing voltage) applied to the heating element 21 is divided by the resistors 41 and 42, and the divided voltage is obtained. Are applied to the time constant determination terminal of the monostable multivibrator 44. When the strobe signal SB having the pulse width Ts is supplied from the external circuit, the monostable multivibrator 44 is driven by the rise of the strobe signal SB.
Is triggered. Then, the monostable multivibrator 4
4, the control pulse C having a pulse width Tm having a time constant determined by the voltage dividing resistors 41 and 42 and the capacitor 43.
P is generated and applied to the NAND gate 50.

In the NAND gate 50, the strobe signal SB having the pulse width Ts and the control pulse CP having the pulse width Tm are provided.
Pulse width Tst (= Ts-Tm)
Output the strobe signal SBa. This strobe signal SBa is inverted by the inverter 34, and the gate circuit 33
Is open and the output of the latch circuit 32 is the switching element 23.
Is supplied to the strobe signal SBa, the pulse width Tst of the strobe signal SBa becomes a pulse width for turning on the switching element 23. For example, when the number of heating elements 21 to be energized at the same time is large, a large amount of current flows in the common electrode line 22, so that the voltage drop due to the resistance value r becomes large. Therefore, the printing voltage applied to the heating element 21 decreases. However, the control pulse CP output from the monostable multivibrator 44
Has a reduced pulse width Tm due to the reduced printing voltage. Therefore, the pulse width Tst of the strobe signal SBa output from the NAND gate 50 becomes large. Therefore, the energy applied to the heating element 21 can be made constant by the reduction of the printing power due to the reduction of the printing voltage applied to the heating element 21 and the effect of increasing the energization time due to the increased strobe pulse width Tst. ..

On the other hand, when the number of heating elements 21 to be energized at the same time is small, the voltage drop due to the resistance value r is small, so that the pulse width Tm of the control pulse CP output from the monostable multivibrator 44 becomes large. Therefore, the pulse width Tst of the strobe pulse SBa decreases. Therefore, the energy applied to the heating element 21 becomes constant due to these effects. As described above, in the present embodiment, when the electric power applied to the heating element 21 decreases due to the voltage drop of the resistance value r of the common electrode line 22, the pulse width Tst of the strobe signal SBa corresponding to the voltage drop. Since it increases, the energy applied to the heating element 21 becomes constant. Therefore, high quality recording can be performed.

The present invention is not limited to the illustrated embodiment, and various modifications can be made. Examples of such modifications include the following. (A) The pulse generating means 40 is composed of the monostable multivibrator 44, but other circuit configurations may be used as long as the pulse width of the control pulse CP generated corresponding to the printing voltage applied to the heating element 21 is changed. Good. In addition, the NAND gate 50 is ANDed according to the polarity of the control pulse CP.
It may be configured by other gate means such as a gate. (B) The gate circuit 33 in FIG. 1 may be configured by another gate means such as a NAND gate, or the switching element 23 may be configured by another transistor such as a field effect transistor.

[0018]

As described in detail above, according to the first aspect of the invention, when the power applied to the heating element is lowered due to the voltage drop of the resistance value of the common electrode line, the voltage drop is dealt with. Since the pulse width of the strobe signal is increased by the pulse generating means and the gate means, the energy applied to the heating element can be kept constant. Here, with a strobe signal supplied from the outside as a reference, a strobe pulse width corresponding to the printing voltage is generated by a control pulse having a pulse width smaller than the pulse width, and the switching element is turned on and off by the strobe pulse width. I am trying to make it work. Therefore, high-quality recording can be performed by simply replacing the conventional thermal head peripheral circuit with the thermal head of the present invention. According to the second invention, the pulse generating means is constituted by a monostable multivibrator, and the gate means is N-type.
Since it is configured by the AND gate, the strobe pulse width corresponding to the change of the printing voltage can be accurately generated with a simple configuration.

[Brief description of drawings]

FIG. 1 is a schematic configuration block diagram of a thermal head showing an embodiment of the present invention.

FIG. 2 is a schematic configuration block diagram of a conventional thermal head.

FIG. 3 is a time chart showing the operation of FIG.

[Explanation of symbols]

 21 heating element 22 common electrode line 23 switching element 24 driving power supply 31 shift register 32 latch circuit 33 gate circuit 40 pulse generating means 44 monostable multivibrator 50 NAND gate CP control pulse SB, SBa strobe signal

Claims (2)

[Claims] 1. A plurality of heating elements for printing connected in parallel to a common electrode line for supplying a power supply current, a gate circuit for outputting printing data by being gate-controlled by a strobe signal given from the outside, In a thermal head provided with a plurality of switching elements that are respectively connected in series to the heating elements and that are turned on and off by the output of the gate circuit to switch the power supply current flowing to the heating elements, in synchronization with the strobe signal, Pulse generating means for generating a control pulse having a pulse width smaller than the pulse width of the strobe signal, the pulse width varying in accordance with the printing voltage applied to the heating element; and the conduction of the strobe signal by the control pulse. And a gate means for controlling the state to control the gate circuit. Maruhead. 2. The pulse generating means comprises a monostable multivibrator which is triggered by an edge of the strobe signal and generates a control pulse having a pulse width corresponding to the printing voltage, and the gate means comprises the control pulse. The thermal head according to claim 1, wherein the thermal head comprises a NAND gate that calculates a NAND of the strobe signal and the strobe signal.