JPH0453761A - Drive circuit of thermal head - Google Patents

Abstract

PURPOSE:To ensure that the variation of a voltage supplied to a thermal head does not result in the deterioration of print quality by connecting a pulse width correction circuit to a thermal head, then connecting a head drive voltage from the power supply terminal to a time constant circuit, and connecting a signal obtained by the time constant to a comparison circuit. CONSTITUTION:Pulse width correction circuits 30a to 30d for correction of the pulse width of a strobe signal to be supplied to a thermal head are connected to the thermal head 1. The pulse width correction circuits connect a head drive voltage from the power supply terminal of the thermal head 1, to a time constant circuit through the time constant circuit consisting of a CR circuit and a comparison circuit consisting of a comparator 21. Next, a signal obtained from the time constant circuit is entered into the comparison circuit and then is compared with a reference voltage. Finally a strobe signal with a pulse width corrected against the head drive voltage of the thermal head 1 is output to the thermal head 1 from the comparison circuit.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a driving circuit for a thermal head in a thermal printer.

(Prior Art) Conventionally, in a thermal printer, a thermal head consisting of a large number of heat-generating resistive elements arranged in a line is placed to face a thermal roll paper on a platen, and the heat-generating resistive elements are selectively driven to print dots. It is designed to do this.

FIG. 4 is a schematic diagram showing a cross section of a conventional line thermal printer.

In the figure, 1 is a line-type thermal head 7F constructed by arranging a large number of heating resistive elements in a row, 2 is a platen disposed opposite to the thermal head 1, and 3 is between the thermal head 1 and platen 2. It is a thermal roll paper that is fed.

The platen 2 is rotated by a pulse motor (not shown) and feeds the thermal roll paper 3. At this time, by selectively heating the heat-sensitive resistance element of the thermal head 1, characters and figures consisting of dots are printed on the heat-sensitive roll paper 3.

FIG. 5 is a circuit block diagram for operating a conventional thermal printer.

In the figure, 4 is a microprocessor, ROM, and RAM.
, a control unit consisting of a timer, etc., 5 is an ink face line for inputting print data from an external device, 6 is a control unit 17 for controlling the thermal head 1, pulse motor 7, etc.
0 port, and is connected to the control section 4 by a common bus 8.

The pulse motor 7 is connected to the I10 port 6 via a motor driver circuit 9 and is driven by the motor driver circuit 9.

Reference numeral 10 denotes a power supply device for supplying power to the control section 4 and the thermal head 1. Power is supplied to the thermal head 1 via a switch circuit 11, and the switch circuit 11 is turned on only when the thermal head 1 is used. The switch circuit 11 is controlled via the I10 port 6.

Next, the operation of the line thermal printer having the above configuration will be explained.

FIG. 6 is an operation time chart of a conventional line thermal printer.

When the pulse motor 7 (Fig. 5) is driven, the paper is fed from the t-1 line to the i line, and when printing on the i line is started, the print data is transferred to D in synchronization with the CLOCK signal.
It is input and transferred as an ATA signal.

When one line of print data is stored in a shift register circuit (not shown), a short pulse is input to the LATCH signal, and the print data stored in the shift register circuit is taken into the thermal head 1.

Next, the signals 1 to 1 with a constant pulse width are sent sequentially in a time-division manner, and the heating resistor element of the thermal head 1 is caused to generate heat in response.

With the above operations, the printing operation for one line is completed, and then the pulse motor 7 is rotated to feed the paper to the i+1 line, and the same operation is performed continuously to print characters and figures. .

It should be noted that ■ indicates the voltage of the power supply terminal of the thermal head 1.

(Problem to be Solved by the Invention) However, in the conventional thermal head drive circuit described above, when the 5TBI to punishment signal turns on (low level), the voltage drop of the wiring resistance and the voltage drop of the switch circuit 11 occur. , Voltage ■ due to voltage fluctuations due to load fluctuations in the power supply itself. decreases.

FIG. 7 is an explanatory diagram of voltage drop.

The figure shows the collector power supply ■ when a transistor is used in the switch circuit 11. It shows the collector emitter saturation voltage VCt for
5^, and the voltage drop in the switch circuit 11 (
Collector emic saturation voltage■. ) will be 1-2v.

If the voltage supplied to the thermal radar 1 fluctuates in this way, the print density will vary and the print quality will deteriorate, especially when printing with gradation in graphic printing. The decline is significant.

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of the conventional thermal head drive circuit described above, and to provide a thermal head drive circuit that does not cause the print quality to deteriorate due to fluctuations in the voltage supplied to the thermal head. shall be.

(Means for Solving the Problem) To achieve this, the present invention provides a line-type thermal head in which a large number of heat-generating resistive elements are arranged in a line, and a line in which printing is performed by selectively driving the heat-generating resistive elements. In a thermal head drive circuit of a thermal printer, a pulse width correction circuit for correcting the pulse width of a strobe signal supplied to the thermal head is provided and connected to the thermal head. The pulse width correction circuit is provided with a time constant circuit made of a CR circuit and a comparison circuit made of a comparator, and the head drive voltage of the power supply terminal of the thermal head is connected to the time constant circuit.

The signal obtained by the time constant circuit is input to a comparison circuit and compared with a reference voltage.

A strobe signal whose pulse width has been corrected in accordance with the head drive voltage of the thermal head is output from the comparison circuit to the thermal head.

(Function) According to the present invention, a line thermal printer is provided which has a line-type thermal head in which a large number of heat-generating resistive elements are arranged in a line as described above, and prints by selectively driving the heat-generating resistive elements. In the thermal head drive circuit, a pulse width correction circuit that corrects the pulse width of the strobe signal supplied to/from the thermal head is connected to the thermal head, and a time constant circuit and a comparison circuit are connected to the pulse width correction circuit. The head drive voltage of the power supply terminal of the thermal head is connected to a time constant circuit, and the signal obtained by the time constant circuit is connected to a comparison circuit, so that the head drive voltage supplied to the thermal head is When it changes, the signal output by the time constant circuit changes correspondingly.

The second change is input to the comparator circuit and given as a change in the pulse width of the strobe signal.

(Example) Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a diagram showing a driving circuit for a thermal head according to the present invention.

In the figure, 1 is a thermal head, 6 is a 170 boat for controlling the thermal head 1, a pulse motor, etc.

Further, 10 is a power supply device that supplies power to the thermal head l, 11 is a switch circuit, and the thermal head l is supplied with t.
When paying S for 1 sen, it is turned on upon receiving a signal from the I10 boat 6.

Between I10 port 6 and thermal head l, STB 1
~ “■ Correction circuit 30a to correct the pulse width of the signal
~30d is provided. Each pulse width correction circuit 308
30d, the head drive voltage V□ is fed back from the power supply terminal of the thermal head 1.

FIG. 2 is an internal circuit diagram of the pulse width correction circuit.

Only the pulse width correction circuit 30a is shown.

In the figure, 12 is an inverter circuit, STB, IN
The strobe pulse input to the terminal is inverted by the inverter circuit 12 and passed through the resistor 13 to the transistor 1.
Flows into the base of 4. A Zener diode 15, a resistor 16, and a capacitor 7 are connected in series to the vH terminal. The capacitor 7 and the resistor 16 form a time constant circuit, but when the transistor 4 is on, the load on the capacitor 7 is instantaneously discharged via the transistor 14. On the other hand, when the transistor 14 is turned off, charge is accumulated in the capacitor 7 via the Zener diode 15 and the resistor 16. The voltage vL of the capacitor 7 at this time is expressed as follows.

Here, VH is the head drive voltage of the thermal head supply terminal, 2 is the Zener voltage of the Zener diode 15,
constant. In addition, C1 is the capacitance of the capacitor 17,
R1 is the resistance value of the resistor 16, and L is the resistance value of the transistor 14.
This is the elapsed time since the switch was turned off. That is, voltage ■,
The time t for the voltage to reach a certain voltage value is ■. changes due to fluctuations in

The value of the power supply V is determined by the comparator 21 via the resistor IB.
is input to the (+) side of A voltage V obtained by dividing +5ν by a resistor 19.20 is input to the (-) side, and a comparator 21 compares both voltages vr and VL. Then, when V<VL, the comparator 21 is at a high level,
In the opposite case, a low level is output.

FIG. 3 is an operation time chart of the pulse width correction circuit.

As shown in the figure, when a low level signal is input to the STB and IN terminals, the inverter circuit 12 inverts and outputs the signal, and the transistor 14 becomes an on state ζ(v,==0ν), and the comparator 21 outputs Since VL <v, it becomes a low level, and the thermal head 1 is driven. At this time, the head drive voltage V at the power supply terminal of the thermal head l. gradually decreases due to a voltage drop in the switch circuit 11, a voltage drop due to wiring resistance, a drop in the voltage of the power supply, etc.

At time L0, when the STB and IN signals are changed from a low level to a high level, the transistor 14 is turned off, and charging of the capacitor 17 via the resistor 16 and the Zener diode 15 by the head drive voltages v and l begins. At this time, if the voltage value of the head drive voltage (■8) is high, the time for the value of V to exceed the value of V will be faster, and conversely, the head drive voltage (■) will exceed the value of V. If the voltage value of is low, it will be slow.

In Figure 3, STB and 0LIT are head drive voltages
.

becomes high level at the time t when it drops to vl, and ■2
Time to descend to 1. becomes a high level.

In other words, the drop in the head drive voltage (8) is compensated for by the drive time, and even if the head drive conditions change, the thermal head 1
The pulse width is corrected to keep the energy given constant.

Note that the present invention is not limited to the above embodiments,
Various modifications are possible based on the spirit of the present invention, and these are not excluded from the scope of the present invention.

For example, the above invention can be applied to thermal printers, thermal transfer printers, and the like.

(Effects of the Invention) As described above in detail, the present invention includes a line type thermal head in which a large number of heat generating resistive elements are arranged in a line, and selectively drives the heat generating resistive elements. In the drive circuit for the thermal head of a line thermal printer that performs printing, a pulse width correction circuit that corrects the pulse width of the strobe signal supplied to the thermal head is connected to the thermal head. A time constant circuit and a comparison circuit are provided in the thermal head, the head drive voltage of the power supply terminal of the thermal head is connected to the time constant circuit, and the signal obtained by the time constant circuit is connected to the comparison circuit. When the head drive voltage supplied to the head drive voltage changes, the signal output by the time constant circuit changes correspondingly, and is output by the comparison circuit as a change in the pulse width of the strobe signal.

Therefore, even if the head drive voltage fluctuates, by correcting the pulse width of the strobe signal, the energy given to the thermal head can be kept constant, and it is possible to prevent print quality from deteriorating.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a drive circuit for a thermal head according to the present invention;
Fig. 2 is an internal circuit diagram of the pulse width correction circuit, Fig. 3 is an operation time chart of the pulse width correction circuit, Fig. 4 is a schematic diagram showing a cross section of a conventional line thermal printer, and Fig. 5 is a conventional thermal printer. 6 is an operation time chart of a conventional line thermal printer, and FIG. 7 is an explanatory diagram of voltage drop. DESCRIPTION OF SYMBOLS 1...Thermal head, 4...Control unit, 5...Interface line, 6...I10 boat, 8...Common bus, 9...Motor driver circuit. Patent applicant Oki Electric Industry Co., Ltd.

Claims (1)

[Scope of Claims] A thermal head drive circuit for a line thermal printer that has a line-type thermal head in which a large number of heat-generating resistive elements are arranged in a line, and prints by selectively driving the heat-generating resistive elements, (a) A pulse width correction circuit for correcting the pulse width of the strobe signal supplied to the thermal head is connected to the thermal head, (b) A time constant circuit and a comparison circuit are provided in the pulse width correction circuit, (c) The above A head drive voltage of a power supply terminal of the thermal head is connected to a time constant circuit, (d) a signal obtained by the time constant circuit is connected to a comparison circuit, and (e) a signal corresponding to the head drive voltage of the thermal head is connected to a time constant circuit. A driving circuit for a thermal head, characterized in that a strobe signal whose pulse width has been corrected is output from the comparison circuit to the thermal head.