JPS618364A - Temperature controller for thermal print head - Google Patents

Abstract

PURPOSE:To perform a temperature control of a thermal print head smoothly without adding a special function, by controlling the power source voltage to be applied to the thermal print head utilizing a voltage drop compensating function of the power source circuit. CONSTITUTION:The temperature of a thermal print head 1 is detected with a temperature detector 2 and an output of a temperature detection circuit 3 is converted into an digital signal with an A/D conversion circuit 4 to be inputted into an arithmetic processing circuit 5, which performs computation of a set voltage value corresponding to the detection temperature and the results are converted into an analog signal with a D/A conversion circuit 6 to be inputted into a thermal print head power source circuit 7, which controls the power source voltage to be applied to the thermal print head 1 according to the signal inputted into the power source circuit 7. As the voltage to be applied to the thermal print head 1 varies, the heating value of the thermal print head 1 changes to vary the temperature thereof 1 and the changes in the temperature is detected to be fed back with temperature detector 2. Thus, the temperature of the thermal print head 1 is controlled.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a temperature control device for a thermal print head that controls the temperature of a thermal print head used in, for example, a thermal transfer color copying machine or a thermal printer.

[Technical background of the invention and its problems] Conventionally, temperature control means for this type of thermal print head (so-called thermal head) detects the temperature of the thermal print head and controls the pulse width applied to the thermal print head. There are two methods, including a method of controlling the thermal print head and a method of controlling the voltage applied to the thermal print head. Among these methods, the method of controlling the voltage applied to the thermal print head requires a very complicated power supply circuit for the thermal print head, resulting in high cost.

Therefore, there is a strong demand for something that is as simple as possible and also allows for easy changes in control contents.

[Object of the Invention] The present invention has been made in view of the above circumstances, and its first object is to smoothly and accurately control the temperature of a thermal print head without adding any special functions to the power supply circuit. It is an object of the present invention to provide a temperature control device for a thermal print head which can be controlled at low cost and has excellent performance.

A second object of the present invention is to provide a temperature control device for a thermal print head that prevents uneven print density and enables clear printing at all times.

[Summary of the Invention] In order to achieve the first object, the present invention utilizes a voltage drop compensation function provided in the power supply circuit of the thermal print head to control the power supply voltage applied to the thermal print head. This is what I did.

Further, in order to achieve the second object of the present invention, the power supply voltage applied to the thermal print head is controlled at regular intervals in accordance with the detected temperature value.

[Embodiment of the Invention] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

FIG. 1 schematically shows the configuration of a temperature control device for a thermal print head according to the present invention. That is, the temperature of the thermal print head 1 is detected by a temperature detector 2, which is connected to a temperature detection circuit 3. Since the output of the temperature detection circuit 3 is an analog signal, it is converted into an A/
Convert to digital signal in D conversion step 4. The output of the A/D converter circuit 4 is input to an arithmetic processing circuit 5, and the arithmetic processing circuit 5 performs arithmetic processing on a voltage setting value corresponding to the detected temperature 7. Since the calculation result of the calculation processing circuit 5 is a digital signal, it is converted into an analog signal by the D/A conversion circuit 6. The output of the D/A conversion circuit 6 is input to a thermal print head power supply circuit 7, and this power supply circuit 7 controls the power supply voltage applied to the thermal print head 1 according to the input signal. When the voltage applied to the thermal print head 1 changes, the amount of heat generated by the thermal print head 1 changes, and the temperature of the thermal print head 1 also changes, which is detected by the density detector 2 and fed back. In this way, the temperature of the thermal print head 1 is controlled.

Incidentally, the thermal print head has its own thermal response characteristics, and the response is delayed. Therefore, it is preferable that the cycle of temperature detection and voltage control be equal to a certain period corresponding to this response characteristic. If this period is too long, there will be no effect, and if it is too short, the response of the thermal print head will drop, resulting in an oscillation state. In both cases, uneven print density occurs.

Therefore, the present invention p prevents this by performing a control cycle of the power supply voltage applied to the thermal print head at a constant cycle in accordance with the detected temperature value, as described above. According to experiments conducted by the present inventors, the optimum period was set to 1 oois.

Figure 2 (a) and (b) show a temperature sensor 2 on a thermal print head 1.
This shows the state in which the is installed. That is, the thermal print head 1 includes a heat dissipation plate 11 formed of an iron plate or the like, a ceramic substrate 12 provided on the heat dissipation plate 11, and a large number of resistors provided in a straight line approximately in the center of the ceramic substrate 12. It is composed of a body 13 and the like. In addition, on both sides of the resistor 13, there are integrated circuits (ICs) such as a shift register for data transfer and a driver for driving the resistor, although this has not been disclosed orally.
integrated circuits) are provided 1 and these are protected by a protective cover 1
4.14. Note that the resistor 1
Although 3 is shown as a single straight line in the drawing, it actually consists of a large number of very small dot-shaped resistors arranged in a straight line at minute intervals. The temperature detector 2 is mounted on a so-called counterbore in the heat dissipation plate 11 under the ceramic substrate 12 near the resistor 13 at approximately the center of the thermal print head 1 configured as described above. A diode type thermistor is used as the temperature detector 2, and its resistance value is, for example, IOKΩ at 25°C.

FIG. 3 is a circuit diagram specifically showing the temperature detector 2, the temperature detection circuit 3, and the A/D conversion circuit 4. As shown in FIG.

That is, both ends of the thermistor TH as the temperature detector 2 are connected to both ends of the resistor R of the temperature detection circuit 3.

One end of this resistor Rs is grounded, and the other end is connected to a +5V power supply via a resistor R2 and a variable resistor VR in series.
Ru. As a result, when the temperature of the thermal print head 1 changes, the resistance value of the thermistor TH changes, and therefore the voltage across the resistor R1 changes. Therefore, this voltage corresponds to the temperature, and temperature is detected. The variable resistor VR is used to correct variations in the resistance value of the thermistor TH and to vary the voltage across the resistor R1.

The A7/'D conversion circuit 4 is constituted by an A/D conversion IC (for example, product number TC5090BP manufactured by Toshiba Corporation) 15. This ICl3 has an lNTl terminal,
External resistor R connected to INTJ and INTO terminals
3 and the value of capacitor C determine the A/D conversion time. In this embodiment, the constant is set to approximately 1111S.

When a clock pulse (approximately 750 K H2) is supplied to the OS C, 1, and N terminals as an external signal, and a pulse is applied to the ST and C terminals, A/D conversion is started for the analog signal input to the ArN terminal. and when the conversion is finished, E
The level of the OC terminal changes, allowing you to know the conversion. In this case, since there are only 4 bits of digital output, by switching the level of the R8EL terminal, a total of 8 bits of output, including the upper 4 bits and the lower 4 bits, can be obtained.

FIG. 4 is a block diagram specifically showing the arithmetic processing circuit 5. As shown in FIG. This arithmetic processing circuit 5 includes a CPU (Central Processing Unit, e.g. product number @t8o85.A manufactured by Intel Corporation) 21, a buffer circuit (e.g. product number SN' manufactured by Texas Instruments Inc.) 74LS244), 22, latch circuit (for example, Texas Instruments product number 5N74LS)
373) 23, both 1f'tl buffer circuits (for example, Texas Instruments product number 5N74LS2
45) 24, ROM (read only memory, for example Intel product number i 2764) 25, RAM
(random access memory) and input/output ports (
For example, it has an 8-bit configuration using product number 18155)26 manufactured by Intel Corporation. That is, CPLI2
1 generates an internal clock using a ceramic oscillator (not shown), and starts operation after receiving a reset pulse from an initialization circuit (not shown). Also,
The cPU 21 receives an interrupt signal from an interrupt control circuit (not shown).

The upper 8 bits of the address bus are then supplied from the CPU 21 to the ROM-25 via the buffer circuit 22.
Addresses from RO to 125 are specified. Also, C
From the address/data bus of the PLI 21, the lower "8 bits I" of the gRO, M address is sent via the latch circuit 23
- is supplied to the ROM 25. Zarani, CPU 21
The address/data bus is connected to the RAM and input/output port 2G via the bidirectional buffer circuit 2'4'.
It is connected to the port and performs addressing of RAM and input/output ports 2-6 and writing to ports 1 to 1 in output mode. On the other hand, RA as input mode
Transfers the contents of the RAM of the M and input/output ports 1-26, and transfers the contents of the input port. Also, ROM2
5 has its output connected to the address/data bus, and sends the contents of the designated address to that bus. Note that the input ports of the RAM and input/output boat 26 are connected to the A/D conversion circuit 4, and the output ports are connected to the D/A conversion circuit 6. In the above configuration, digital data from the A/D conversion circuit 4 is sent to the CPLJ 21 via the RAM, input/output board 26, and bidirectional buffer circuit 24, and the CPU 21 performs predetermined calculations and reads data from the ROM table. By performing the above processing, digital data corresponding to the power supply voltage of the power supply circuit 7 is created. The created digital data is sent to the D, -'A conversion circuit 6 via the bidirectional buffer circuit 24, RAM, and input/output port 26.

FIG. 5 is a circuit diagram specifically showing the D/A conversion circuit 6 and the power supply circuit 7. As shown in FIG. As for the overall connection, the arithmetic processing circuit 5
The outputs of the RAMJ 5 and the input/output port 26 are applied to the input of the D/A conversion circuit 6, converted into D/A, and applied to the power supply circuit 7. First, in the D/A conversion circuit 6, 8-bit digital data from the RAM and the input/output port 26 is input to a driver (for example, product number TD62 manufactured by Toshiba Corporation) via an input terminal 31.
003) added to 32 inputs. Note that the driver 32
A pull-up resistor 33 is connected between the input and the +5V power supply for protection in case the input is open. The driver 32 turns on and off depending on the input digital data, and when it is turned on, the photocoupler connected to its output (for example, product number T manufactured by Toshiba Corporation)
The light emitting diode of LP521-4) 34 is turned on and the phototransistor is turned on. When the driver 32 is off, the light emitting diode is not lit and the photo transistor is off. Note that the pull-up resistor 35 is used to limit the current of the light emitting diode.

When the phototransistor of the photocoupler 34 is turned on, the input of the buffer (for example, product number TC4050 manufactured by Toshiba Corporation) 36 connected to its output becomes low level, the output also becomes low level, and the buffer connected to the output becomes low level. The level of the ladder resistor 37 also becomes low level. When the phototransistor of the photocoupler 34 is turned off, the input of the buffer 36 becomes high level due to the pull-up resistor 38, the output also becomes high level, and the level of the ladder resistor 37 becomes high level. As above,
When the D/A conversion input level is low level, the level of the ladder resistor 37 is high level, and when the D/A conversion input level is high level, the level of the ladder resistor 37 is low level. And one end of the ladder resistor 37 is 0Vc
Since it is at the Nviil level, it becomes a D/A conversion circuit that divides the power supply voltage (5V) of the buffer 36 by 8 bits. That is, the D,/A conversion input level is 8C
When the D display is roOHJ, the output level of the ladder resistor 37 is +5■, and when the D/A conversion input level is BCD display and rFFl-IJ, the output level of the ladder resistor 37 is OV. Note that the reason for using the photocoupler 34 is to isolate the power sources of the D/'A conversion input and the D,/, and A conversion outputs, and the reason for this will be described later. However,
The output of the ladder resistor 37 is applied to the input of a buffer amplifier (for example, product number TA5358 manufactured by Toshiba Corporation) 39, and the output is taken out as a D/A conversion output (analog signal). Note that the buffer amplifier 39 is used as a buffer and for impedance conversion.

The power supply circuit 7 is constituted by, for example, a switching power supply device (for example, product number RM15-23RGB manufactured by TDK) 40, the terminal portion of which is shown in FIG. This power supply device 40 connects a commercial AC power source to AC (alternating current input) terminals 51 and 52, and connects to an RC (remote control)
By leaving the terminals 53 and 54 in the A-open state, the DC (direct current output) terminals 55, 561! A predetermined DC voltage is output to ffi. R8(
Remote sensing) terminals 57 and 58 were originally terminals for correcting the voltage drop caused by the wire between the terminal and the load, and can be corrected by connecting them as shown in Figure 7. . This is due to the potential difference between R8 terminal 57 and DC terminal 55 and the potential difference between R8 terminal 57 and DC terminal 55.
8 and the DC terminal 56, and corrects the power supply voltage by the amount of the potential difference.

Therefore, the fifth method uses this remote sensing function (voltage drop compensation function) to control the power supply voltage.
The connections shown in the figure. That is, the output of the buffer amplifier 39 that detects the D/A ratio is connected to the DC terminal 5 of the power supply 40.
5, and the R8 terminal 57 is connected to the 0VCNVI source. In this connection, when the output voltage of the buffer amplifier 39 increases, the potential difference between the RS terminal 57 and the DC terminal 55 increases by that amount, and an apparent negative voltage is applied to the R8 terminal 57. It is assumed that there is a voltage drop in the output by that amount, and the voltage at the DC terminal 55 is increased. Since a predetermined constant voltage is originally applied to the DC terminal 55, the voltage increases by this increase. Conversely, when the output voltage of the buffer amplifier 39 decreases, the voltage of the DC# terminal 55 decreases by that amount. In this way, D/
Thermal print head power supply circuit 7 according to the output of the A conversion circuit 6
The output voltage can be controlled. Furthermore, since a remote sensing function (voltage drop compensation function) is used for the control, there is no need for any special additional circuitry, and the system can be realized at low cost. Note that by setting the level of the RC terminal 53 to low level by the driver 41, the power supply device 40
The output of is turned off, and the output is turned on by setting it to high level.

Next, the reason why the photocoupler 34 is used in the D/A conversion circuit 6 to isolate the power source will be explained. As described above, by means of changing the power supply voltage applied to the thermal print head 1; since the spider-1 sensing function of the Nm device 40 is used, a control voltage is further added to the voltage applied to the thermal print head 1. There is a need. Therefore, this control voltage needs to be a floating power supply independent of other power supplies. This power supply is connected to a photocoupler 34, a pull-up resistor FL38, a buffer 36, a ladder resistor 37, and a buffer amplifier 39, respectively, in FIG. Figure 8 shows an example of the power supply, for example +5
■+5 from power input by DC/DC converter 61
■Power output is obtained. This is a power supply isolated from the input.

With the configuration described above, by controlling the power supply voltage applied to the thermal printing head 1 by using the remote sensing function (voltage drop compensation function) provided in the thermal printing head power supply circuit 7, Temperature control of the thermal print head 1 can be performed smoothly and accurately without adding any special circuit to the thermal print head power supply circuit 7, resulting in low cost (and excellent performance). By controlling the power supply voltage applied to the thermal print head 1 at a constant cycle of #Rjil corresponding to the response characteristics unique to the thermal print head, uneven printing density can be prevented and clear printing is always possible. .

[Effects of the Invention] As detailed above, the temperature tl of the thermal print head of the present invention
According to the 1m device, the temperature of the thermal print head can be smoothly and accurately controlled without adding any special W1 function to the power supply circuit, and therefore, it is possible to obtain a product with low cost and excellent performance. It also prevents uneven printing density.
Clear printing is always possible.

[Brief explanation of the drawing]

The drawings show one embodiment of the present invention, in which Fig. 1 is a block diagram schematically showing the overall configuration, and Fig. 2 is a top view and side view showing how the temperature sensor is attached to the thermal print head. Figure 3 shows the temperature detector, temperature detection circuit and
'The circuit diagram specifically showing the D conversion circuit, Figure 4, is al! i
A block diagram specifically showing the arithmetic processing circuit, Fig. 5 is a D/A
Figure 6 is a plan view showing the terminal section of the switching power supply; Figure 7 is a connection diagram for explaining the remote sensing function of the switching power supply; Figure 8 is a circuit diagram specifically showing the power supply circuit. The figure shows 70- of the D/A conversion circuit.
FIG. 2 is a circuit diagram showing a power supply; 1...Thermal print head, 2...Temperature detector, 3...
・Temperature detection circuit, 4...A,,'D conversion circuit, 5
...Arithmetic processing circuit, 6...D/A conversion circuit, 7
...Thermal print head power supply circuit. Applicant's Representative Patent Attorney Takehiko Suzue Figure 1 Figure 2 Figure 3

Claims (3)

[Claims] (1) In a device that controls the temperature of a thermal print head that performs printing by applying power to a resistor and applying pressure to a heat-sensitive medium, the temperature of at least one location near the resistor of the thermal print head temperature detection means for detecting;
A converter that converts the output of this temperature detection means into a digital signal.
D conversion means, arithmetic processing means that performs predetermined arithmetic processing using the output of this A/D conversion means, D/A conversion means that converts the output of this arithmetic processing means into an analog signal, and this D/A conversion means and a power supply circuit having a voltage drop compensation function for controlling the power supply voltage applied to the thermal print head by the output of the power supply circuit, and the power supply voltage applied to the thermal print head is controlled using the voltage drop compensation function of the power supply circuit. A temperature control device for a thermal print head, characterized in that: (2) In a device that controls the temperature of a thermal print head that performs printing by applying power to a resistor and applying pressure to a heat-sensitive medium, the temperature of at least one location near the resistor of the thermal print head temperature detection means for detecting;
A converter that converts the output of this temperature detection means into a digital signal.
D conversion means, arithmetic processing means that performs predetermined arithmetic processing using the output of this A/D conversion means, D/A conversion means that converts the output of this arithmetic processing means into an analog signal, and this D/A conversion means and a power supply circuit that controls the power supply voltage applied to the thermal print head according to the output of the temperature sensor, and the power supply voltage applied to the thermal print head is controlled at regular intervals in accordance with the detected temperature value. Temperature control device for thermal print head. (3) The temperature control device for a thermal print head according to claim 2, wherein the constant period is a constant time range corresponding to the unique response characteristics of the thermal print head.