JP2004001256A - Method and apparatus for measuring resistance of thermal head - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To measure the resistance of a heating element with high accuracy without increasing the cost. <P>SOLUTION: In a resistance measurement mode, a transistor Tr1 is turned on and transistors 18b-18n are turned off. Consequently, a switch Sb is turned on to start charging of a capacitor 20. A microcomputer 13 matches the applying voltage of a head VH to the discharge start voltage E by varying a control signal for the applying voltage of the head. After the charge voltage of the capacitor 20 reached the E, the switch Sb is turned off and the capacitor 20 begins to be discharged. Terminal voltage of the capacitor 20 is subjected to digital conversion continuously by means of an A/D converter 24 and the lowered voltage level V at the A/D input terminal of a resistance measuring section 13a is compared with a reference voltage Vref. When they match each other, the required discharge time Ta is measured by means of a timer 13c and then the resistance Ra of a heating element 18a is calculated and written in a RAM 13b. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method and an apparatus for measuring a resistance value of a heating element constituting a thermal head.
[0002]
[Prior art]
It is known that the resistance value of the heating element of the thermal head changes with time and printing. In addition, the change in resistance value varies from one heating element to another, which causes uneven printing. For this reason, the applicant of the present invention has proposed a method of measuring the resistance value of each heating element and controlling the energization time of each heating element based on the result (for example, JP-A-6-79897, JP-A-Hei. 9-193437).
[0003]
In the method described in the above publication, a capacitor is connected in parallel with the heating element and the reference resistor, and the discharge time is measured to calculate the resistance value of the heating element. In order to calculate the resistance value R of the heating element from the discharge time T, the following formula (2) is used. When the discharge start voltage is E, the reference voltage is Vref, the capacitance of the capacitor is C, the resistance value of the heating element is R, and the discharge time from E to Vref is T, the following equation (1) is established.
Vref / E = exp (-T / CR) (1)
∴R = −T / C / ln (Vref / E) (2)
If C is known, R can be calculated. Even if C is not known, it can be calculated from the following equation (3) by separately measuring the discharge time Ts discharged with the known resistance Rs.
R = Rs * T / Ts (3)
The discharge time T can be expressed by the following formula (4).
T = −CR · ln (Vref / E) (4)
[0004]
[Problems to be solved by the invention]
In the above equation (4), the discharge start voltage E, the reference voltage Vref, the capacitor capacitance C, and the resistance value R of the heating element have deviations. In particular, the influence of the discharge start voltage E and the reference voltage Vref is large. If the discharge time is set too short so that the counter for counting the discharge time does not overflow, the measurement accuracy decreases. In order to suppress this, since the discharge start voltage E and the reference voltage Vref must be set with high accuracy, the adjustment cost and the use of high-accuracy parts cause a cost increase.
[0005]
It is an object of the present invention to provide a resistance measurement method for a thermal head that can accurately measure the resistance of a heating element without increasing the cost.
[0006]
[Means for Solving the Problems]
In order to achieve the above object, the thermal head resistance measurement method of the present invention observes the head applied voltage applied to the thermal head from the power supply unit, and controls the power supply unit so that the head applied voltage becomes a predetermined voltage. In addition, a capacitor is connected in parallel with a plurality of heating elements arranged in a line on the thermal head, and the capacitor is charged by the power supply unit, and then the capacitor starts discharging with the predetermined voltage as a discharge start voltage, The time required to reach a predetermined reference voltage is measured for each heating element, and the resistance value of each heating element is calculated based on the measurement result. The head applied voltage is converted into a digital signal by the A / D conversion means and input to the control means, and the control means controls the power supply unit based on the digital signal.
[0007]
The thermal head resistance measurement apparatus according to the present invention includes a plurality of heating elements arranged in a line on the thermal head, one end connected to the heating element, and the connection between the heating element and a power supply terminal of the thermal head is turned on. A first switch means group to be turned off, a control circuit for controlling the first switch means group so that each heating element is connected to a power supply, a capacitor connected in parallel to a power supply terminal of the thermal head, and the heat generation Based on a power source for applying a head applied voltage to the thermal head to drive the element, A / D conversion means for converting the head applied voltage into a digital signal, and a digital signal input from the A / D conversion means Control means for controlling the power supply unit so that the head applied voltage becomes a predetermined voltage, and second switch means connected in series to the power supply unit and the capacitor After the second switch means is turned on and the capacitor is charged to the predetermined voltage, the second switch means is turned off, and the charge of the capacitor is started to be discharged by the heating element using the predetermined voltage as the discharge start voltage. It comprises discharge time measuring means for measuring the time required to reach a predetermined reference voltage for each heating element, and resistance value calculating means for calculating the resistance value of each heating element based on the measurement result.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
In FIG. 1 showing the electrical configuration of a color thermal printer to which the resistance value measuring method of the present invention is applied, image data of one frame is written in the frame memory 10 in a state of being separated for each color. At the time of gradation expression heating, image data of a color to be printed is read from the frame memory 10 line by line and written to the line memory 11. The image data in the line memory 11 is read for each pixel and sent to the comparator 12. The comparator 12 compares the image data of each pixel and the gradation data (comparison data), and outputs a signal “1” if the image data is larger.
[0009]
For example, in the case of 64 gradations, the microcomputer 13 sequentially generates gradation data of “0” to “3F” in hexadecimal. When the gradation data of “0” is sent from the microcomputer 13, the comparator 12 compares the image data of each pixel with the gradation data in order. As a result, the comparison result for one line is output as a serial signal from the comparator 12 and sent to the shift register 14 of the thermal head 1 via the switch Sa for switching between the print mode and the resistance measurement mode. When the comparison of the image data for one line is completed, the microcomputer 13 generates gradation data “1” and sends it to the comparator 12. Therefore, by using the gradation data of “0” to “3F”, the image data of each pixel is compared 64 times and converted into 64-bit drive data. The 64-bit drive data is sent to the shift register 14 in 64 steps.
[0010]
Serial drive data is shifted in the shift register 14 by a clock and converted into a parallel signal. The drive data converted into the parallel signal by the shift register 14 is latched in the latch array 15 in synchronization with the latch signal. When the strobe signal is input, the AND gate array 16 outputs an “H” signal when the input drive signal is “1”. The latch array 15 and the AND gate array 16 are provided with circuit elements for each pixel.
[0011]
Transistors Tr1 to Trn are connected to each output terminal of the AND gate array 16, and the transistor is turned on when the output signal is “H”. Heat generating elements 18a to 18n are connected in series to these transistors Tr1 to Trn. Resistance elements are used as the heating elements 18a to 18n.
[0012]
A noise absorbing capacitor 20 is connected in parallel with the heating elements 18 a to 18 n, and the capacitor 20 is connected to a power supply unit 21. The power supply unit 21 includes a switch Sb, a rectifier circuit 22, and a voltage stabilization circuit 23, and applies a head application voltage VH to the thermal head 1. The switch Sb is always closed in the print mode, and in the resistance measurement mode, the opening and closing of the switch Sb is controlled by the microcomputer 13 each time the resistance values Ra to Rn of the heating elements 18a to 18n are measured.
[0013]
A 10-bit A / D converter 24 is connected to one terminal of the capacitor 20, and an output from the A / D converter 24 is input to an A / D input terminal of the resistance measurement unit 13 a of the microcomputer 13. The microcomputer 13 is connected to the voltage stabilization circuit 23 of the power supply unit 21 via a 10-bit D / A converter 25 and inputs a D / A control signal to the D / A converter 25. Thus, the head application voltage control signal is input from the D / A converter 25 to the voltage stabilization circuit 23 of the power supply unit 21.
[0014]
In FIG. 2 showing the power supply unit 21, the AC power source is converted into a DC power source by a bridge type rectifier circuit 22 using four rectifier elements 31 a to 31 d. This DC power supply is reconverted into an AC power supply having a high frequency by the inverter 32 of the voltage stabilization circuit 23, and then converted into DC of two different voltages by the transformer 33 and the rectifying elements 34a and 34b.
[0015]
The output having the lower voltage is smoothed by the capacitors 35 and 36 and the coil 37, then divided by the resistors 38 and 39, and applied to the non-inverting input terminal of the comparator 40. On the other hand, the head application voltage control signal is input from the D / A converter 25 to the inverting input terminal of the comparator 40, and the output terminal of the comparator 40 is connected to the inverter 32. That is, the frequency of the inverter 32 is specified by the head application voltage control signal, and the head application voltage VH output from the voltage stabilization circuit 23 is controlled. The connection point between the coil 37 and the capacitor 36 is connected to the thermal head 1 via the FET 42. The FET 42 is connected between a gate and a source with a Zener diode 43 for maintaining a constant voltage therebetween and a diode 44 for preventing a reverse flow when the FET 42 is OFF.
[0016]
Normally, since the transistor 45 is off, the FET 42 is on. Accordingly, since the switch Sb is turned on, the constant voltage VH controlled by the inverter 32 is supplied to the thermal head 1 via the line 46. In the resistance measurement mode, the transistor 45 is turned on by a start signal from the resistance measurement unit 13a. When the transistor 45 is turned on, the gate voltage of the FET 42 is lowered and the FET 42 is turned off, so that the switch Sb is turned off. At this time, the discharge power from the capacitor 20 of the thermal head 1 tends to flow through the Zener diode 43 and the transistor 45, but this is prevented because the diode 44 is provided. Therefore, all the discharge current of the capacitor 20 is supplied to the heating element.
[0017]
The microcomputer 13 changes the head application voltage control signal output from the D / A converter 25 according to the D / A control output signal, controls the head application voltage VH output from the power supply unit 21, and sets the head application voltage VH. It is made to coincide with a predetermined discharge start voltage E.
[0018]
In the resistance measurement mode, after charging the capacitor 20 to the discharge start voltage E, the switch Sb is opened, and only the transistor Tr1 of the heating element 18a to be measured is turned on, for example. Immediately after this, the voltage value V of the A / D input terminal of the resistance measuring unit 13a is the discharge start voltage E, but as the charge accumulated in the capacitor 20 is discharged by the heating element 18a, the A / D input terminal. Voltage value V decreases and eventually coincides with the reference voltage Vref stored in the resistance measurement unit 13a. During this time, the A / D converter 24 continuously converts the voltage input from the capacitor 20 into a digital signal.
[0019]
The resistance measurement unit 13a of the microcomputer 13 observes the voltage value V of the A / D input terminal immediately after the switch Sb is opened, and measures the time Ta until the voltage value V matches the reference voltage Vref by the timer 13c. Based on this, the resistance value Ra of the heating element 18a is calculated and written to the RAM 13b.
[0020]
In order to calculate the resistance value Ra from the discharge time Ta, the following equation (5) is used. When the discharge start voltage is E, the discharge time is T, the capacitance of the capacitor 20 is C, and the resistance value of the heating element is R, the terminal voltage V of the capacitor 20 is expressed by the following equation (1).
V / E = exp (-T / CR) (1)
Assuming V = (1/2) E, Equation (1) is as follows.
exp (-T / CR) = 1/2
∴R = T / 0.693C (5)
[0021]
Here, since the capacitance C of the capacitor 20 is a constant, the resistance value R of the heating element is determined by the discharge time T. In the print mode, the image data is corrected by the resistance value R calculated according to the equation (5) and printed. The resistance values Ra to Rn written in the RAM 13b are backed up by the battery 26.
[0022]
Next, the operation of the above embodiment will be described with reference to FIGS. During the initial setup of the color thermal printer, the switch Sa is switched to the resistance measurement mode, and the shift register 14 is connected to the microcomputer 13. The microcomputer 13 outputs data for one line in which the transistor Tr1 is on and the other transistors 18b to 18n are off. Then, the switch Sb is turned on by the resistance measuring unit 13a, and charging of the capacitor 20 is started.
[0023]
The microcomputer 13 changes the head application voltage control signal output from the D / A converter 25, controls the head application voltage VH output from the power supply unit 21, and matches the head application voltage VH with the discharge start voltage E. Let After the charge voltage of the capacitor 20 reaches E, the switch Sb is turned off. As a result, the electric charge of the capacitor 20 starts to be discharged by the heating element 18a, and the terminal voltage of the capacitor 20 is continuously digitally converted by the A / D converter 24, and the voltage value V of the A / D input terminal of the resistance measuring unit 13a. Is reduced. The resistance measuring unit 13a compares the voltage value V of the A / D input terminal with the reference voltage Vref, and when they match, the discharge time Ta required for this is measured by the timer 13c, and the heating element 18a is calculated by the equation (5). Resistance value Ra is calculated and written in the RAM 13b.
[0024]
Next, the transistor 18b is turned on, and the other transistors Tr1 and transistors 18c to 18n are turned off. After the discharge time Tb by the heating element 18b is measured by the resistance measuring unit 13a, the resistance value Rb of the heating element 18b is calculated and written in the RAM 13b. Similarly, the resistance values Rc to Rn of the heating elements 18c to 18n are calculated and written in the RAM 13b. These resistance values Rc to Rn are retained until the battery 26 is exhausted thereafter. Alternatively, it is stored in a non-volatile memory (not shown).
[0025]
In the print mode, the shift register 14 is connected to the comparator 12 by the switch Sa. In this print mode, first, image data of three colors is taken into the frame memory 10. For these image data, correction data is calculated from the difference between the ideal resistance value when the heating elements 18a to 18n are completely uniform and the actually measured resistance values Ra to Rn, and recorded by the heating elements 18a to 18n. The image data is corrected by the correction data so that the image to be printed is accurately printed.
[0026]
When the leading end of the recording area of a known color thermosensitive recording material in which a magenta thermosensitive coloring layer, a cyan thermosensitive coloring layer, and a yellow thermosensitive coloring layer are sequentially formed on a support reaches the thermal head 1, thermal recording is started. At the time of this thermal recording, image data of a yellow image is read from the frame memory 10 for one line and is once written in the line memory 11.
[0027]
Next, the corrected image data of each pixel is sequentially read out from the line memory 11 and sent to the comparator 12, where it is compared with the gradation data of the gradation level “0”. The output of the comparator 12 is “1” for a pixel that records a yellow image, and “0” for a pixel that does not record a yellow image. The comparison result of each pixel is sent to the shift register 14 as serial drive data, and is shifted in the shift register 14 by a clock to be converted into parallel drive data. The parallel drive data is latched by the latch array 15 and then sent to the AND gate array 16.
[0028]
The microcomputer 13 generates a bias heating pulse having a long width and sends it to the AND gate array 16 as a strobe signal. Since the AND gate array 16 outputs a logical product of the strobe signal and the output signal of the latch array 15, the output terminal of the latch array 15 among the output terminals of the AND gate array 16 is “1”. 1 "is output. For example, when the first output terminal of the AND gate array 16 is “1”, since the transistor Tr1 is turned on, the heating element 18a is energized to generate heat. As a result, the heating element 18a is energized for a time corresponding to the bias heating pulse, and bias thermal energy is applied to the color thermal recording material.
[0029]
Before the bias heating is completed, the microcomputer 13 generates gradation data having a gradation level of “0” and sends it to the comparator 12 for comparison with the image data of each pixel again. By this comparison, serial drive data is formed, and this drive data is written into the shift register 14. When the bias heating is completed, the microcomputer 13 generates a gradation expression pulse having a short pulse width. This gradation expression pulse is sent to the AND gate array 16 as a strobe signal. The heating element is energized for a short time by the strobe signal, and the yellow thermosensitive coloring layer is colored to the density of the gradation level “1”.
[0030]
Hereinafter, in order for the microcomputer 13 to sequentially change the gradation level from “1” to “3F”, drive data corresponding to each gradation level is output from the comparator 12. As a result, the heat generating elements 18a to 18n are energized a number of times according to the corrected image data, and tone expression thermal energy is applied to the color thermosensitive recording material to develop a desired density. For example, in the case of 64 gradations, 64 pulse currents are supplied to the heat generating elements for gradation expression for the pixels with the maximum density.
[0031]
When the first line of the yellow image is recorded, the color thermosensitive recording material is fed by one pixel, and at the same time, the image data of the second line of the yellow image is read from the frame memory 10. Based on the image data of the second line of the yellow image, the second line is thermally recorded on the color thermosensitive recording material. Near ultraviolet rays having a wavelength of about 420 nm are irradiated on the portion where the yellow image is thermally recorded. As a result, the diazonium salt compound contained in the yellow thermosensitive recording material is decomposed and the color developing ability is lost.
[0032]
When the recording area is again at the position of the thermal head 1, a magenta image is recorded on the magenta thermosensitive coloring layer line by line. The coloring heat energy of the magenta image is larger than the coloring heat energy of the yellow image. However, since the yellow thermosensitive coloring layer has already been photofixed, the yellow thermosensitive coloring layer will not develop color again. The color thermosensitive recording material on which the magenta image is recorded is irradiated with ultraviolet rays of around 365 nm, and the magenta thermosensitive coloring layer is photofixed.
[0033]
When the recording area is again at the position of the thermal head 1, a cyan image is recorded on the cyan thermosensitive coloring layer line by line. This cyan thermosensitive coloring layer has a value that does not develop color under normal storage conditions, so that the cyan thermosensitive coloring layer is not given photofixability. The color thermal recording material in which the thermal recording of the yellow image, the magenta image, and the cyan image is completed in this manner is discharged out of the color thermal printer.
[0034]
The resistance values Ra to Rn of the heating elements 18a to 18n written in the RAM 13b are held by the built-in backup battery 26. However, when the battery 26 is consumed or has changed over time, the resistance value Ra to Rn is measured again to measure the RAM 13b. Write to. Further, backup power may be supplied from, for example, the power supply unit 21 without using the battery 26. In this case, since the data in the RAM 13b is lost immediately after the thermal printer is turned off, the resistance values Ra to Rn are measured every time the thermal printer is set up. If the data in the RAM 13b is stored in the nonvolatile memory, the resistance values Ra to Rn may be measured only when the data is destroyed.
[0035]
Next, another embodiment will be described with reference to FIGS. In this embodiment, a reference resistor 19 and a transistor Trs are connected in parallel with the heating elements 18a to 18n and the transistors Tr1 to Trn. The reference resistor 19 has a known resistance value Rs and an error of about 1%. Is used. Further, as the reference voltage Vref of the comparator 50, an appropriate voltage lower than the discharge start voltage E can be used, but it can be specified as follows. That is, the microcomputer 13 decrements the head application voltage control signal (D / A conversion value) from the D / A converter 25 by one from MAX (value when the head application voltage VH = E), one by one. The applied voltage VH is gradually lowered, and when the stop signal from the comparator 50 becomes active, the voltage value of the A / D input terminal input from the A / D converter 24 to the resistance measuring unit 13 is referred to as a reference voltage Vref. To do.
[0036]
The discharge start voltage E can be calculated from the following equation (6).
E = Vref / exp (−Tmax / CmaxRmax) (6)
Tmax is the maximum measurement time of the timer 13c, Cmax is the capacitance of the capacitor 20 to be used, and Rmax is the resistance value Rs of the reference resistor 19. The following equation (7) with a margin so that the timer 13c does not overflow may be used.
E = Vref / 0.9exp (−Tmax / CmaxRmax) (7)
[0037]
The microcomputer 13 converts the head applied voltage VH by the A / D converter 24 while changing the head applied voltage VH by the D / A converter 25, and performs D / A conversion so that VH = E. D / A control output signal of the device 25 is set.
[0038]
For example, in order to measure the resistance value Ra of the heating element 18a, first, the switch Sa is switched to the resistance measurement mode, and only the transistor Trs is turned on while the transistors Tr1 to Trn are turned off, and the switch Sb is closed. After the capacitor 20 is charged to the discharge start voltage E, the switch Sb is opened, and the time Ts during which the charge of the capacitor 20 is discharged by the reference resistor 19 is measured. Next, the transistors 18b to 18n and the transistor Trs are turned off, and only the transistor Tr1 is turned on. After the switch Sb is closed and the capacitor 20 is charged, the switch Sb is opened and the discharge time Ta by the heating element 18a is measured.
[0039]
The resistance value Ra of the heating element 18a is expressed by the formula Ra = (Ta / Ts) R. _S Therefore, the calculation accuracy of the resistance value Ra does not depend on the capacitance C of the capacitor 20, and the resistance value R of the reference resistor 19 is calculated. _S It depends only on the accuracy of Resistance value R _S Is an error of about 1%, the resistance value Ra of the heating element 18a can be calculated with high accuracy. The resistance values Rb to Rn of the other heating elements 18b to 18n can be calculated similarly.
[0040]
In the above embodiment, the stop signal for stopping the measurement of the discharge time is generated using the comparator 50. However, if the A / D converter 24 is high speed, the terminal voltage of the capacitor 20 being discharged by the reference resistor 19 is used. Since the microcomputer 13 can monitor A by continuously A / D converting V, the time Ts until V = Vref may be measured (see FIGS. 9 and 10). The reference voltage Vref is a predetermined voltage value stored in advance in the microcomputer 13.
[0041]
It is also possible to take out only the function of measuring the resistance value of the heating element of the thermal head from the color thermal printer described above, and to make it independent as a resistance measuring device of the thermal head. In this case, for example, it can be used as a tester for inspecting the thermal head.
[0042]
In the above embodiment, both A / D converters and D / A converters have 10 bits. However, the present invention is not limited to this, and for example, 16 bits or 8 bits may be used depending on the purpose. But you can. Further, since only the A / D converter requires high accuracy, the A / D converter may be 10 bits and the D / A converter may be 8 bits.
[0043]
In the above embodiment, (1/2) E is used as the reference voltage Vref. When the reference voltage Vref is close to the head applied voltage E, the measurement accuracy is lowered instead of shortening the time for measuring the discharge time of the capacitor, and even when the reference voltage Vref is set to “0” or a value close thereto. The discharge curve becomes smoother, the measurement accuracy becomes worse, and the measurement time becomes longer. In the present embodiment, Vref = (1/2) E is set in order to perform quick and highly accurate measurement. However, the present invention is not limited to this, and may be (1/3) E or (2/3) E, for example.
[0044]
Although a color thermal printer is taken as an example, the present invention can also be applied to a monochrome thermal printer, a color thermal transfer printer, and the like. In the description of the embodiment, the heating element or the reference resistor 19 is turned on in advance, and the capacitor is charged. However, conversely, the capacitor is charged first, and then the heating element or the reference resistor is charged. The battery 19 may be turned on to cause discharge.
[0045]
【The invention's effect】
As described above in detail, the present invention observes the head applied voltage applied to the thermal head from the power supply unit, controls the power supply unit so that the head applied voltage becomes a predetermined voltage, and forms a line on the thermal head. Using the charging / discharging of a capacitor connected in parallel with a plurality of heating elements arranged at the same time, the time until this capacitor discharges and reaches a predetermined potential is measured for each heating element. Since the resistance value of each heating element is calculated, it is unnecessary to adjust the discharge start voltage and the reference voltage, which are conventionally required, and the resistance measurement of the heating element can be performed with high accuracy without increasing the cost. Also, the head applied voltage is converted into a digital signal by the A / D conversion means and input to the control means, and the control means controls the power supply unit based on the digital signal, so that the above effect can be achieved with a very simple configuration. Can be achieved.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an electric circuit of a color thermal printer.
FIG. 2 is a circuit diagram illustrating an example of a power supply unit.
FIG. 3 is a flowchart of a resistance measurement mode.
4 is a waveform diagram of signals supplied to the respective units shown in FIG. 1. FIG.
FIG. 5 is a flowchart of a print mode.
FIG. 6 is a block diagram showing another example of an electric circuit.
7 is a waveform diagram of signals supplied to the respective units shown in FIG. 6. FIG.
FIG. 8 is a flowchart of a resistance measurement mode of the embodiment shown in FIG.
FIG. 9 is a block diagram showing an example of an electric circuit that does not require a circuit for generating a stop signal.
10 is a flowchart of a resistance measurement mode of the embodiment shown in FIG.
[Explanation of symbols]
1 Thermal head
12, 40, 50 comparator
13 Microcomputer
13a Resistance measurement unit
13b RAM
13c timer
18a-18n Heating element
19 Reference resistor
20 capacitors
21 Power supply
23 Voltage stabilization circuit
24 A / D converter
25 D / A converter
Sa, Sb switch

Claims (3)

The head application voltage applied to the thermal head from the power supply unit is observed, the power supply unit is controlled so that the head application voltage becomes a predetermined voltage, and a capacitor is connected in parallel with a plurality of heating elements arranged in a line on the thermal head. After the capacitor is charged by the power supply unit, the capacitor starts discharging with the predetermined voltage as the discharge start voltage, and the time until the capacitor reaches the predetermined reference voltage is measured for each heating element. A method for measuring a resistance value of a thermal head, wherein the resistance value of each heating element is calculated based on the result. 2. The thermal head according to claim 1, wherein the head applied voltage is converted into a digital signal by an A / D conversion means and inputted to the control means, and the control means controls the power supply unit based on the digital signal. Resistance value measurement method. A plurality of heating elements arranged in a line on the thermal head, a first switch means group having one end connected to the heating element and turning on / off the connection between the heating element and a power supply terminal of the thermal head; A control circuit for controlling the first switch means group so that the heating element is connected to the power source, a capacitor connected in parallel to the power supply terminal of the thermal head, and a head applied voltage for driving the heating element. A power supply unit for applying power to the head, A / D conversion means for converting the head application voltage into a digital signal, and a power supply so that the head application voltage becomes a predetermined voltage based on the digital signal input from the A / D conversion means. Control means for controlling the power supply section, second switch means connected in series to the power supply section and the capacitor, and the second switch means to turn on the condenser. After the battery is charged to the predetermined voltage, the second switch means is turned off, and the time from the start of discharge of the capacitor by the heating element to the arrival of the predetermined reference voltage is determined with the predetermined voltage as the discharge start voltage. A thermal head resistance value measuring device comprising: a discharge time measuring means for measuring each heating element; and a resistance value calculating means for calculating a resistance value of each heating element based on the measurement result.

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